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The Official Scientific Journal of Delhi Ophthalmological Society
Optic Nerve Aplasia – A Rare Entity
Debarpita Chaudhury, Meenakshi Ravindran, Rengappa Ramakrishnan, Arijit Mitra
Aravind Eye Hospital & Postgraduate Institute of Ophthalmology, S.N. High Road, Tamil Nadu, India
Corresponding Author:
Debarpita Chaudhury
DO, DNB
Department of Paediatric Ophthalmology
Aravind Eye Hospital & 
Postgraduate Institute of Ophthalmology,
Tamil Nadu, India
Email id: debarpita.c@rediffmail.com

Published Online: 30-JUL-2018
DOI: http://dx.doi.org/10.7869/djo.378
Abstract
Optic nerve (ON) aplasia is a rare developmental anomaly comprising of absence of the ON, ganglion cells and the central retinal vessels. It may be 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 four year old girl presented to us with the parents noticing the right eye to be smaller than the left one since birth. On examination, the right eye had no perception of light with microcornea and mild enophthalmos. The right pupil did not react to direct light stimulation (APD) and there was no consensual reaction. Fundus of the right eye showed absence of the optic disc and central retinal vessels. Spectral Domain Optical coherence tomography (SD-OCT) of the right eye showed significant thinning of retinal nerve fibre layers. Ultrasound A-Scan showed decreased axial length of the right eye. Ultrasound B-Scan showed absence of optic nerve shadow. Magnetic resonance imaging (MRI) revealed that there was no recognizable right optic nerve in the anterior aspect of the orbital course, however in the posterior part, the optic nerve was seen only as a thin cord. The left eye was normal. Flash visual evoked potential (VEP)was unrecordable in the right eye and normal in the left eye. The child did not have any other associated central nervous system (CNS) abnormalities.
Keywords : Optic nerve aplasia, optic disc, microcornea, enophthalmos, magnetic resonance imaging.
Introduction
Aplasia of the optic nerve is a very rare ocular abnormality. It may be unilateral or bilateral. The unilateral variety tends to be more benign, most commonly associated with ipsilateral microphthalmos in an otherwise normal patient. For true aplasia to exist, there must be complete absence of the optic nerve (including the optic disc), retinal ganglion and nerve fibre layers and central retinal vessels. Magnetic resonance imaging (MRI) in unilateral aplasia show microphthalmos, absence of optic nerve, chiasma and tracts on the side of aplasia. Histologically, there is absence of the optic nerve, retinal ganglion cells, nerve fiber layer, and optic nerve vessels.1 We report an extremely rare case of right sided unilateral true optic nerve aplasia in a four year old girl occurring as an isolated Central Nervous System (CNS) anomaly.

Case Report
A 4 year old girl presented to us with the parents noticing her right eye to be small since birth. She was the first child born out of a non consanguineous marriage, term pregnancy and normal delivery with a birth weight of 2900 grams. Antenatal as well as birth history was normal. Family history was negative for ocular or other birth defects. Except for the ocular finding of the right eye being smaller than the left, the child had no other ocular or systemic abnormalities and had normal developmental milestones (Figure 1).


Ophthalmic examination demonstrated that there was no light perception in the right eye. Left eye had a vision of 20/20. The right eye had microcornea with horizontal and vertical diameters being 9.5 mm and 8 mm respectively. The right pupil did not react to direct light stimulation (APD) and there was no consensual reaction. The right eye had a normal anterior chamber depth, iris had normal colour and pattern and lens was clear. The left eye was normal with a clear cornea with the horizontal and vertical diameters being 12 mm and 11 mm respectively. The left pupil reacted normally to direct light stimulation but there was no consensual light reflex. Intraocular pressures were 16 mm Hg in both eyes. Gonioscopic examination showed open angles in both eyes. Dilated fundus examination of the right eye showed absence of optic disc and central retinal vessels. Numerous large, round to oval, circumscribed, whitish to yellow areas of retinochoroidal hypopigmentation were visible on the posterior pole. There was marked choroidal tessellation with multiple areas of chorioretinal atrophy. Choroidal vessels were prominent in areas of decreased retinal pigment epithelium (RPE) (Figure: 2a, 2b & 2c). The left eye fundus examination revealed normal disc, macula, retinal vessels and normal peripheral retina (Figure 2d).



