November 2019
Volume 60, Issue 14
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Visual Neuroscience  |   November 2019
Retinal Function in X-Linked Juvenile Retinoschisis
Author Affiliations & Notes
  • Lucia Ambrosio
    Department of Ophthalmology, Boston Children's Hospital & Harvard Medical School, Boston, Massachusetts, United States
  • Ronald M. Hansen
    Department of Ophthalmology, Boston Children's Hospital & Harvard Medical School, Boston, Massachusetts, United States
  • Rotem Kimia
    Department of Ophthalmology, Boston Children's Hospital & Harvard Medical School, Boston, Massachusetts, United States
  • Anne B. Fulton
    Department of Ophthalmology, Boston Children's Hospital & Harvard Medical School, Boston, Massachusetts, United States
  • Correspondence: Lucia Ambrosio, Department of Ophthalmology, Boston Children's Hospital & Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115, USA; Lucia.Ambrosio@childrens.harvard.edu
Investigative Ophthalmology & Visual Science November 2019, Vol.60, 4872-4881. doi:https://doi.org/10.1167/iovs.19-27897
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      Lucia Ambrosio, Ronald M. Hansen, Rotem Kimia, Anne B. Fulton; Retinal Function in X-Linked Juvenile Retinoschisis. Invest. Ophthalmol. Vis. Sci. 2019;60(14):4872-4881. doi: https://doi.org/10.1167/iovs.19-27897.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose: To assess retinal function in young patients with X-linked juvenile retinoschisis (XLRS), a disorder that is known to alter ERG postreceptor retinal components and also possibly photoreceptor components.

Methods: ERG responses to full-field stimuli were recorded under scotopic and photopic conditions in 12 XLRS patients aged 1 to 15 (median 8) years. A- and b-wave amplitudes and implicit times were examined over a range of stimulus intensities. Rod and cone photoreceptor (SROD, RROD, SCONE, RCONE) and rod-driven postreceptor (log σ, VMAX) response parameters were calculated from the a- and b-waves. Data from XLRS patients were evaluated for significant change with age.

Results: A- and b-wave amplitudes were smaller in XLRS patients compared with controls under both scotopic and photopic conditions. Saturated photoresponse amplitude (RROD), postreceptor b-wave (log σ), and saturated b-wave amplitude (VMAX) were significantly lower in XLRS patients than in controls; SROD did not differ between the two groups. SCONE and RCONE values were normal. In XLRS patients, neither a- and b-wave amplitudes nor calculated parameters (SROD, RROD, log σ, VMAX, SCONE, and RCONE) changed with age.

Conclusions: In these young XLRS patients, RROD and a-wave amplitudes were significantly smaller than in controls. Thus, in addition to XLRS causing postreceptor dysfunction, an effect of XLRS on rod photoreceptors cannot be ignored.

X-linked juvenile retinoschisis (XLRS), a monogenic retinal disorder due to changes in RS1, is characterized by intralaminar retinal cavities that, by ophthalmoscopy, may appear as a spoke-like pattern in the macula, even during infancy and childhood.13 
The XLRS ERG a-wave, representing photoreceptor activity, tends to be relatively more robust than the b-wave, representing postreceptor activity. Nonetheless, in some XLRS patients, smaller than normal a-wave amplitudes46 and small rod–saturated photoresponse amplitudes have been reported7; the median age of participants in those studies was 21, 25.5, and 31 years, respectively. 
Between ages 4.5 and 55 years, there is some evidence of age-related decline in the amplitude of the b-wave response from dark-adapted XLRS eyes6; although Bradshaw et al.4 found no significant change in XLRS a-wave amplitude between age 6 and 70 years, and Cukras et al.3 showed stability of the ERG parameters over time. 
In a knockout mouse model of XLRS,8 both rods and cones degenerate. In view of this evidence of age-related changes and of photoreceptor degeneration, we decided to re-examine photoreceptor and postreceptor responses in young patients with XLRS. 
Methods
Patients
Twelve patients with XLRS (aged 1–15 years; median 8 years) were included (Table 1). The diagnosis of XLRS was based on family history, ophthalmoscopic findings, and results of spectral-domain optical coherence tomography (SD-OCT). By ophthalmoscopy, all patients had foveal schisis and a faint, reticular pattern in peripheral retina suggestive of peripheral schisis; by OCT of the maculas, the presence of cavities between the outer plexiform layer and the outer nuclear layer was documented. 
Table 1
 
XLRS Patients and Their Visual Acuities and Spherical Equivalents Close to the Time of ERG Test
Table 1
 
