To the Editors:
We report characteristics of the optic nerve head, retinal nerve fiber layer (RNFL), and macular thickness on optic disk hypoplasia in a patient with optic disk hypoplasia. All optic nerve head dimensions were reduced. In addition, mean RNFL thickness was only 13 urn compared with 103 urn in the fellow eye. Parafoveal macular thickness was also decreased. In optic disk hypoplasia, optical coherence tomography (OCT) shows not only decreased optic disk dimensions, but also grossly reduced RNFL and parafoveal thickness.
Optic disk hypoplasia has been reported as the most common congenital anomaly of the optic disk.1 The condition is usually bilateral, but unilateral cases are also known to occur. Bilateral cases have been reported to have mutations in the PAX 6 gene.2 Although a host of factors have been implicated in the causation of this condition, most often, the cause is not known. Optic disk hypoplasia shows wide variation in presentation in terms of morphologic features, visual acuity, visual field defects, and associated neurologic and endocrinologie manifestations. We herein report the OCT features of macular thickness, optic nerve head dimensions, and RNFL thickness in a patient with unilateral optic disk hypoplasia of unknown cause.
A 14-year-old girl had a history of absence of vision in the right eye since birth. This was associated with outward deviation of the right eye since early childhood. She had no other ocular or systemic symptoms. The patient showed normal growth and development, and findings on physical examination were unremarkable.
Ophthalmic evaluation showed visual acuity of no perception of light in the right eye and 6/6 unaided in the left eye. She was found to have a right exotropia of 30° and nystagmoid movements of the right eye. The anterior segment was normal. Fundus evaluation showed a small optic disk with a surrounding ring of hypopigmentation (Fig. 1). The retinal blood vessels in this eye were straight, with few branching vessels. The optic disk to fovea distance was also increased. The left eye fundus was normal.
In both eyes, OCT was performed, and on line scan, the fbveal depression was absent and flattening of the adjacent retina was observed. Although macular thickness analysis showed comparable thickness within the innermost circle (1 mm), the adjacent retina was of uniform thickness. This was in contrast to the normal fellow eye, wherein retinal thickness in the middle and outer circles was greater than that in the innermost circle (Fig. 2). In the optic nerve head scan, all dimensions were grossly decreased in the right eye and normal in the left eye. The disk diameter was only 0.88 mm, with a cup measuring 0.42 mm. In the right eye, RNFL thickness was grossly reduced in all sectors, with an average value of 13.23 µm. In contrast, in the left eye, it was normal at 103.69 µm.
Evaluation of optic nerve hypoplasia can be difficult in subtle cases, but the diagnosis is obvious in severely affected patients, such as the one described herein. When the diagnosis is uncertain, high-resolution magnetic resonance imaging can be a useful diagnostic modality.3,4
A relatively new modality, OCT provides accurate and reproducible measurements of the optic nerve and macula. So far, there are few reports of detecting optic disk hypoplasia using OCT. Unoki et al. studied patients with superior segmental hypoplasia using OCT, but have commented only on decreases in peripapillary RNFL thickness in the superior quadrant.5 To the best of our knowledge (after a literature search), this is the first quantitative report of macular thickness, optic disk dimensions, and RNFL features on OCT assessment in a patient with optic disk hypoplasia.
We found that all of the parameters, including average retinal thickness, RNFL thickness, average nerve width, disk diameter, cup diameter, rim length, vertical and horizontal integrated rim area, disk area, cup area, and rim area, were significantly reduced in the affected eye compared with the fellow eye. This case shows the dimensional changes in the optic nerve and suggests that both RNFL and overall macular thickness are decreased in optic disk hypoplasia.
1. Birkebeck NH, Patel L, Wright NB, et al. Endocrine status in patients with optic nerve hypoplasia: relationship to midline central nervous system abnormalities and appearance of the hypothalamic-pituitary axis on magnetic resonance imaging. J Clin Endocrinol Metab 2003;88:5281-5286.
2. Azuma N, Yamaguchi Y, Randa H, et al. Mutations of the PAX6 gene detected in pariente with a variety of optic-nerve malformations. Am J Hum Genet 2003;72:1565-1570.
3. Romano PE. Simple photogrammetric diagnosis of optic nerve hypoplasia. Arch Ophthalmol 1989;107:824-826.
4. Brodsky MC, Glasier CM, Pollock SC, Angtuago EJ. Optic nerve hypoplasia: identification by magnetic resonance imaging. Arch Ophthalmol 1990; 108: 1562-1567.
5. Unoki K, Ohba N, Hoyt WF. Optical coherence tomography of superior segmental optic hypoplasia. Br J Ophthaimol 2002;86:910-914.