Documenting Fixation at an Extrafoveal Locus with a Modified Slit Lamp in Age-Related Macular Degeneration

Abstract

purpose. To develop a simple and clinically useful technique for observing fixation at an extrafoveal locus (preferred retinal locus [PRL]) with different targets and texts in age-related macular degeneration (AMD).

methods. A standard slit lamp was modified by adding several fixation targets in the illumination pathway for direct observation and documentation of fixation during fundus examination. Fixation patterns were analyzed in 30 subjects with AMD.

results. The location and stability of fixation with various stimuli was possible to record in each subject. In 23 subjects, there was no difference between the fixations at star and wagon wheel stimuli; in seven subjects, they were in clearly different retinal locations. Fixation was unstable in three subjects. The PRL for reading words was detectable in all subjects.

conclusions. The present assessment technique seems to offer a simple, clinically available technique to record fixation patterns to different targets and texts.

Patients with a central scotoma frequently develop an eccentric area of fixation commonly referred to as a preferred retinal locus (PRL). ^{1 2 3 4} The PRL can be localized precisely with a scanning laser ophthalmoscope (SLO). ^{5 6 7 8 9} Aulhorn ¹⁰ showed that eccentric fixation can be demonstrated perimetrically by the shift of the blind spot and scotoma. Currently, there are no clinically available simple techniques to document the PRL or the trained retinal locus (TRL) ^{11 12} used by patients with advanced age-related macular degeneration (AMD). The present situation is discussed by Stelmack et al. ¹³ who stated that the “work of greatest significance in the study of EV [eccentric viewing] is the documentation of the preferred retinal locus (PRL), characteristics of size and shape of scotomas surrounding the PRL, and PRL ability measured with the scanning laser ophthalmoscope (SLO).” Déruaz et al. ¹⁴ have documented use of more than one PRL during reading, which means that eccentric fixation must be examined by using both usual fixation targets and texts. Both the structure of the central scotoma and preference to certain parts of the central visual field may play a role in the choice of the preferred locus. ¹⁵ The anatomic location of the PRL must be taken into consideration also when planning retinal laser surgery but more important, it is to know the location and the stability of the fixation in the rehabilitation of the patients with AMD when helping them to find their PRL and to train to use it efficiently in reading and other visual tasks. Patients may see better if they use a TRL for their “pseudofovea.” ^{11 12}  

During the EU Project AMD-READ, ¹⁶ a new technique of observing the fixation at an extrafoveal locus was developed ¹⁷ and is reported in detail in this article. 

Materials and Methods

The subjects (n = 30), age 64 to 84 years (mean, 77.8), included in the study had AMD. They were recruited from the groups of patients with macular degeneration at the University Eye Hospital in Turku and had a mean visual acuity less than 0.3 (0.5 logMAR; range 0.5–1.3) and were thus likely to have eccentric fixation (Table 1) . Eleven of the subjects used a CCTV for reading and 18 used 2× to 6× magnifiers. One subject did not care to read at all. Each subject was informed about the experimental nature of this project and signed the consent form before entering the study. This study adhered to the tenets of the Declaration of Helsinki and was approved by the ethics committee of the Turku University Hospital. 

The new technique of observing fixation was based on a modification of a commercial standard slit lamp (BQ-900; Haag-Streit, Bern, Switzerland) with a video camera (4 × 0411; Panasonic, Osaka, Japan). On the metal plate where the height of the illuminating slit is controlled, extra holes were drilled for fixation targets (Fig. 1) . The fixation targets were a star (estimated diameter, 11° in the visual field), a wagon wheel (diameter 22°), and two texts of different sizes: a six-line text with letter height 1° 7′ corresponding to a visual acuity of 0.05 (1.3 logMAR) and a three-line text with letter height 2° 14′ corresponding to a visual acuity of 0.025 (1.6 logMAR). The size of the wagon wheel stimulus was chosen to match the size of the fixation stimulus in a macular mapping test (MMTest; The Smith-Kettlewell Eye Research Institute, San Francisco, CA). ^{18 19}  

The pupil of the better eye of the subject was dilated with drops, and the eye fundus was examined with the slit lamp using a +90 D lens. The observations and the conversations with the subjects were recorded digitally on a computer hard disc. 