Ultrasound B-scan showed no optic nerve (ON) shadow in the right eye. On A-scan ultrasound, the mean axial length was 19.9 mm in the right eye and 21.4 mm in the left eye. MRI scan of the orbits and brain disclosed that there was no recognizable right optic nerve in the anterior aspect of the orbital course, however, in the posterior part, remnants of the ON sheaths with some glial tissue were present as a thin cord. The optic chiasma appeared asymmetric due to chiasmal aplasia on the right side. There was mild enophthalmos of the right globe. The left globe and optic nerve were normal (Figure 3). Spectral domain optical coherence tomography (SD-OCT) scan was normal for the left eye while there was significant thinning of retinal nerve fibre layer (RNFL), inner retinal layers (IRL) and outer retinal layers (ORL) in the right eye (Figure 4). Flash visual evoked potential (VEP) was unrecordable in the right eye and normal in the left eye. The remaining ocular and cerebral structures including septum pellucidum were normal. Toxoplasma, rubella, cytomegalovirus and herpes (TORCH) titres were negative. Brainstem auditory evoked potential (BAEP) were within normal limits on both sides. Growth hormone,thyroid stimulating hormone and thyroxine levels were normal. She had no other systemic abnormalities.





Discussion
Optic nerve aplasia is a very rare condition that is characterized by the complete absence of the optic nerve head on clinical examination. There is no light perception along with afferent pupillary defect (APD) on the affected side. Fundus examination of the affected eye shows absent optic disc and retinal vessels. MRI scan shows absent optic nerve and asymmetry of the optic chiasma due to aplasia of one side.1 Histologically, there is absence of the optic nerve, retinal ganglion cells, nerve fiber layer and optic nerve vessels.1 Optic nerve aplasia was thought to be part of a spectrum of optic nerve hypoplasia and prior to a clear definition of this disorder in literature,many early reports of optic nerve hypoplasia were included incorrectly in discussions of optic nerve aplasia.1-3 Family history is not consistent with Mendelian inheritance and results of chromosomal examination in cases of ON aplasia are normal2-5 Males and females are similarly affected.1,2
The pathogenesis of optic nerve aplasia remains speculative. In early reports, optic nerve aplasia was attributed to failure of the mesoderm to enter the foetal fissure and provide vascularization of the retina and nerve tissue.1 Weiter and associates expressed doubt that mesodermal development was defective, because the dural sheath (a mesodermal derivative) was present in most of their cases.6 They suggested that ventral invagination of the optic vesicle caused nerve fibre misdirection and secondary atrophy. Yanoff and colleagues postulated a primary failure of ganglion cells to develop and send out axons as the cause of ON agenesis.7 Thus, based on the current understanding, ON aplasia may be due to defective formation of the embryonal fissure, failure of the mesenchymal anlage of the hyaloid system to enter the embryonal fissure, or primary agenesis of the retinal ganglion cells.4-7
Published reports have described a spectrum of eye abnormalities on the affected side. Frequently, these eyes are microphthalmic and enophthalmic, and the eyelid may be ptotic. Esotropia of the affected eye is common. The corneal diameter is usually small (5 to 10 mm).1 An immature iridocorneal angle may be present, iris hypoplasia, iris coloboma, or aniridia may occur.2,5-7 The retina is usually avascular. No definite optic nerve is present. The RPE may be grossly abnormal, with geographic areas of hyperplasia, hypoplasia, or normal pigmentation. The choroidal vasculature is easily seen in regions of deficient RPE. Cystoid retinal degeneration is identified in regions of detachment. Colobomas, anterior to the equator and anterior or posterior staphylomas occur in the majority of eyes.6 No light perception is present in the affected eye, and light stimulation of the eye with aplasia elicits no direct or consensual pupillary response.
Visual fields are not recordable on the side of aplasia and are normal in the unaffected eye as was seen in our case. Fluorescein angiography demonstrates absence of retinal blood vessels on the side of aplasia.1,8,9 With A-scan ultrasound, the globe is usually found to be small. B-scan ultrasound reveals no optic nerve shadow. In certain cases B-scan ultrasound may detect an area of retinal detachment.3 Flash VEP is unrecordable on the side of aplasia as was seen in our patient. A computerized tomography (CT)scan may show the globe and bony orbit to be smaller than normal.4 MRI can document the absent optic nerve on the affected side.The chiasma and lateral geniculate body may appear small and remnants of the dural sheath are usually identified as was seen in our case.4 Bilateral cases are more often associated with other major congenital defects that often result in death of the infant. These patients can also have septo-optic dysplasia, a rare congenital birth defect.2
Microphthalmos has been reported to occur with optic nerve aplasia, similar to our patient.2-4 In addition, optic nerve aplasia has been associated with retinal pigment epithelial and choroidal colobomas, persistent fetal vasculature, and retinal dysplasia.
There have been reports of posterior segment neovascularization, thought to be a response to the ischemia resulting from the absence of retinal vasculature.10 ON aplasia cases may be associated with multiple central nervous system (CNS) anomalies such as hydrancephaly and hypopituitarism.3 Our patient did not have any abnormalities of the cranium and she had no CNS abnormalities other than ON aplasia.
Management of such cases is directed towards any associated neurological problems. The parents are counseled regarding education, vocational rehabilitation and mobility training in cases of bilateral ON aplasia. In certain cases, it would be useful to implant an ocular prosthesis in patients with unilateral microphthalmos with ON aplasia.