XLRS Patients and Their Visual Acuities and Spherical Equivalents Close to the Time of ERG Test
Visual acuity, measured using age-appropriate tests, and spherical equivalent (diopters [D]), derived from results of cycloplegic retinoscopy, are shown in Table 1. Families of nine patients consented to genetic testing. Five unrelated patients had the same mutation in RS1–Trp96Arg. 
The patients' ERG results were compared with those in 13 healthy controls (aged 20–31 years; median 22 years). Normal ERG response parameters are stable in this age range and have completely matured by age 1 year.14,15 An additional study provided control data from other healthy young adults for comparison to the XLRS results for ERG responses to a photopic 150-ms stimulus.14 
This study adhered to the tenets of the Declaration of Helsinki for research involving human participants and was approved by the Boston Children's Hospital Committee on Clinical Investigation. 
ERG
ERG responses to full-field stimuli were recorded using previously described methods.16 In brief, phenylephrine 2.5% and cyclopentolate 1% were administered to dilate the pupils. After 30 minutes of dark adaptation from room light, the participants were prepared for testing under a dim red light. Following instillation of proparacaine 0.5%, a Burian Allen bipolar electrode (Hansen Ophthalmic Development Laboratory, Coralville, IA, USA) was placed on each eye and a ground electrode was placed on the skin over one mastoid. ERG responses were recorded using Espion e2 system (Diagnosys LLC, Lowell, MA, USA). 
Three patients (#10, #11, #12) were using a topical carbonic anhydrase inhibitor17 (dorzolamide, Trusopt 2%, 3×/day, each eye; Merck, Kenilworth, NJ, USA) at the time of the ERG test. Responses from four patients (#1, #2, #3, and #4) were recorded under brief, light anesthesia. All other patients and controls were tested awake. 
Rod and Rod-Driven Activity
Responses to a range (from −2 to +3.3 log scot td s) of blue (λ = 470 ± 30 nm), brief (<3 ms) stimuli were recorded. Stimuli were incremented in 0.3-log unit steps. Two to 16 responses were averaged in each stimulus condition. The amplitude and implicit times of the a- and b-wave responses were examined as a function of stimulus intensity. The a-wave amplitude was measured from the baseline to the a-wave trough. The b-wave amplitude was measured from the a-wave trough to the peak of the b-wave. The a- and b-wave implicit times were measured from stimulus onset to the maximum amplitude of the response. 
For all participants, rod-response parameters were estimated using a model of the activation of rod phototransduction fit to the a-wave responses to the most intense stimuli (∼2.1–3.3 log scot td s; Equation 1),18,19  
\(\def\upalpha{\unicode[Times]{x3B1}}\)\(\def\upbeta{\unicode[Times]{x3B2}}\)\(\def\upgamma{\unicode[Times]{x3B3}}\)\(\def\updelta{\unicode[Times]{x3B4}}\)\(\def\upvarepsilon{\unicode[Times]{x3B5}}\)\(\def\upzeta{\unicode[Times]{x3B6}}\)\(\def\upeta{\unicode[Times]{x3B7}}\)\(\def\uptheta{\unicode[Times]{x3B8}}\)\(\def\upiota{\unicode[Times]{x3B9}}\)\(\def\upkappa{\unicode[Times]{x3BA}}\)\(\def\uplambda{\unicode[Times]{x3BB}}\)\(\def\upmu{\unicode[Times]{x3BC}}\)\(\def\upnu{\unicode[Times]{x3BD}}\)\(\def\upxi{\unicode[Times]{x3BE}}\)\(\def\upomicron{\unicode[Times]{x3BF}}\)\(\def\uppi{\unicode[Times]{x3C0}}\)\(\def\uprho{\unicode[Times]{x3C1}}\)\(\def\upsigma{\unicode[Times]{x3C3}}\)\(\def\uptau{\unicode[Times]{x3C4}}\)\(\def\upupsilon{\unicode[Times]{x3C5}}\)\(\def\upphi{\unicode[Times]{x3C6}}\)\(\def\upchi{\unicode[Times]{x3C7}}\)\(\def\uppsy{\unicode[Times]{x3C8}}\)\(\def\upomega{\unicode[Times]{x3C9}}\)\(\def\bialpha{\boldsymbol{\alpha}}\)\(\def\bibeta{\boldsymbol{\beta}}\)\(\def\bigamma{\boldsymbol{\gamma}}\)\(\def\bidelta{\boldsymbol{\delta}}\)\(\def\bivarepsilon{\boldsymbol{\varepsilon}}\)\(\def\bizeta{\boldsymbol{\zeta}}\)\(\def\bieta{\boldsymbol{\eta}}\)\(\def\bitheta{\boldsymbol{\theta}}\)\(\def\biiota{\boldsymbol{\iota}}\)\(\def\bikappa{\boldsymbol{\kappa}}\)\(\def\bilambda{\boldsymbol{\lambda}}\)\(\def\bimu{\boldsymbol{\mu}}\)\(\def\binu{\boldsymbol{\nu}}\)\(\def\bixi{\boldsymbol{\xi}}\)\(\def\biomicron{\boldsymbol{\micron}}\)\(\def\bipi{\boldsymbol{\pi}}\)\(\def\birho{\boldsymbol{\rho}}\)\(\def\bisigma{\boldsymbol{\sigma}}\)\(\def\bitau{\boldsymbol{\tau}}\)\(\def\biupsilon{\boldsymbol{\upsilon}}\)\(\def\biphi{\boldsymbol{\phi}}\)\(\def\bichi{\boldsymbol{\chi}}\)\(\def\bipsy{\boldsymbol{\psy}}\)\(\def\biomega{\boldsymbol{\omega}}\)\(\def\bupalpha{\unicode[Times]{x1D6C2}}\)\(\def\bupbeta{\unicode[Times]{x1D6C3}}\)\(\def\bupgamma{\unicode[Times]{x1D6C4}}\)\(\def\bupdelta{\unicode[Times]{x1D6C5}}\)\(\def\bupepsilon{\unicode[Times]{x1D6C6}}\)\(\def\bupvarepsilon{\unicode[Times]{x1D6DC}}\)\(\def\bupzeta{\unicode[Times]{x1D6C7}}\)\(\def\bupeta{\unicode[Times]{x1D6C8}}\)\(\def\buptheta{\unicode[Times]{x1D6C9}}\)\(\def\bupiota{\unicode[Times]{x1D6CA}}\)\(\def\bupkappa{\unicode[Times]{x1D6CB}}\)\(\def\buplambda{\unicode[Times]{x1D6CC}}\)\(\def\bupmu{\unicode[Times]{x1D6CD}}\)\(\def\bupnu{\unicode[Times]{x1D6CE}}\)\(\def\bupxi{\unicode[Times]{x1D6CF}}\)\(\def\bupomicron{\unicode[Times]{x1D6D0}}\)\(\def\buppi{\unicode[Times]{x1D6D1}}\)\(\def\buprho{\unicode[Times]{x1D6D2}}\)\(\def\bupsigma{\unicode[Times]{x1D6D4}}\)\(\def\buptau{\unicode[Times]{x1D6D5}}\)\(\def\bupupsilon{\unicode[Times]{x1D6D6}}\)\(\def\bupphi{\unicode[Times]{x1D6D7}}\)\(\def\bupchi{\unicode[Times]{x1D6D8}}\)\(\def\buppsy{\unicode[Times]{x1D6D9}}\)\(\def\bupomega{\unicode[Times]{x1D6DA}}\)\(\def\bupvartheta{\unicode[Times]{x1D6DD}}\)\(\def\bGamma{\bf{\Gamma}}\)\(\def\bDelta{\bf{\Delta}}\)\(\def\bTheta{\bf{\Theta}}\)\(\def\bLambda{\bf{\Lambda}}\)\(\def\bXi{\bf{\Xi}}\)\(\def\bPi{\bf{\Pi}}\)\(\def\bSigma{\bf{\Sigma}}\)\(\def\bUpsilon{\bf{\Upsilon}}\)\(\def\bPhi{\bf{\Phi}}\)\(\def\bPsi{\bf{\Psi}}\)\(\def\bOmega{\bf{\Omega}}\)\(\def\iGamma{\unicode[Times]{x1D6E4}}\)\(\def\iDelta{\unicode[Times]{x1D6E5}}\)\(\def\iTheta{\unicode[Times]{x1D6E9}}\)\(\def\iLambda{\unicode[Times]{x1D6EC}}\)\(\def\iXi{\unicode[Times]{x1D6EF}}\)\(\def\iPi{\unicode[Times]{x1D6F1}}\)\(\def\iSigma{\unicode[Times]{x1D6F4}}\)\(\def\iUpsilon{\unicode[Times]{x1D6F6}}\)\(\def\iPhi{\unicode[Times]{x1D6F7}}\)\(\def\iPsi{\unicode[Times]{x1D6F9}}\)\(\def\iOmega{\unicode[Times]{x1D6FA}}\)\(\def\biGamma{\unicode[Times]{x1D71E}}\)\(\def\biDelta{\unicode[Times]{x1D71F}}\)\(\def\biTheta{\unicode[Times]{x1D723}}\)\(\def\biLambda{\unicode[Times]{x1D726}}\)\(\def\biXi{\unicode[Times]{x1D729}}\)\(\def\biPi{\unicode[Times]{x1D72B}}\)\(\def\biSigma{\unicode[Times]{x1D72E}}\)\(\def\biUpsilon{\unicode[Times]{x1D730}}\)\(\def\biPhi{\unicode[Times]{x1D731}}\)\(\def\biPsi{\unicode[Times]{x1D733}}\)\(\def\biOmega{\unicode[Times]{x1D734}}\)\begin{equation}\tag{1}R\left( \Phi ,t \right) = R_{\rm{ROD}} \left\{ 1 - \exp \left[ - 0.5 S_{\rm{ROD}} \Phi \left( t - t_d \right)^2 \right] \right\}\quad {\rm{for}}\ t\ \gt\ t_d \end{equation}
where Φ is the flash in scot td s; SROD [(scot td s)−1 s−2] is a sensitivity parameter that depends on the time constants of the molecular processes involved in the activation of phototransduction and is related to the amplification constant for phototransduction; RROD is the saturated response amplitude (μV) that, in normal retina, represents the number of channels in the rod outer segment membrane available for closure by light; and td is a brief delay (seconds).18,19 Fitting was restricted to the leading edge of the a-wave or to a maximum of 15 ms.  
The function (Equation 2),  
\begin{equation}\tag{2}V = {V_{{\rm{MAX}}}}\left[ {\Phi /\left( {\Phi + \sigma } \right)} \right]\end{equation}
was fit to the b-wave amplitudes of the first limb of the b-wave stimulus response function, up to those stimulus values at which a-wave intrusion occurs.20,21 In Equation 2, V is the amplitude (μV) of the b-wave produced by flash Φ (scot td s); VMAX is the saturated b-wave amplitude (μV); and σ (scot td s) is the stimulus that evokes half-maximum response amplitude. Thus, σ is the semisaturation constant and 1/σ is a measure of sensitivity.  
Cone and Cone-Driven Activity
To assess cone and cone-driven function, responses to a range (from 1.4–3.4 log phot td s) of red flashes (λ = 630 ± 30 nm) presented on a steady, white, rod-saturating background (25.5 cd/m2) were recorded. The amplitude and implicit time of the a- and b-wave responses were examined as a function of stimulus intensity. 
A model of the activation of cone phototransduction22 was fit only to the first 5.5 ms of the a-wave to minimize the contribution of postreceptor activity.23 
In the equation (Equation 3),  
\begin{equation}\tag{3}R \left( \Phi ,t \right) = R_{\rm{CONE}}\left\{ 1-\exp\left[ - 0.5\Phi S_{\rm{CONE}}\left( t - t_d \right)^2\right] \right\}*\exp( - t/\tau )\end{equation}
 