Localizing the PRL on the retina was assessed by asking the subjects to fixate the star and the wagon wheel stimuli each for 10 seconds (Fig. 2) . The fixation location on the fundus was identified afterward from the series of five consecutive video frames captured at 1-second intervals using video editor software. ²⁰ With the help of the software, ²¹ the anatomic fixation location with the star and wagon wheel stimuli was marked on a fundus picture (Fundus Camera; Topcon, Tokyo, Japan) which was rotated 180° to have the same view of fundus as in indirect ophthalmoscopy. The fundus picture with the fixation location markings was still flipped vertically to match the visual field directions. Using special software ²² the fundus picture was scaled to visual field dimensions by estimating visually the location of the anatomic fovea and the center of the optic disc and assuming the distance between them to correspond to 15° in the visual field (Fig. 3) . The x–y coordinates of the fixationloci in the visual field were measured with the star and the wagon wheel stimuli with the help of the special software. The fixation stability was subjectively graded as stable or unstable from the video recordings of the fixation. 

The fixation behavior during text reading was assessed with the modified slit lamp (Haag-Streit) and +90 lens by using three- and six-line text stimuli (Fig. 1) . The subjects were asked to read the text aloud, first at their normal reading speed and then slowly, one word at a time. If the fixation location was considered unfavorable for reading (e.g., the retinal location was destroyed by degeneration or was situated very close to a retinal scar), a location where the retina seemed to be less involved was chosen, and the subject was trained in its use, to find out whether the modified slit lamp could be used for training eccentric fixation in future studies. 

The exact timing of reading aloud an individual word was recognized from the video recording and was marked with an asterisk on the video track. The fixation behavior during text reading was analyzed in detail at slower than normal speed. 

Visual acuity was measured with the translucent LEA numbers charts on the ETDRS light box. Word acuity was measured using the Finnish version of the Reading Navigator test. ²³ The reading speed with the current magnifying device of the subject was measured using standardized reading texts. ²⁴  

Results

The range of visual acuity values at 100% contrast was 0.5 to 1.3 logMAR (mean, 0.8), and the word acuity was 0.5 to 1.6 logMAR (mean, 1.0). The range of reading speed was 61 to 707 characters per minute (mean, 326). 

The location and stability of fixation with various stimuli was possible to record in each subject, although small pupils or small rhexis openings after cataract extraction diminished the visibility of the retina in several subjects. 

Fixation on the star stimulus was central (≤2° from the estimated location of the anatomic fovea) in 13 subjects and eccentric in 17 subjects (Table 1) . Fixation on the wagon wheel stimulus was central in 11 subjects and eccentric in 19. In 23 subjects there was no difference between the fixations of these two stimuli; in seven subjects they were in clearly different locations (Figs. 4 5 and 6) . Fixation was unstable in three subjects. The PRL for reading words was detectable in all subjects. One subject had two PRLs for reading. 

Discussion

As stated by Stelmack et al., ¹³ “it is necessary to develop and validate objective and quantitative measures that can be used in clinical settings to evaluate EV [eccentric viewing] behavior, to characterize the visual capabilities of EV loci, and to evaluate both the efficacy and effectiveness of EV training.” The benefit to reading ability of training patients with AMD to use their PRL or the newly trained TRL is emphasized (e.g., by Déruaz et al. ²⁵ and Seiple et al. ²⁶ ) As Déruaz et al. ²⁵ reported, the SLO has been out of production for many years, and for this reason it is not presently a widely used instrument. Microperimetry (MP1; Nidek, Gamagori, Japan) offers one alternative to fixation monitoring, but its drawbacks are the inability to present words or text for fixation and the relatively high price. 

