Optic nerve hypoplasia (including association with septo-optic dysplasia)

Studies of children with ONH have suggested associations with young maternal age, first child, maternal diabetes, prematurity, periventricular leukomalacia and exposure to various toxins during pregnancy (alcohol, anticonvulsants, smoking, SSRIs , and cocaine ) but no firm cause has been established.

It does occur in aniridia , anterior visual system tumours, with some brain malformations (holoprosencephaly, schizencephaly, porencephaly), in septo-optic dysplasia in combination with agenesis of the corpus callosum and/or pituitary insufficiency) and can also be inherited.⁵

Optic nerve hypoplasia may present to the ophthalmologist with primary visual problems, or it may be suspected during endocrine or neurological assessment for associated conditions (eg neonatal hypoglycaemia, developmental impairment). Visual presentation depends on severity.

Bilateral/severe ONH

• Roving eye movements, blindness, sluggish pupil responses.
• May have see-saw nystagmus in septo-optic dysplasia.
• Delayed visual maturation.

Mild ONH

• May be asymptomatic. Nystagmus and/or strabismus may be evident.

Unilateral/asymmetric bilateral

• Strabismus, unsteady fixation, relative afferent pupillary defect (RAPD).

The classic ophthalmological feature of ONH is the ‘double-ring’ sign where the true nerve size is represented by the inner ring.

The size of the disc can be estimated by calculating the disc-to-macula (DM) to disc diameter (DD) ratio. If DM/DD is over 3, this is suspicious of ONH, and if it is greater than 4, then ONH is very likely.⁶

Abnormal disc configurations with reduced nerve fibres can occur with sectoral ONH. The ‘figure 8’ disc is a variant of temporal hypoplasia, it occurs in conjunction with developmental suprasellar tumours. Hypoplasia of the upper part of the optic nerves is seen in children born to mothers with diabetes mellitus.

Ex-premature babies with periventricular leucomalacia (PVL) may show abnormal ON cupping as a variant of optic nerve hypoplasia in normal sized optic discs with reduced axonal numbers. In this case the mechanism is retrograde transynaptic degeneration occurring after 28 weeks.⁷

Children with ophthalmoscopic appearances of ONH have reduced cross-sectional intracranial ON diameters. A cross-sectional area of <2.9mm2 in a child over 12 months of age (by which time the ON has achieved most of its adult size) suggests ONH.⁸

The VEP can be reduced in severe ONH with reduced acuity. Reduced VEP can be predictive of a poor visual prognosis. The flash ERG is usually normal.⁹

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