Φ is the flash in phot td s; SCONE ([phot td s]−1 s−2) a sensitivity parameter that depends on the time constants of the molecular processes involved in the activation of phototransduction; RCONE is the saturated response amplitude (μV); td (seconds) is a brief delay; and τ is the time constant of the RC filter (1.8 ms). The symbol (*) represents the convolution operation. SCONE and RCONE parameters were calculated in 10 of 12 XLRS patients; digital records were not available for two. 
Additionally, a long-duration (150 ms) white flash (200 cd s /m2) presented on a steady background (42 cd/m2) was used to evaluate further cone mediated activity, including the cone-driven OFF response (the d-wave) in 7 of 12 participants.24 The amplitude and implicit time of the d-wave as well as the b-wave (ON response) were measured. 
Calibrations
Calculation of retinal illuminance was based on the luminance measured using a calibrated photodiode (IL 1700; International Light, Newburyport, MA, USA) with a scotopic or photopic filter placed in the position of the subject's eye. The scotopic troland value of the stimulus was calculated, taking each subject's measured pupil diameter into account. For the scotopic ERG, the maximum blue flash produced approximately 1.72 log cd s/m2 and, assuming an 8-mm pupil, approximately 3.4 log scot td s. 
The white, rod-saturating background used in the photopic ERG test was approximately 1.40 log cd/m2. The maximum red flash (1.55 log cd·s/m2) in the light-adapted photopic condition produced approximately 3.2 log phot td s. To calculate the photopic troland value, the Stiles-Crawford effect was taken into account; a pupil area of 20 mm2 was used for the conversion to troland value.23 
Statistical Analyses
Although the ERG test was performed on both eyes of each XLRS patient, data from the eye with the log σ value closer to the normal mean (−0.71 log scot td s) were selected for analysis. ERG parameters (Table 2) in XLRS patients and controls were compared using a Student's t-test. Each ERG component and calculated response parameter was evaluated for significant variation with age (1–15 years). We examined the scotopic ERG photoreceptor (SROD, RROD) for significant relations to the postreceptor parameters (VMAX, Log σ). In this analysis, the parameters were expressed as the log of the difference from the control subject's mean. The criterion level for significance was P < 0.01. 
Table 2
 