References
  1. Little LE, Whitmore PV, Wells TW Jr. Aplasia of the optic nerve. J Pediatr Ophthalmnol 1976; 13:84-88.
  2. Ginsberg J, Bove KE, Cuesta MG. Aplasia of the optic nerve with aniridia. Ann Ophthalmol 1980; 12:433-439.
  3. Storm RL, PeBenito R. Bilateral optic nerve aplasia associated with hydrancephaly. Ann Ophthalmol 1984; 16:982-992.
  4. Margo CE, Hamed LM, Fang E, Dawson WW. Optic nerve aplasia. Arch Ophthalmol 1992;110:1610–13.
  5. Barry DR. Aplasia of the optic nerves. Int Ophthalmol 1985; 7:235-242.
  6. Weiter JJ, McLean IW, Zimmerman LE. Aplasia of the optic nerve and disc. Am J Ophthalmol 1977; 83:569-576.
  7. Yanoff M, Rorke LB, Allman MI. Bilateral optic system aplasia with relatively normal eyes. Arch Ophthlalmol 1978; 96:97-101.
  8. Taniaka E, Tanaka Y. A case of aplasia and hypoplasia of the optic nerve. Jpn J Clin Ophthalmol 1985; 39:1207-1211.
  9. Blanco R, Salvador F, Galan A, et al. Aplasia of the optic nerve: Report of 3 cases. J Pediatric Ophlthalmology and Strabismus 1992; 29:228-231
  10. Lee BL, Bateman JB, Schwartz SD. Posterior segment neovascularization associated with optic nerve aplasia. Am J Ophthalmol 1996; 122:131-133.
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Debarpita Chaudhury, Meenakshi Ravindran, Rengappa Ramakrishnan, Arijit MitraOptic Nerve Aplasia – A Rare Entity.DJO 2018;29:51-54
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Debarpita Chaudhury, Meenakshi Ravindran, Rengappa Ramakrishnan, Arijit MitraOptic Nerve Aplasia – A Rare Entity.DJO [serial online] 2018[cited 2018 Aug 18];29:51-54. Available from: http://www.djo.org.in/articles/29/1/Optic-Nerve-Aplasia.html
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