In practice, fixation monitoring with a modified slit lamp requires good dilation of the pupil and considerable skill in funduscopy. The position of the +90 D examination lens must be continuously adjusted for the eye movements when the subject is trying to fixate the target. In pseudophakic eyes even the small rhexis opening of the anterior capsule of the lens can worsen the visibility. Projecting words and large fixation targets onto the fundus requires a wide light path of the slit lamp. Therefore, in many cases the fundus is seen through a small pupil only monocularly by the examiner. Generally, in funduscopy with small pupils the examiner spontaneously often chooses his dominant eye to look with. That is why it is practical to attach the video camera to the same side of the slit lamp as the dominant eye of the examiner so that the examiner sees the same picture, which is recorded with the video camera. 

The bright light from the slit lamp with the light beam wide open causes discomfort to the patient. In our series, the use of the standard neutral-density filter of the slit lamp reduced the light intensity to a tolerable level. The discomfort can still be reduced by introducing to the light path of the slit lamp an infrared filter that filters visible light but leaves the infrared portion of the spectrum to be recorded by an infrared-sensitive video camera as Seiple et al. ²⁶ did in their study. This, however, requires an infrared-sensitive camera, an electronic video enhancement board, and a monitor for viewing the patient’s fundus image. This system makes the method much more complex and less clinically available than the modified slit lamp presented in this article, which does not require any electronic devices when used in conjunction with clinical fundus examination. 

In most cases, the fixation pattern can already be seen during the clinical examination so that it can be marked manually to separate standard fundus pictures taken in advance. Detailed measurements of the retinal location of the fixation can be made afterward from the video recording running in slow motion or from separate still picture frames if necessary. We did not have access to an SLO to compare the accuracy of the measurements between the modified slit lamp and SLO. Obviously, our method is less accurate in comparison to an SLO but was still sufficient for determining fixation locations and the stability of fixation. 

The slit lamp modification that we have devised offers a simple and easily available technique for recording fixation patterns for different targets and texts during clinical examination of the fundus and thus aids in planning the eccentric viewing training of patients with AMD. Its drawbacks are inferior accuracy compared with SLO and discomfort caused by the bright light of the slit lamp. It also requires considerable skill in funduscopy. 

 Supported by European Commission, Key action No. 6, AMD-READ-Project, QLK 6-CT-2002–00214 and De Blindas Vänner–Sokeain ystävät r.y. (MTL).

 Submitted for publication August 28, 2007; revised January 15 and July 7, 2008; accepted October 3, 2008.

 Disclosure: M.T. Leinonen, None; L. Hyvärinen, None

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Table 1.

 

Type of AMD-Lesion, Visual Acuities, and Fixation Characteristics in Response to a Star Stimulus

ID	Age	Type of AMD	Visual Acuity/logMAR Contrast					Fixation Loci with Star and Wagon	Fixation Loci: Star as Fixation Stimulus
						100%	10%		2.5%		Eccentricity (deg)	Fixation Stability	Quadrant
17	64	Exudative	1.2	1.5	2.0	Same	0.5	Stable	Upper right
27	79	Exudative	0.9	1.1	1.9	Same	0.6	Stable	Upper right
25	79	Dry pigmentary	0.5	0.9	1.5	Same	0.6	Stable	Upper left
26	76	Geographic	0.7	1.1	1.6	Same	0.7	Stable	Lower right
29	81	PED	0.7	0.9	1.6	Same	0.8	Stable	Lower right
24	78	Geographic	0.8	1.2	2.0	Same	1.0	Stable	Lower left
23	77	Geographic	1.0	1.3	1.7	Same	1.3	Unstable	Upper left
18	72	Geographic	0.5	1.0	1.8	Same	1.3	Stable	Upper right
30	73	Geographic	0.9	1.4	2.0	Same	1.4	Stable	Middle right
28	82	Small disciform scar	0.8	1.1	1.5	Same	1.5	Stable	Lower right
19	79	Geographic	0.9	1.3	2.0	Same	1.9	Stable	Lower right
15	79	Geographic	0.8	1.3	1.8	Same	2.0	Stable	Middle left
22	75	Large disciform scar	0.9	1.2	1.8	Same	2.2	Stable	Lower right
21	79	Geographic	0.7	1.0	1.2	Same	2.8	Stable	Lower left
10	84	Geographic	0.7	0.8	1.1	Same	2.9	Stable	Lower left
6	82	Geographic	0.9	0.9	1.5	Same	3.0	Stable	Lower left
5	82	Exudative	0.8	1.0	1.5	Same	3.6	Stable	Lower left
4	74	Geographic	0.7	0.8	1.0	Same	3.6	Stable	Lower left
2	78	Geographic	1.2	1.1	1.8	Same	4.3	Stable	Lower left
3	76	Atrophic scar	0.6	0.9	1.5	Same	4.6	Stable	Lower left
8	79	Exudative	1.1	1.2	1.9	Same	4.9	Stable	Lower left
7	81	Exudative	0.6	0.8	1.5	Same	5.8	Unstable	Lower left
13	83	Geographic	0.5	0.9	1.7	Same	6.2	Stable	Lower right
16	78	Geographic	0.5	0.6	1.2	Different	1.6	Stable	Upper left
9	78	PED	0.8	0.9	1.1	Different	3.6	Stable	Lower left
11	76	Atrophic scar	1.3	1.3	1.4	Different	5.0	Stable	Lower right
14	82	Geographic	0.7	1.4	2.0	Different	5.8	Stable	Lower right
12	73	Large disciform scar	1.2	1.3	1.9	Different	6.5	Unstable	Lower right
1	77	Geographic	0.9	1.2	1.9	Different	8.0	Stable	Upper left
20	79	Geographic	0.7	1.0	1.5	Different	10.2	Stable	Lower left