Comparison of XLRS to Control Data
Table 2
 
Comparison of XLRS to Control Data
Results
Rod and Rod-Driven Activity
Sample ERG records from a dark-adapted XLRS patient and control are shown in Figure 1. The amplitude and implicit time of the a- and b-wave are plotted as a function of stimulus intensity in Figure 2. Over the entire stimulus range, mean amplitude of both a- and b-wave responses are smaller in XLRS patients than in controls, and the XLRS stimulus response functions are shifted to the right (upper panels); the XLRS-control difference is greater for b- than a-wave amplitude. The mean implicit time of the a- and b-wave is longer in XLRS patients than in controls; XLRS a-wave implicit time is only slightly prolonged. At the stimulus corresponding to dark-adapted International Society for Clinical Electrophysiology of Vision (ISCEV) 3.0,25 the average ratio of b- to a-wave amplitude is significantly lower in XLRS patients than in controls (Table 2). Among XLRS patients, there were no age-related changes in b/a ratio, a-wave amplitude, a-wave implicit time, b-wave amplitude, or b-wave implicit time. 
Figure 1
 
Sample ERG records for dark-adapted XLRS patient #6 and a control. The vertical axis gives the stimulus values in log scot td s. The calibration bar pertains to both panels. The control subject has rod ERG parameters near the control mean.
Figure 1
 
Sample ERG records for dark-adapted XLRS patient #6 and a control. The vertical axis gives the stimulus values in log scot td s. The calibration bar pertains to both panels. The control subject has rod ERG parameters near the control mean.
Figure 2
 
The amplitudes of a-wave (upper left panel) and b-waves (upper right panel) in XLRS patients (▴) and controls () are plotted as a function of stimulus intensity. The arrow in the upper panels indicates the stimuli corresponding to dark-adapted ISCEV 3.0. In the lower panels, XLRS and control implicit times are plotted. Means ± SEM are shown. In the left panels, the horizontal bar indicates the range of stimuli over which the a-wave model (Equation 1) was fit.
Figure 2
 
The amplitudes of a-wave (upper left panel) and b-waves (upper right panel) in XLRS patients (▴) and controls () are plotted as a function of stimulus intensity. The arrow in the upper panels indicates the stimuli corresponding to dark-adapted ISCEV 3.0. In the lower panels, XLRS and control implicit times are plotted. Means ± SEM are shown. In the left panels, the horizontal bar indicates the range of stimuli over which the a-wave model (Equation 1) was fit.
Sample model fits for the a- and b-wave in two XLRS patients and a control are shown in Figure 3. The individual values of rod photoresponse parameters (SROD and RROD) and postreceptor parameters (log σ and VMAX) are plotted in Figure 4 and compared in Table 2. Saturated rod photoresponse amplitude (RROD) was significantly smaller in XLRS patients than in controls; in seven with XLRS, RROD was below the control minimum (Fig. 4). Mean rod photoreceptor sensitivity (SROD) did not differ significantly between XLRS patients and controls, although two XLRS patients had SROD below any control value. Saturated b-wave amplitude (VMAX) was significantly lower; log σ shifted to higher values, indicating lower sensitivity in XLRS than in controls. Among those with XLRS, none of the calculated scotopic parameters changed with age. In XLRS, the deficit in VMAX is greater than the deficit in RROD (Fig. 5), consistent with impaired transmission from photoreceptor to postreceptor retina. Similarly, the deficit in postreceptor log σ exceeds that in photoreceptor SROD. The deficit in VMAX is not significantly related to the deficit in RROD. 
Figure 3
 
Sample model fits for dark-adapted a- and b-waves. In the upper panels, a-waves (solid) and model (Equation 1) fits (dashed lines) for XLRS #6 (left panels), XLRS #4 (middle panels), and controls (right panels) are shown. XLRS #6 had the smallest and XLRS #4 the largest value of RROD. The control had log σ value at the mean for the group. The calculated values of SROD and RROD are given in each panel. Note that Equation 1 is fit to the a-wave responses to the five or six most intense stimuli. In the lower panels, b-wave amplitude is plotted as a function of stimulus strength for these participants. The smooth curve represents Equation 2 fit to the data, up to those stimulus values at which a-wave intrusion occurs. The calculated values of log σ and VMAX are shown.
Figure 3
 
Sample model fits for dark-adapted a- and b-waves. In the upper panels, a-waves (solid) and model (Equation 1) fits (dashed lines) for XLRS #6 (left panels), XLRS #4 (middle panels), and controls (right panels) are shown. XLRS #6 had the smallest and XLRS #4 the largest value of RROD. The control had log σ value at the mean for the group. The calculated values of SROD and RROD are given in each panel. Note that Equation 1 is fit to the a-wave responses to the five or six most intense stimuli. In the lower panels, b-wave amplitude is plotted as a function of stimulus strength for these participants. The smooth curve represents Equation 2 fit to the data, up to those stimulus values at which a-wave intrusion occurs. The calculated values of log σ and VMAX are shown.
Figure 4
 
Comparison of rod and rod-driven ERG response parameters in XLRS patients and controls. In the upper panels, rod photoreceptor sensitivity (SROD) and saturated response amplitude (RROD) are shown for XLRS patients (▴ recorded awake, △ recorded under anesthesia, ▪ treated with dorzolamide) and controls (○). In the lower panels, postreceptor b-wave log σ and saturated b-wave amplitudes (VMAX) are shown. Horizontal bars represent the mean for each group.
Figure 4
 
Comparison of rod and rod-driven ERG response parameters in XLRS patients and controls. In the upper panels, rod photoreceptor sensitivity (SROD) and saturated response amplitude (RROD) are shown for XLRS patients (▴ recorded awake, △ recorded under anesthesia, ▪ treated with dorzolamide) and controls (○). In the lower panels, postreceptor b-wave log σ and saturated b-wave amplitudes (VMAX) are shown. Horizontal bars represent the mean for each group.
Figure 5
 
Relation of photoreceptor and postreceptor parameters. In left panel, deficits in postreceptor sensitivity (Δ log σ) are plotted as a function of deficits in rod sensitivity (Δ log SROD) for controls (○) and XLRS patients (▴). In right panel, deficits in saturated b-wave amplitude (Δ log VMAX) are plotted as a function of deficits in saturated photoresponse amplitude (Δ log RROD) for controls and XLRS patients. Dashed lines indicate the range of parameters values for controls; diagonal lines have slope +1.
Figure 5
 
