Optical coherence tomographic findings at the fixation point in a case of bilateral congenital macular coloboma

Introduction

Congenital macular coloboma appears as atrophic lesions with well-circumscribed borders unilaterally or bilaterally, which result in a nonprogressive decrease in visual acuity. These findings are also seen in patients with intrauterine infection, especially toxoplasmosis. Spectral domain (SD) optical coherence tomography (OCT) is a beneficial tool to make a diagnosis of macular coloboma. We report on the SD-OCT findings in a patient with bilateral macular coloboma. Furthermore, we show the retinal structures at the fixation point.

Case report

The subject was a 4-year-old boy. He visited our hospital at age 1 year and 4 months for the evaluation of strabismus. He had medical history of ventricular septal defect. The anterior segments were normal. The Hirschberg test revealed an exotropia of 10°. The fundus examination of both eyes showed round-shaped sharply-demarcated atrophic lesions involving the macula with large choroidal vessels and the bared sclera at the base (Figure 1). The atrophic lesion was approximately 1 disc diameter (DD) in height and 1.2 DD in width in the right eye, and 1.2 DD in height and 1.5 DD in width in the left eye. Some retinal vessels were disturbed at the margin. Immunologic tests including toxoplasmosis, rubella, varicella, herpes virus, and human T-cell leukemia virus were all negative. Anti-toxoplasma antibody was also negative in his mother. At age 4 year and 1 month, cycloplegic refraction showed insignificant refractive errors and his best corrected visual acuity was 0.6 bilaterally. No stereoacuity was detected with TNO stereo test and Titmus Stereo Test. The fundus examination showed no significant changes except for some pigmentation in the atrophic lesions (Figure 2). Examination of fixation behavior by visuscope showed steady fixation in the normal area close to the nasal edge of the atrophic lesion bilaterally (Figure 2). The SD-OCT (Cirrus HD-OCT; Carl Zeiss Meditec AG, Jena, Germany) showed a crater-like depression accompanying atrophic neurosensory retina, and the absence of the retinal pigment epithelium and choroid (Figure 3). The retinal structures of the fixation area were normal involving IS/OS junction line (Figure 3). In addition, it was not difficult to get the SD-OCT images on him with dilated pupil and without any anesthesia. At age 5 years and 3 months, his best corrected visual acuity was 1.0 OD and 0.8 OS.

Figure 1 Fundus photographs taken at age 1 year and 4 months. Notes: (A) Right eye. (B) Left eye.

Figure2 Fundus photographs taken at age 4 years and 1 month. Notes: (A) Right eye. (B) Left eye. The white crosses show the fixation points.

Figure 3 SD-OCT images at age 4 years and 1 month. Notes:(A) Right eye. (B) Left eye. (A and B) ILM-RPE map on the macular cube 200×200. Cross points of vertical and horizontal lines show fixation points (white crosses). (C) B-scans. The bars show the fixation points (white arrows). Abbreviations: ILM-RPE, internal limiting membrane – retinal pigment epithelium; SD-OCT, spectral domain optical coherence tomography.

Discussion

Congenital macular coloboma is thought to result from incomplete differentiation of the arcuate bundles along the horizontal raphe during development.¹ However, its morphology of the macular lesion is very similar to that of postinflammatory congenital macular scars resulting from ocular toxoplasmosis. Differentiation between macular coloboma and postinflammatory scarring is important to decide the course of treatment. SD-OCT is a beneficial tool in making a differential diagnosis of macular degeneration.²

The OCT findings of this case showed absence of the retina and choroid. This structure conformed well to the findings of previous reports.^2–5 On the other hand, the OCT features of congenital toxoplasmosis macular scars are retinal thinning, retinal pigment epithelium hyper-reflectivity, excavation, intraretinal cysts, and fibrosis.^6,7 Added to the results of immunologic test, the OCT findings showed that the diagnosis of this patient was congenital macular coloboma.

The fixation points of the patient were located close to the nasal edge of the atrophic lesion. The OCT findings suggested that the retinal structures of the fixation points were well conserved involving IS/OS junction line. It was thought that the patient made adaptation to reflect the image on the comparatively healthy peripheral retina. This phenomenon is known as eccentric viewing.^8,9 There have been no reports on the structure of the fixation point, although the retinal structure of the fixation point influences the visual acuity and binocularity of patients.

The visual acuity from previous reports^{1–4,10–12} ranged from light perception to 0.8. However, the atrophic lesions of this case were comparatively smaller than other cases and the conserved normal retinal structures near the fovea resulted in the patient having better visual acuity than those of other cases, although he had exotropia and no stereoacuity. When his dominant eye perceived an image on the fixation point, the fellow eye perceived the image on the atrophic lesion, but the image on the atrophic lesion did not reach the visual cortex. This suggests that his monofixation is not caused by suppression.

Conclusion

The SD-OCT imaging is helpful in making a diagnosis and determining the course of treatments. Furthermore, the structure of the fixation point has a critical influence on the visual acuity and prognosis of congenital macular coloboma.

Disclosure

The authors report no conflicts of interest in this work.

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