 The results are ordered by columns Fixation Loci with Star and Wagon and Fixation Stability.

Figure 1.

 

Standard Haag-Streit slit lamp and with lamp housing removed (A, B). (C) Modified, unfinished rotating aperture disc of a standard Haag-Streit slit lamp with five extra holes drilled. Three of the holes were already drilled coniform and two of them painted black. (D) The fixation targets, star (estimated diameter in the visual field, 11°), wagon wheel (estimated diameter, 22°), and two texts in Finnish, were contact copied to graphics film. (E) The film strips with the fixation targets were fastened to the rotating disc with adhesive tape. (F) The words were horizontally flipped when projected to the focusing rod of the slit lamp so that the subject could see the text normally during indirect ophthalmoscopy with +90 D lens.

Figure 2.

 

Screen capture from the video recording of a subject fixating the star target (Movie S1).

Figure 3.

 

Two screen capture images of the software which was used to measure the fixation location. The fixation location of the left eye of the subject during fixation on the star stimulus was identified from the video recording and marked on the fundus picture which is rotated 180° and flipped vertically to match the visual field directions. Thus, the temporal location of the blind spot of the visual field of the left eye matches the optic disc of the fundus picture. (A) Cross hair: estimated location of the fovea and the star the fixation location. (B) With the help of the cross hair, the location of the star in the visual field is measured: 11° (arrow) inferiorly in the lower visual field of the left eye, meridian 265°.

Figure 4.

 

(A) Fixation loci of 23 AMD-subjects assessed by the modified slit lamp. The documented retinal loci were converted to locations in the visual field. Star and a wagon wheel patterns were used as the fixation stimuli. (B) Fixation loci of seven subjects with AMD were different (≥3°) between the star and wagon wheel stimuli. These locations are connected with a dashed line for each subject. The fundus picture of the subject marked with an arrow can be seen in Figure 5 .

Figure 5.

 

Difference in the location of the fixation targets in one subject (marked with an arrow in Fig. 4B ). At first, the subject could read only the large text using the location defined by the wagon wheel. After a few minutes of training, the subject was also able to read the small text using the locus where the target star was fixated (Fig. 6) .

Figure 6.

 

Screen capture from the video recording of a subject reading with eccentric fixation location (Movie S2).

Supplementary Materials

Movie S1 - 5.3 MB (QuickTime Movie) 

Accompanies Figure 2. 

Movie S2 - 8.5 MB (QuickTime Movie) 

Accompanies Figure 6. 

The authors thank Lea-Test, Ltd. (Helsinki, Finland) for providing the light boxes and visual acuity tests used in the study and Marja Tammikallio and Leena Kivi-Tuominen for performing all vision testing of the subjects.