Relation of photoreceptor and postreceptor parameters. In left panel, deficits in postreceptor sensitivity (Δ log σ) are plotted as a function of deficits in rod sensitivity (Δ log SROD) for controls (○) and XLRS patients (▴). In right panel, deficits in saturated b-wave amplitude (Δ log VMAX) are plotted as a function of deficits in saturated photoresponse amplitude (Δ log RROD) for controls and XLRS patients. Dashed lines indicate the range of parameters values for controls; diagonal lines have slope +1.
RROD values of the three patients (#10, #11, and #12) on dorzolamide were among the largest. Of the four tested under anesthesia, two were above (patients #3 and #4) and two were below (patients #1 and #2) the RROD results of the group. 
Cone and Cone-Driven Activity
Sample ERG records from a light-adapted XLRS patient and a control are shown in Figure 6. The mean amplitude and implicit time of the a- and b-waves are plotted as a function of stimulus intensity in Figure 7. At all intensities, the amplitudes of both a- and b-waves were smaller in XLRS patients than in controls (upper panels), and the implicit times were longer (lower panels). The mean b-wave amplitude increased monotonically with stimulus intensity, as it did in each XLRS individual. The photopic hill,26 characteristic of controls, was not seen in XLRS.7 
Figure 6
 
Sample ERG records for light-adapted, 1-year-old XLRS patient #2 and a control. The vertical axis indicates the stimulus values in log phot td s. The calibration bar pertains to both panels.
Figure 6
 
Sample ERG records for light-adapted, 1-year-old XLRS patient #2 and a control. The vertical axis indicates the stimulus values in log phot td s. The calibration bar pertains to both panels.
Figure 7
 
The amplitudes of the cone-mediated a- (upper left panel) and b-wave (upper right panel) in XLRS patients (▴) and controls (○) are plotted as a function of stimulus intensity. In the lower panels, implicit time in XLRS patients and controls are plotted. Means ± SEM are shown.
Figure 7
 
The amplitudes of the cone-mediated a- (upper left panel) and b-wave (upper right panel) in XLRS patients (▴) and controls (○) are plotted as a function of stimulus intensity. In the lower panels, implicit time in XLRS patients and controls are plotted. Means ± SEM are shown.
Sample cone model fits for an XLRS patient and a control are plotted in Figure 8 (upper panels). The cone photoresponse parameters (SCONE and RCONE) are plotted in Figure 8 (lower panels) and compared in Table 2. All XLRS SCONE values were within the normal range. Although four XLRS patients had RCONE values below that in any control, mean RCONE did not differ significantly between XLRS patients and controls. Among those with XLRS, these calculated photopic parameters did not change with age. 
Figure 8
 
Sample model fits cone-mediated a-waves. In the upper panels, a-waves (solid black lines) and Equation 3 (dotted red lines) for XLRS patient #4 and a control are plotted. Equation 3 was fit only to the 5.5 ms of the response after a-wave onset (solid red lines). The calculated values of SCONE and RCONE are given in each panel. The vertical axis of XLRS was expanded for visibility. The control participant had cone a-wave parameters near the control mean. In the lower panels, cone photoreceptor sensitivity (SCONE) (left) and saturated amplitude (RCONE) (right) are shown for XLRS patients (▴, recorded awake, △ recorded under anesthesia, ▪ treated with dorzolamide) and controls (○). Horizontal bars represent the mean for each group.
Figure 8
 
Sample model fits cone-mediated a-waves. In the upper panels, a-waves (solid black lines) and Equation 3 (dotted red lines) for XLRS patient #4 and a control are plotted. Equation 3 was fit only to the 5.5 ms of the response after a-wave onset (solid red lines). The calculated values of SCONE and RCONE are given in each panel. The vertical axis of XLRS was expanded for visibility. The control participant had cone a-wave parameters near the control mean. In the lower panels, cone photoreceptor sensitivity (SCONE) (left) and saturated amplitude (RCONE) (right) are shown for XLRS patients (▴, recorded awake, △ recorded under anesthesia, ▪ treated with dorzolamide) and controls (○). Horizontal bars represent the mean for each group.
Sample records of responses to a long flash (150 ms) from an XLRS patient and a control are shown in Figure 9. The b-wave amplitude was significantly smaller in XLRS patients than controls; d-wave amplitudes did not differ significantly between groups (Table 2). The ratio of b- to d-wave amplitude was similar in XLRS patients and controls. Both the b- and d-wave implicit times were significantly longer in XLRS patients than controls (Table 2). In XLRS, these ERG components did not change with age. 
Figure 9
 
ERG responses to 150-ms full-field flash presented on a steady background. In the upper panel, sample responses to the long flash in XLRS patient #11 (red) and a control (black) are shown. The peak of the d-wave (OFF) component in the control is indicated by the vertical dashed line. The time course of the stimulus (time after flash onset in ms) is represented by the horizontal line above the x-axis. In the lower left panel, amplitude of the b- and d-wave are plotted for XLRS patients and controls. Implicit times of the b- and d-wave are plotted in the lower right panel. In each panel, horizontal bars represent the mean for the groups.
Figure 9
 
