Anomalies in the optic nerve
Five congenital optic nerve anomalies to watch out for
Yair Morad MD
There are five important congenital hereditary and non-hereditary optic nerve anomalies that require early diagnosis in order to preserve and enhance vision, Yair Morad MD, Shamir Medical Centre, Tel Aviv University, Israel, told the 2020 WSPOS Virtual Meeting.
The first of these is optic nerve coloboma. It results from incomplete closure of the embryonic fissure during the fifth-to-seventh week of gestation. It usually involves unilateral or bilateral infero-nasal defects that may include the optic nerve, choroid, retina, iris and lens. Extended defects usually cause microphthalmia. Optical correction and patching are necessary in these children in addition to genetic counselling and renal ultrasound. Serous retinal detachment occurs in up to 45% of patients.
Optic pit is another important optic nerve anomaly. It is a congenital excavation of the optic nerve head that appears to result from defected fissure closure. It can appear in any sector, but is usually located temporally. Secondary serous retinal detachment occurs in 25-to-75% of affected eyes and becomes symptomatic in the third or fourth decade of life, though it can occur in children.
Morning glory disc anomaly is characterised by an enlarged disc, with a pink orange colour and white glial tissue in its centre. Other features of this condition include peripapillary excavation and pigmentation and an increased number of blood vessels. Visual acuity in affected eyes usually ranges from 6/60 to finger counting, although some eyes can have normal visual acuity.
Optical correction and a trial of patching is indicated in these patients. They should also be monitored for serous detachment. In addition, magnetic resonance imaging is also advisable because of the condition’s association with basal encephalocele and the rare vascular disorder Moyamoya disease.
Another optic nerve anomaly is tilted disc syndrome, in which the superotemporal part of the disc is elevated and the inferonasal part is posteriorly displaced, resulting in an oval-shaped disc with its long axis obliquely oriented. Many patients have a bi-temporal hemianopia that results from posterior bowing of parts of the retina causing a refractive scotoma due to myopic shift in these areas. However, this visual defect can be eliminated by additional minus lenses.
Finally, there is the optic disc with myelinated nerve fiber layer, in which there is myelination of the afferent visual pathways starting from the lateral geniculate body and terminating in the lamina cribrosa. Some patients with the condition have normal vision but most have myopia and amblyopia. In some cases, the amblyopia does not respond to aggressive patching due to hypoplastic optic nerve or hypoplastic macula.