Pathogenesis

Ischaemic optic neuropathy occurs in two forms, non-arteritic (median age 56 years) and arteritic (median age 74 years).

Non-arteritic ischaemic optic neuropathy

The non-arteritic form, AION or PION, may occur in patients with carotid artery disease and may be the initial manifestation of an ICA occlusion.76 Rarely, patients with ICA disease experience the simultaneous occurrence of cerebral infarction and ipsilateral ischaemic optic neuropathy. This combination is called the opticocerebral syndrome.77 Three patients described by Bogousslavsky77 had AION and cerebral signs. Other patients reported had ICA occlusion or severe stenosis, acute hemispheric stroke, and simultaneous monocular blindness caused by PION in the ipsilateral eye.78 No patient gained improvement in vision in the affected eye and all patients developed optic atrophy.

The pathogenesis of AION in the setting of disease of the ipsilateral ICA is multifactorial. In some cases, optic nerve infarction results from reduced blood flow secondary to carotid occlusion, poor collateral blood supply, and local changes in the pial circulation of the optic nerve. In other cases AION or PION may be due to embolism to multiple posterior ciliary arteries, pial vessels, or both.79,80 In the majority of cases, however, non-arteritic AION is a disease of small vessels and not directly related to carotid disease but rather reflects shared risk factors, such as hypertension, diabetes mellitus, or cigarette smoking.66,81-83

Perioperative non-arteritic AION is also the result of multiple factors and occurs in the setting of various surgical interventions. The two most important factors in conjunction with surgery are hypotension and blood loss (for review see Williams et al.84). Severe anaemia alone may not cause AION, but even a short episode of hypotension in an already anaemic patient may predispose to AION induced visual loss.85

Loss of vision after a severe spontaneous haemorrhage is usually bilateral but it may affect both eyes asymmetrically or be unilateral. The severity ranges from mild or transient blurred vision in one eye to irreversible total blindness in both eyes. The visual loss often occurs at the time of haemorrhage but it may, in rare instances, be delayed beyond 10 days. Most patients are debilitated and over 40 years of age. Many have intercurrent systemic illnesses but they do not necessarily have risk factors for atherosclerosis. Visual loss typically follows repeated haemorrhage although visual loss can occur after a single massive bleed.86 Ophthalmoscopy typically shows the characteristic changes of AION but the discs can appear normal initially when infarction of the optic nerve(s) occurs in the midorbital portion.87 About 50% of patients who lose vision after an acute bleed experience some recovery of vision, but only 10-15% recover completely.

Clinical features that profile patients at risk of PION (infarction of the optic nerve posterior to the lamina cribrosa) include middle aged men (mean age of 50 years) who undergo spine surgery, blood loss ranging from 2000 to 16 000 ml with a drop in haematocrit from 95% to 19% (mean 14%), and intraoperative systemic hypotension in all patients. Prevention or immediate correction of hypovolaemic hypotension can reduce the incidence of PION.

Anterior ischaemic optic neuropathy occurring in chronic uraemic patients on dialysis has some common risk factors, including hypotension and anaemia. Uraemic children can be affected. What is striking, in the three published paediatric cases, is that they all had polycystic kidney disease.88

Arteritic ischaemic optic neuropathy

In arteritic AION or PION, constitutional symptoms of giant cell arteritis may be absent and acute visual loss is the herald symptom. The stroke to the optic nerve is caused by inflammatory occlusion of the short posterior ciliary arteries that supply the immediate retrolaminar and laminar portions of the optic disc. The inflammation may also produce sectorial areas of choroidal ischaemia.

In some cases of arteritic AION or PION, the constitutional symptoms of giant cell arteritis are insidious. They include fatigue, anorexia, weight loss, and alterations in mental status, including depression and memory impairment. Severe persistent headache is present in 40-90% of patients with or without tenderness over the temporal arteries or scalp, intermittent claudication of the masseters, and facial swelling. Severe systemic manifestations include respiratory tract symptoms, myocardial infarction, and gastrointestinal complications due to generalised vasculitis.

Acute visual loss is usually unilateral but it may be bilateral and simultaneous or the second eye may be affected days, weeks, or even months after the first eye, particularly if corticosteroid treatment is not begun immediately or is stopped while the disease is still active. Episodes of transient monocular visual loss may precede persistent visual loss caused by AION and occasionally transient visual loss may be induced by exertion or changes in posture. The appearance of the ischaemic optic disc in giant cell arteritis resembles that of non-arteritic AION.

Giant cell arteritis causing PION must also be suspected in acutely blind elderly patients with a normal appearing optic disc in the affected eye due to interruption of blood flow to the retrolaminar optic nerve. Infarction in these cases may even affect the intracranial portion and in most cases of giant cell arteritis-associated PION, histopathological examination reveals inflammatory occlusion of the ophthalmic artery and short posterior ciliary arteries.68

Ocular ischaemia from polyarteritis nodosa (PAN) is rare but PAN can produce ischaemia of a variety of ocular structures, including the retina, choroid, and optic nerve. In one patient, all three structures were affected. The patient, a 70 year old woman with hand and foot numbness, suddenly lost vision in the right eye from a CRAO and then developed a left AION and bilateral triangular areas of pigment epithelial disturbance caused by choroidal infarction - the triangular sign of Amalric. Initially, giant cell arteritis was suspected because of the combination of bilateral ocular ischaemia in an elderly patient with an elevated ESR and CRP. However the patient's peripheral neuropathy made this less likely, and a multiple mononeuropathy was demonstrated by EMG and nerve conduction velocity studies. Biopsy specimens from her sural nerve and biceps muscle showed a necrotising vasculitis with fibrinoid necrosis consistent with PAN.89 Treatment of PAN consists primarily of corticosteroids with the addition of cyclophosphamide therapy if there is significant visceral organ involvement, continued progression of disease, or both. Bilateral ocular ischaemia AION is also reported in Takayasu arteritis.90

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