ERG responses to 150-ms full-field flash presented on a steady background. In the upper panel, sample responses to the long flash in XLRS patient #11 (red) and a control (black) are shown. The peak of the d-wave (OFF) component in the control is indicated by the vertical dashed line. The time course of the stimulus (time after flash onset in ms) is represented by the horizontal line above the x-axis. In the lower left panel, amplitude of the b- and d-wave are plotted for XLRS patients and controls. Implicit times of the b- and d-wave are plotted in the lower right panel. In each panel, horizontal bars represent the mean for the groups.
Discussion
None of the ERG parameters (Table 2) changed with age (1–15 years), and all were within the range reported for an older sample.7 Furthermore, the mean amplitudes of the a-waves in dark-adapted XLRS eyes were lower than in controls throughout the stimulus range (Fig. 2) that covers the range of stimuli used in previous studies.4,6,7 The saturated amplitude of the rod photoresponse, RROD (derived from the a-wave responses to the brightest flashes [Equation 1]) was significantly below that in the controls—our values of XLRS RROD, at 31% to 92% of the normal mean, overlap broadly those of Khan et al.7 (54%–123% of the normal mean). Despite the similar values, Khan et al.7 found that the mean XLRS RROD did not differ significantly between XLRS and their controls. These a-waves from the dark-adapted eye must be rod-dominated, although some cone contamination cannot be excluded.27 Because our RROD values were low, even in our youngest participants and even in those tested awake without anesthesia, compromise of the dark-adapted a-wave response, the RROD parameter, and explicitly rod activity, may be an inherent dysfunction of the XLRS retina. Interestingly, the patients on dorzolamide had among the largest RROD values; however, in another study of carbonic anhydrase inhibitor on isolated retina, saturated rod photoresponse amplitude was decreased.28 
Contamination by postreceptoral activity29,30 precludes parallel comments on the cone-mediated a-wave, but at the same time the possibility of impaired XLRS cone phototransduction cannot be excluded, as XLRS cone-mediated a-wave amplitudes were lower and implicit times slower than in controls (Fig. 7). The calculated values of the cone photoresponse parameters, SCONE and RCONE, did not show significant deficits (Table 2; Fig. 8), although the OCT study of Bennett et al.31 showed shortened photoreceptor outer segments in the central 10° of XLRS retina compared with controls. This central retina region has many cones and few rods. 
Indisputably, XLRS postreceptor activity, represented by the b-wave, had more marked deficits than did the a-wave in both scotopic and photopic conditions (Figs. 2, 5, 7, and Table 2). While acknowledging the evidence of significant age-related decline in b-wave amplitudes in some dark-adapted XLRS eyes,6 decline was not seen among our young participants, including boys with mutations (Table 1) that may be considered more severe.6 
The biological basis for the apparent rod dysfunction is not currently known; retinoschisin is expressed in photoreceptor inner segments and at the synapse between photoreceptor and bipolar cells.3234 But the manner in which abnormal retinoschisin might impair photoreceptor activity cannot be specified. Possibly impaired diffusion of molecules in the XLRS photoreceptor discs and channel closure occurs, but the reality is that only two of our XLRS patients had SROD values below the range found in the control subjects (Fig. 4), and dark-adapted XLRS a-wave implicit time was only minimally prolonged (Fig. 2). RROD (representing the number of channels available for closure by light) is significantly reduced in XLRS—possibly due to abnormality in the circulating current. 
Photoreceptor dysfunction, characterized by deficits in both saturated rod-response amplitude and amplification parameters has been demonstrated in mouse models of XLRS, even those as young as age 15 days.35 While being wary of too facile a transfer across species, we additionally note that analyses of proteomic data from the retinas of a knockout mouse model of XLRS show that the phototransduction pathway is highly regulated (Ambrosio L, et al. IOVS 2019;60:ARVO E-Abstract 467). In short, an effect of XLRS on photoreceptor function cannot be dismissed. The mechanisms by which this effect occurs, and its implications, remain to be specified. 
Acknowledgements
Supported by grants from Christ Family Fund (Boston, MA, USA), Massachusetts Lions Eye Research Fund (Feeding Hill, MA, USA), and the CSL Foundation, Inc. (Burlington, MA, USA). 
Disclosure: L. Ambrosio, None; R.M. Hansen, None; R. Kimia, None; A.B. Fulton, None 
References
Walia S, Fishman GA, Molday RS, et al. Relation of response to treatment with dorzolamide in X-linked retinoschisis to the mechanism of functional loss in retinoschisin. Am J Ophthalmol. 2009; 147: 111–115.
Sieving PA, MacDonald IM, Chan S. X-linked juvenile retinoschisis. In: Adam MP, Ardinger HH, Pagon RA, et al., eds. GeneReviews®. Seattle, WA: University of Washington; 1993–2019. Available at: http://www.ncbi.nom.nih.gov/books/NBK26471/.
Cukras CA, Huryn LA, Jeffrey BG, Turriff A, Sieving PA. Analysis of anatomic and functional measures in X-Linked retinoschisis. Invest Ophthalmol Vis Sci. 2018; 59: 2841–2847.
Bradshaw K, George N, Moore A, Trump D. Mutations of the XLRS1 gene cause abnormalities of photoreceptor as well as inner retinal responses of the ERG. Doc Ophthalmol. 1999; 98: 153–173.
Pennesi ME, Birch DG, Jayasundera KT, et al. Prospective evaluation of patients with X-linked retinoschisis during 18 months. Invest Ophthalmol Vis Sci. 2018; 59: 5941–5956.
Bowles K, Cukras C, Turriff A, et al. X-linked retinoschisis: RS1 mutation severity and age affect the ERG phenotype in a cohort of 68 affected male subjects. Invest Ophthalmol Vis Sci. 2011; 52: 9250–9256.
Khan NW, Jamison JA, Kemp JA, Sieving PA. Analysis of photoreceptor function and inner retinal activity in juvenile X-linked retinoschisis. Vis Res. 2001; 41: 3931–3942.
Weber BH, Schrewe H, Molday LL, et al. Inactivation of the murine X-linked juvenile retinoschisis gene, Rs1h, suggests a role of retinoschisin in retinal cell layer organization and synaptic structure. Proc Natl Acad Sci U S A. 2002; 99: 6222–6227.
Leone JF, Mitchell P, Kifley A, Rose KA. Normative visual acuity in infants and preschool-aged children in Sydney. Acta Ophthalmol. 2014; 92: e521–e529.
Dobson V, Maguire M, Orel-Bixler D, Quinn G, Ying GS. Visual acuity results in school-aged children and adults: Lea Symbols chart versus Bailey-Lovie chart. Optom Vis Sci. 2003; 80: 650–654.
Dobson V, Clifford-Donaldson CE, Green TK, Miller JM, Harvey EM. Normative monocular visual acuity for early treatment diabetic retinopathy study charts in emmetropic children 5 to 12 years of age. Ophthalmology. 2009; 116: 1397–1401.
Mayer DL, Hansen RM, Moore BD, Kim S, Fulton AB. Cycloplegic refractions in healthy children aged 1 through 48 months. Arch Ophthalmol. 2001; 119: 1625–1628.
Zadnik K, Manny RE, Yu JA, et al. Ocular component data in schoolchildren as a function of age and gender. Optom Vis Sci. 2003; 80: 226–236.
Hansen RM, Moskowitz A, Akula JD, Fulton AB. The neural retina in retinopathy of prematurity. Prog Retin Eye Res. 2017; 56: 32–57.
Fulton AB, Hansen RM. The development of scotopic sensitivity. Invest Ophthalmol Vis Sci. 2000; 41: 1588–1596.
Moskowitz A, Hansen RM, Eklund SE, Fulton AB. Electroretinographic (ERG) responses in pediatric patients using vigabatrin. Doc Ophthalmol. 2012; 124: 197–209.
Genead MA, Fishman GA, Walia S. Efficacy of sustained topical dorzolamide therapy for cystic macular lesions in patients with X-linked retinoschisis. Arch Ophthalmol. 2010; 128: 190–197.
Hood DC, Birch DG. Rod phototransduction in retinitis pigmentosa: estimation and interpretation of parameters derived from the rod a-wave. Invest Ophthalmol Vis Sci. 1994; 35: 2948–2961.
Lamb TD, Pugh ENJr. A quantitative account of the activation steps involved in phototransduction in amphibian photoreceptors. J Physiol. 1992; 449: 719–758.
Peachey NS, Alexander KR, Fishman GA. The luminance-response function of the dark-adapted human electroretinogram. Vision Res. 1989; 29: 263–270.
Fulton AB, Rushton WA. The human rod ERG: correlation with psychophysical responses in light and dark adaptation. Vision Res. 1978; 18: 793–800.
Hood DC, Birch DG. Human cone receptor activity: the leading edge of the a-wave and models of receptor activity. Vis Neurosci. 1993; 10: 857–871.
Friedburg C, Allen CP, Mason PJ, Lamb TD. Contribution of cone photoreceptors and post-receptoral mechanisms to the human photopic electroretinogram. J Physiol. 2004; 556: 819–834.
Sieving PA. Photopic ON- and OFF-pathway abnormalities in retinal dystrophies. Trans Am Ophthalmol Soc. 1993; 91: 701–773.
McCulloch DL, Marmor MF, Brigell MG, et al. ISCEV Standard for full-field clinical electroretinography (2015 update). Doc Ophthalmol. 2015; 130: 1–12.
Wali N, Leguire LE. The photopic hill: a new phenomenon of the light adapted electroretinogram. Doc Ophthalmol. 1992; 80: 335–345.
Hood DC, Birch DG. Assessing abnormal rod photoreceptor activity with the a-wave of the electroretinogram: applications and methods. Doc Ophthalmol. 1996; 92: 253–267.
Findl O, Hansen RM, Fulton AB. The effects of acetazolamide on the electroretinographic responses in rats. Invest Ophthalmol Vis Sci. 1995; 36: 1019–1026.
Bush RA, Sieving PA. A proximal retinal component in the primate photopic ERG a-wave. Invest Ophthalmol Vis Sci. 1994; 35: 635–645.
Robson JG, Frishman LJ. Sampling and interpolation of the a-wave of the electroretinogram. Doc Ophthalmol. 2004; 108: 171–179.
Bennett LD, Wang YZ, Klein M, Pennesi ME, Jayasundera T, Birch DG. Structure/psychophysical relationships in X-linked retinoschisis. Invest Ophthalmol Vis Sci. 2016; 57: 332–337.
Molday LL, Hicks D, Sauer CG, Weber BH, Molday RS. Expression of X-linked retinoschisis protein RS1 in photoreceptor and bipolar cells. Invest Ophthalmol Vis Sci. 2001; 42: 816–825.
Reid SN, Yamashita C, Farber DB. Retinoschisin, a photoreceptor-secreted protein, and its interaction with bipolar and Muller cells. J Neurosci. 2003; 23: 6030–6040.
Reid SN, Akhmedov NB, Piriev NI, Kozak CA, Danciger M, Farber DB. The mouse X-linked juvenile retinoschisis cDNA: expression in photoreceptors. Gene. 1999; 227: 257–266.
Liu Y, Kinoshita J, Ivanova E, et al. Mouse models of X-linked juvenile retinoschisis have an early onset phenotype, the severity of which varies with genotype. Hum Mol Genet. 2019; 28: 3072–3090.
Figure 1
 
