Non-Arteritic Anterior Ischaemic Optic Neuropathy Associated with Optic Nerve Hypoplasia and Elevated Intraocular Pressure

ABSTRACT

A 46-year-old woman with no known past medical history presented with sudden painless visual impairment in the left eye. Ocular examination showed a swollen and hyperaemic left optic disc with a small and crowded right optic disc. Intraocular pressures were moderately elevated. Investigations for underlying ischaemic and inflammatory markers were normal. A diagnosis of left non-arteritic anterior ischaemic optic neuropathy (NAAION) was made, with associated ocular hypertension. On follow-up, NAAION resolved with anti-glaucoma medication and repeat fundus examination confirmed the presence of an underlying hypoplastic disc. The coexistence of optic disc hypoplasia and elevated intraocular pressure may further augment the risk of developing NAAION.

Case

A 46-year-old woman presented with a two day history of sudden onset of gross diminution of vision of her left eye. Her systemic examination was within normal limits. Ocular examination revealed a best-corrected visual acuity of 6/9 with her right eye and 3/60 with her left eye. There was a grade one relative afferent pupillary defect (RAPD) in the left eye. Intraocular pressure (IOP), measured with Goldmann applanation tonometry, was 28 mmHg in each eye. Colour vision and contrast sensitivity were normal in the right eye but could not be assessed in the left eye due to poor vision. Humphrey visual field analysis was normal in the right eye and showed a high number of fixation losses in the left eye. Fundus examination showed a small crowded disc in the right eye with a cup:disc ratio of 0.1 along with venous tortuosity. The left optic disc was oedematous with blurred margins and an obliterated cup (Figure 1a,b). A B scan ruled out optic disc drusen. Fundus fluorescein angiography (FFA) showed normal disc perfusion in the right eye and disc hypoperfusion with late disc staining suggestive of non-arteritic ischaemic optic neuropathy (NAAION) in the left eye (Figure 1c,d). In addition, FFA in the left eye also showed peripapillary patchy areas of choroidal hypoperfusion in the early phases surrounding the inferior half of the disc (Figure 1c), which filled up gradually during the late phases (Figure 1d) – a finding again suggestive of NAAION.

Figure 1.

Fundus photographs of the (a) right and (b) left eyes acutely showing optic nerve hypoplasia in the right eye and optic disc oedema in the left eye. (c) FFA showing left eye optic disc hypoperfusion (red arrow) and peripapillary choroidal hypoperfusion in the early phase, and (d) optic disc staining in the late phase (blue arrow) suggestive of NAAION in the left eye

Her blood pressure, fasting lipid profile and blood sugar levels were in the normal range and the erythrocyte sedimentation rate was 20 mm/hr. Carotid Doppler showed mild intimal thickening in the left internal carotid artery with no evidence of any plaques, calcifications or stenosis. Visual evoked potentials should a P100 latency of 80 msec in the right eye and prolonged to 210 msec with decreased amplitude in the left eye. Her central corneal thickness measured 503 μm and 509 μm in the right and left eye, respectively. She was diagnosed with ocular hypertension and NAAION in the left eye. She was treated with oral aspirin, corticosteroids, oral acetazolamide 250 mg bd along with timolol eye drops. Subsequently the intraocular pressures were found to be 32 mmHg in the right eye and 29 mmHg in the left eye. Therefore, dorzolamide and latanoprost eye drops were added. One month later the vision in her right eye had improved to 6/12 and the intraocular pressure was maintained at 20 mmHg, although the grade one RAPD persisted. On fundus examination, there had been resolution of the disc oedema but this revealed the presence of optic nerve hypoplasia with the disc diameter measuring less than 1.3 mm, the presence of a double-ring sign and increased disc-to-macula distance (Figure 2a,b). Retinal optical coherence tomography (OCT) of the retinal nerve fibre layer (RNFL) showed thickening in the left eye, which resolved on follow-up (Figure 2c,d).

Figure 2.

