Bilateral optic nerve aplasia: a rare isolated central nervous system anomaly.

Optic nerve (ON) aplasia is a rare developmental anomaly comprising of absence of the ON, ganglion cells and the central retinal vessels. It is usually accompanied by a variety of central nervous system (CNS) malformations. We report an extremely rare case of bilateral true ON aplasia occurring as an isolated CNS anomaly. A 10-month-old female child was detected with bilateral absence of fixation, associated with multiple ocular findings of microcornea, anterior embryotoxon, pupillary corectopia, partial aniridia and lens coloboma. The optic disc locations were visible as avascular whitish areas. There were no retinal vessels seen in the disc area or elsewhere. There were bilateral multiple chorioretinal atrophic patches present. B-scan ultrasonography showed bilateral absence of ONs. Magnetic resonance imaging showed thinned out bilateral ONs with a hypoplastic optic chiasma and optic tract. There were no other CNS anomalies. Flash visually evoked potential was unrecordable.

INTRODUCTION

Optic nerve (ON) aplasia is a rare, congenital developmental anomaly, comprising of absence of ON, central retinal vessels and ganglion cells.12 It is generally, accompanied by other central nervous system (CNS) anomalies. A search of the medical literature has revealed only two previous reports of bilateral ON aplasia in otherwise normal children.34]

CASE REPORT

A 10-month-old female child, born of a third degree consanguineous marriage, presented with an inability to make eye contact and follow objects since birth. There was no significant history regarding the antenatal, natal and post-natal period. Developmental milestones had been reached normally.

General examination revealed platycephaly of the skull. Neurological and other systemic examinations were within normal limits. On ophthalmic examination, an absence of fixation with up-rolling of each eye was noted. The child did not follow any light or visual target and the pupillary reflexes were absent in both eyes. Ocular movements as tested by Doll's eye maneuver were normal. The ocular adnexa were also normal.

Anterior segment examination revealed bilateral anterior embryotoxon with bilateral microcorneas.

The right eye had an inferiorly placed pupil (corectopia) while the left eye had partial aniridia associated with a lens coloboma in the inferotemporal quadrant. There were no abnormalities seen on gonioscopy and intraocular pressures were within normal limits. Axial lengths were also within normal limits.

Ophthalmoscopy revealed the absence of both optic discs, which were represented by whitish, undefined round structures. Both, the disc area and the retina were devoid of retinal blood vessels. There was marked choroidal tessellation with multiple areas of chorioretinal atrophy seen bilaterally [Figures 1 and 2].

Figure 1

Right eye fundus shows whitish area representing disc, absent retinal vessels, multiple chorioretinal atrophic patches. Inset shows a view of inferior retina

Figure 2

Left eye fundus shows whitish area representing disc, absent retinal vessels and a large chorioretinal atrophic patch

The ONs could not be seen on B-scan ultrasonography. Magnetic resonance imaging of the brain showed the presence of markedly thinned out ONs [Figure 3]. The optic chiasma and optic tracts were hypoplastic. Flash visually evoked potential (VEP) was unrecordable. There were no other CNS abnormalities detected.

Figure 3

T2 weighted magnetic resonance imaging showing thinned out optic nerves with hypoplastic chiasma

DISCUSSION

The ON is formed of the axons of the retinal ganglion cells, which are derived embryologically from the inner neuroblastic layer of the optic cup.5 Failure of development of these cells is rare. When there is accompanying failure of development of mesodermal elements as well, it is termed aplasia of the ON which is an absence of optic disc, ganglion cells and central retinal vessels.12 Many cases, which have been reported in literature as ON aplasia are, in fact, cases of ON hypoplasia as some of the features above are present.6

Our case had true bilateral ON aplasia as there were no retinal vessels seen in either eye and the optic discs were visible as ill-defined avascular whitish structures. The radiological report of thinned ONs, were likely to be merely ON sheaths with some glial tissue. Flash VEP was performed to distinguish ON hypoplasia from ON aplasia. VEP was unrecordable.

Variable ophthalmic features associated with ON aplasia include micro-ophthalmos, enophthalmos, ptotic lids, squint, microcornea, trabeculodysgenesis, iris hypoplasia, iris coloboma, aniridia and persistent hyperplastic primary vitreous.12]

In the current case, there were several associated ocular abnormalities, such as microcornea, anterior embryotoxon, corectopia, partial aniridia, lens coloboma and multiple chorioretinal atrophic areas.

ON aplasia cases are generally associated with multiple CNS anomalies such as hydrancephaly and hypopituitarism.78 Our patient had a platycephaly, which was not accompanied by any CNS abnormalities other than ON aplasia.

Management of such cases is directed towards any associated neurological problems. Since there is no specific ophthalmic management, the parents are counseled regarding education, vocational rehabilitation and mobility training of blind children.

To the best of our knowledge, this is only the third case of bilateral ON aplasia in a normal child, which is an extremely rare developmental anomaly.