Sample ERG records for dark-adapted XLRS patient #6 and a control. The vertical axis gives the stimulus values in log scot td s. The calibration bar pertains to both panels. The control subject has rod ERG parameters near the control mean.
Figure 1
 
Sample ERG records for dark-adapted XLRS patient #6 and a control. The vertical axis gives the stimulus values in log scot td s. The calibration bar pertains to both panels. The control subject has rod ERG parameters near the control mean.
Figure 2
 
The amplitudes of a-wave (upper left panel) and b-waves (upper right panel) in XLRS patients (▴) and controls () are plotted as a function of stimulus intensity. The arrow in the upper panels indicates the stimuli corresponding to dark-adapted ISCEV 3.0. In the lower panels, XLRS and control implicit times are plotted. Means ± SEM are shown. In the left panels, the horizontal bar indicates the range of stimuli over which the a-wave model (Equation 1) was fit.
Figure 2
 
The amplitudes of a-wave (upper left panel) and b-waves (upper right panel) in XLRS patients (▴) and controls () are plotted as a function of stimulus intensity. The arrow in the upper panels indicates the stimuli corresponding to dark-adapted ISCEV 3.0. In the lower panels, XLRS and control implicit times are plotted. Means ± SEM are shown. In the left panels, the horizontal bar indicates the range of stimuli over which the a-wave model (Equation 1) was fit.
Figure 3
 
Sample model fits for dark-adapted a- and b-waves. In the upper panels, a-waves (solid) and model (Equation 1) fits (dashed lines) for XLRS #6 (left panels), XLRS #4 (middle panels), and controls (right panels) are shown. XLRS #6 had the smallest and XLRS #4 the largest value of RROD. The control had log σ value at the mean for the group. The calculated values of SROD and RROD are given in each panel. Note that Equation 1 is fit to the a-wave responses to the five or six most intense stimuli. In the lower panels, b-wave amplitude is plotted as a function of stimulus strength for these participants. The smooth curve represents Equation 2 fit to the data, up to those stimulus values at which a-wave intrusion occurs. The calculated values of log σ and VMAX are shown.
Figure 3
 