Fundus photographs of the (a) right and (b) left eyes on follow up showing resolution of optic disc oedema in the left eye (black arrow). (c) OCT-RNFL scans in both eyes showing RNFL thickening in the left eye (red arrow) acutely and (d) showing resolution of optic disc oedema and RNFL thinning in the left eye (green arrow) on follow-up

Magnetic resonance imaging (MRI) of the brain showed hypoplastic optic nerves with an associated empty sella turcica (Figure 3a,b). Optic disc drusen were ruled out by ultrasound B-scan showing the absence of calcification in the horizontal axial scan (Figure 3c).

Figure 3.

(a,b): Axial and sagittal MRI brain showing hypoplastic optic nerves and an empty sella turcica (red arrow). (c) Normal B scan of the left eye with no evidence of calcification in the horizontal axial scan. (d) Normal Humphrey visual field in the right eye

Discussion

NAAION is characterised by a reduction in the perfusion to the optic nerve head, which is supplied by the branches of the short posterior ciliary artery and central retinal artery.¹ This leads to a painless, unilateral loss of vision that may or may not recover. Systemic risk factors include diabetes mellitus, hypertension, hyperlipidaemia, sleep apnoea, and nocturnal hypotension. Ocular risk factors include absent or small cup in the optic disc, and the term “disc at risk” refers to the greater prevalence of small crowded optic discs in the fellow eyes of patients with NAAION.²

Optic nerve hypoplasia is characterised by a small optic nerve head and a significantly reduced number of axons occurring due to irreversible axonal damage to the visual pathway at some time before the full development of the eye. It may be unilateral but often occurs bilaterally (in 65% to 75% of cases). An ophthalmoscopically detectable sign of optic nerve head hypoplasia is the “double-ring sign”, in which a peripapillary ring surrounds the small optic nerve head. Further, optic nerve hypoplasia is confirmed if the distance from the disc to the macula is equal to or greater than 3 disc diameters.³ Visual acuity depends on the extent of axonal loss and visual field defects can range from normal to localised defects in the nasal and inferior quadrants or diffuse constriction. The spectrum of optic nerve hypoplasia appears to be very wide also with respect to the extent of damage.⁴

NAAION has been reported previously in association with optic disc drusen and marked optic disc oedema due to any cause. To the best of our knowledge, this is the first case reported in a hypoplastic disc.

The mean ocular perfusion pressure (MOPP) = ⅔(mean arterial pressure − IOP). In NAAION, the optic nerve head (ONH) already has poor and precarious circulation. Under those circumstances, even a transient or sustained rise in IOP has the potential to further compromise the circulation and result in further ONH damage and visual loss. Although the prevalence of ocular hypertension in ischaemic optic neuropathy is not known, NAAION has been reported in patients with acute angle closure attacks, Posner- Schlossman syndrome, and neovascular glaucoma suggesting that an elevated IOP may be a risk factor in structurally congested discs.^5–8 In optic nerve hypoplasia there are fewer nerve fibres compared with glial and other supporting cells, which compromises the microcirculation of the structurally congested optic discs. Increasing perfusion of the optic nerve by reducing intraocular pressure may reduce damage and prevent progression. Topical or oral brimonidine tartrate has been reported to provide neuroprotection for retinal ganglion cells following experimental glaucoma and optic nerve injury in animal models.⁹

In summary, we report a case of NAAION associated with optic nerve hypoplasia and IOP elevation. The mild IOP elevation concurrent with the disc crowding appears to have reduced circulation in the vulnerable optic nerve, causing NAAION.^10,11 Although this combination occurs rarely, patients with optic nerve hypoplasia and raised IOP may be protected against NAAION by prophylactic treatment with antiglaucoma medications.

Conclusion

NAAION is known to occur in small crowded discs, the authors report the first case of NAAION in a case of optic nerve hypoplasia. IOP monitoring and management are recommended for individuals at risk for NAAION.

Declaration of interest statement

Author declares no conflict of interest or funding received for this case study.