Retinal Function in X-Linked Juvenile Retinoschisis

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 (S_ROD, R_ROD, S_CONE, R_CONE) and rod-driven postreceptor (log σ, V_MAX) 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 (R_ROD), postreceptor b-wave (log σ), and saturated b-wave amplitude (V_MAX) were significantly lower in XLRS patients than in controls; S_ROD did not differ between the two groups. S_CONE and R_CONE values were normal. In XLRS patients, neither a- and b-wave amplitudes nor calculated parameters (S_ROD, R_ROD, log σ, V_MAX, S_CONE, and R_CONE) changed with age.

Conclusions: In these young XLRS patients, R_ROD 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.^1–3 

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 amplitudes^4–6 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

 

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; S_ROD [(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; R_ROD 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 t_d 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); V_MAX 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; S_CONE ([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; R_CONE is the saturated response amplitude (μV); t_d (seconds) is a brief delay; and τ is the time constant of the RC filter (1.8 ms). The symbol (*) represents the convolution operation. S_CONE and R_CONE 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 (S_ROD, R_ROD) for significant relations to the postreceptor parameters (V_MAX, 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

 

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 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 (S_ROD and R_ROD) and postreceptor parameters (log σ and V_MAX) are plotted in Figure 4 and compared in Table 2. Saturated rod photoresponse amplitude (R_ROD) was significantly smaller in XLRS patients than in controls; in seven with XLRS, R_ROD was below the control minimum (Fig. 4). Mean rod photoreceptor sensitivity (S_ROD) did not differ significantly between XLRS patients and controls, although two XLRS patients had S_ROD below any control value. Saturated b-wave amplitude (V_MAX) 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 V_MAX is greater than the deficit in R_ROD (Fig. 5), consistent with impaired transmission from photoreceptor to postreceptor retina. Similarly, the deficit in postreceptor log σ exceeds that in photoreceptor S_ROD. The deficit in V_MAX is not significantly related to the deficit in R_ROD. 

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 R_ROD. The control had log σ value at the mean for the group. The calculated values of S_ROD and R_ROD 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 V_MAX 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 (S_ROD) and saturated response amplitude (R_ROD) 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 (V_MAX) 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 S_ROD) for controls (○) and XLRS patients (▴). In right panel, deficits in saturated b-wave amplitude (Δ log V_MAX) are plotted as a function of deficits in saturated photoresponse amplitude (Δ log R_ROD) for controls and XLRS patients. Dashed lines indicate the range of parameters values for controls; diagonal lines have slope +1.

R_ROD 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 R_ROD 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 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 (S_CONE and R_CONE) are plotted in Figure 8 (lower panels) and compared in Table 2. All XLRS S_CONE values were within the normal range. Although four XLRS patients had R_CONE values below that in any control, mean R_CONE 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 S_CONE and R_CONE 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 (S_CONE) (left) and saturated amplitude (R_CONE) (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.

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, R_ROD (derived from the a-wave responses to the brightest flashes [Equation 1]) was significantly below that in the controls—our values of XLRS R_ROD, 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 R_ROD 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 R_ROD 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 R_ROD parameter, and explicitly rod activity, may be an inherent dysfunction of the XLRS retina. Interestingly, the patients on dorzolamide had among the largest R_ROD values; however, in another study of carbonic anhydrase inhibitor on isolated retina, saturated rod photoresponse amplitude was decreased.²⁸ 

Contamination by postreceptoral activity^29,30 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, S_CONE and R_CONE, 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.^32–34 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 S_ROD 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). R_ROD (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