Sample model fits for dark-adapted a- and b-waves. In the upper panels, a-waves (solid) and model (Equation 1) fits (dashed lines) for XLRS #6 (left panels), XLRS #4 (middle panels), and controls (right panels) are shown. XLRS #6 had the smallest and XLRS #4 the largest value of RROD. The control had log σ value at the mean for the group. The calculated values of SROD and RROD are given in each panel. Note that Equation 1 is fit to the a-wave responses to the five or six most intense stimuli. In the lower panels, b-wave amplitude is plotted as a function of stimulus strength for these participants. The smooth curve represents Equation 2 fit to the data, up to those stimulus values at which a-wave intrusion occurs. The calculated values of log σ and VMAX are shown.
Figure 4
 
Comparison of rod and rod-driven ERG response parameters in XLRS patients and controls. In the upper panels, rod photoreceptor sensitivity (SROD) and saturated response amplitude (RROD) are shown for XLRS patients (▴ recorded awake, △ recorded under anesthesia, ▪ treated with dorzolamide) and controls (○). In the lower panels, postreceptor b-wave log σ and saturated b-wave amplitudes (VMAX) are shown. Horizontal bars represent the mean for each group.
Figure 4
 
Comparison of rod and rod-driven ERG response parameters in XLRS patients and controls. In the upper panels, rod photoreceptor sensitivity (SROD) and saturated response amplitude (RROD) are shown for XLRS patients (▴ recorded awake, △ recorded under anesthesia, ▪ treated with dorzolamide) and controls (○). In the lower panels, postreceptor b-wave log σ and saturated b-wave amplitudes (VMAX) are shown. Horizontal bars represent the mean for each group.
Figure 5
 
Relation of photoreceptor and postreceptor parameters. In left panel, deficits in postreceptor sensitivity (Δ log σ) are plotted as a function of deficits in rod sensitivity (Δ log SROD) for controls (○) and XLRS patients (▴). In right panel, deficits in saturated b-wave amplitude (Δ log VMAX) are plotted as a function of deficits in saturated photoresponse amplitude (Δ log RROD) for controls and XLRS patients. Dashed lines indicate the range of parameters values for controls; diagonal lines have slope +1.
Figure 5
 
Relation of photoreceptor and postreceptor parameters. In left panel, deficits in postreceptor sensitivity (Δ log σ) are plotted as a function of deficits in rod sensitivity (Δ log SROD) for controls (○) and XLRS patients (▴). In right panel, deficits in saturated b-wave amplitude (Δ log VMAX) are plotted as a function of deficits in saturated photoresponse amplitude (Δ log RROD) for controls and XLRS patients. Dashed lines indicate the range of parameters values for controls; diagonal lines have slope +1.
Figure 6
 
Sample ERG records for light-adapted, 1-year-old XLRS patient #2 and a control. The vertical axis indicates the stimulus values in log phot td s. The calibration bar pertains to both panels.
Figure 6
 
Sample ERG records for light-adapted, 1-year-old XLRS patient #2 and a control. The vertical axis indicates the stimulus values in log phot td s. The calibration bar pertains to both panels.
Figure 7
 
The amplitudes of the cone-mediated a- (upper left panel) and b-wave (upper right panel) in XLRS patients (▴) and controls (○) are plotted as a function of stimulus intensity. In the lower panels, implicit time in XLRS patients and controls are plotted. Means ± SEM are shown.
Figure 7
 
The amplitudes of the cone-mediated a- (upper left panel) and b-wave (upper right panel) in XLRS patients (▴) and controls (○) are plotted as a function of stimulus intensity. In the lower panels, implicit time in XLRS patients and controls are plotted. Means ± SEM are shown.
Figure 8
 
Sample model fits cone-mediated a-waves. In the upper panels, a-waves (solid black lines) and Equation 3 (dotted red lines) for XLRS patient #4 and a control are plotted. Equation 3 was fit only to the 5.5 ms of the response after a-wave onset (solid red lines). The calculated values of SCONE and RCONE are given in each panel. The vertical axis of XLRS was expanded for visibility. The control participant had cone a-wave parameters near the control mean. In the lower panels, cone photoreceptor sensitivity (SCONE) (left) and saturated amplitude (RCONE) (right) are shown for XLRS patients (▴, recorded awake, △ recorded under anesthesia, ▪ treated with dorzolamide) and controls (○). Horizontal bars represent the mean for each group.
Figure 8
 
Sample model fits cone-mediated a-waves. In the upper panels, a-waves (solid black lines) and Equation 3 (dotted red lines) for XLRS patient #4 and a control are plotted. Equation 3 was fit only to the 5.5 ms of the response after a-wave onset (solid red lines). The calculated values of SCONE and RCONE are given in each panel. The vertical axis of XLRS was expanded for visibility. The control participant had cone a-wave parameters near the control mean. In the lower panels, cone photoreceptor sensitivity (SCONE) (left) and saturated amplitude (RCONE) (right) are shown for XLRS patients (▴, recorded awake, △ recorded under anesthesia, ▪ treated with dorzolamide) and controls (○). Horizontal bars represent the mean for each group.
Figure 9
 
ERG responses to 150-ms full-field flash presented on a steady background. In the upper panel, sample responses to the long flash in XLRS patient #11 (red) and a control (black) are shown. The peak of the d-wave (OFF) component in the control is indicated by the vertical dashed line. The time course of the stimulus (time after flash onset in ms) is represented by the horizontal line above the x-axis. In the lower left panel, amplitude of the b- and d-wave are plotted for XLRS patients and controls. Implicit times of the b- and d-wave are plotted in the lower right panel. In each panel, horizontal bars represent the mean for the groups.
Figure 9
 
ERG responses to 150-ms full-field flash presented on a steady background. In the upper panel, sample responses to the long flash in XLRS patient #11 (red) and a control (black) are shown. The peak of the d-wave (OFF) component in the control is indicated by the vertical dashed line. The time course of the stimulus (time after flash onset in ms) is represented by the horizontal line above the x-axis. In the lower left panel, amplitude of the b- and d-wave are plotted for XLRS patients and controls. Implicit times of the b- and d-wave are plotted in the lower right panel. In each panel, horizontal bars represent the mean for the groups.
Table 1
 
XLRS Patients and Their Visual Acuities and Spherical Equivalents Close to the Time of ERG Test
Table 1
 
XLRS Patients and Their Visual Acuities and Spherical Equivalents Close to the Time of ERG Test
Table 2
 
Comparison of XLRS to Control Data
Table 2
 
Comparison of XLRS to Control Data
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