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Ischemic optic neuropathy (ION) is a condition in which there is significant impairment or loss of vision in the middle-aged or elderly population, especially Caucasians, due to ischemia of the optic nerve head. The incidence in the USA is between 2 and 10 per 100 000 with 6000 new cases being diagnosed each year.
There are two distinct types of IONs, anterior ION (AION) and posterior ION (PION). These are differentiated by the part of the optic nerve head that is affected. There are two distinct sources of arterial supply for the two parts of the nerve head, according to which the lesions are classified, as well as based upon the etiology. The anterior part is supplied by the posterior ciliary artery but the posterior segment has other sources of blood supply.
Arteritic AION is due mainly to giant cell arteritis, an inflammatory condition of the large or medium-sized arteries, which often attacks the posterior ciliary artery. In rare cases, this condition may be associated with panarteritis nodosa or systemic lupus erythematosus.
Non-arteritic AION is due to other causes which lead to a temporary interruption of supply to the optic nerve, or to embolism of the arteries of the optic nerve head. Predisposing conditions for non-embolic non-arteritic AION include arterial hypertension, diabetes mellitus, cardiovascular disease, atherosclerosis, migraine, sleep apnea, and hyperlipidemia. Local factors such as marked papillitis, raised intraocular pressure or abnormal vascular patterns are also linked to a higher incidence of non-arteritic AION. The autoregulatory factors that keep the pressure in the arteries that perfuse the optic nerve head also fail. The final straw may be nocturnal hypotension as occurs during sleep, especially on patients who are on hypertension-lowering drugs. This precipitates ischemia of the optic nerve in patients who already have borderline perfusion.
Embolic AION is rare but its effects are severe and permanent.
Similarly, arteritic PION is caused by giant cell arteritis of the orbital arteries supplying part of the optic nerve, and is far less frequent than arteritic AION.
Non-arteritic PION is due to various other conditions, roughly approximating those factors which precipitate non-arteritic AION by producing hypotension of the optic nerve supply and creating ischemia.
Surgical PION, on the other hand, is a complication of certain surgical procedures. These include prolonged spinal surgeries, radical cervical lymph node dissection, coronary bypasses and venous grafts in the lower limbs. It is due to too long a period of hypotension, complicated by hemodilution, and orbital and eye swelling including chemosis. The loss of vision is both bilateral and permanent, and since it tends to be catastrophic, even resulting in blindness, it is essential to protect the optic nerve supply during such procedures.
Many patients, approximately 3 of every 4, clearly remember that they experienced the visual loss as soon as they awoke from sleep, or the first time during the day that they tried to see something.It is both painless and sudden, and not preceded by any warning symptoms. Sometimes the visual loss is progressive. Blurring or dimness of the inferior visual field is the most common way in which vision loss is recognized.
Diagnosis of AION is based on the clinical history with the finding of the characteristic inferior nasal field defects. Ophthalmoscopy shows an edematous optic disc which becomes pale in approximately 2 months. Splinter hemorrhages may be seen near the disc.
Tests in patients with ION include:
The arteritic and non-arteritic types of AION may be distinguished by the presence of the following in the former:
These features indicate an arteritic AION.
PION shows a normal fundoscopy and fluorescein angiography, but pallor of the disc develops within about 8 weeks. Central field defects usually occur with or without other defects. In a few patients, the central vision is spared while peripheral vision is impaired. A relative afferent pupillary defect may also be observed.
Both arteritic AION and arteritic PION are dire emergencies that result in rapid onset of permanent blindness. Therefore, they must be treated with high doses of corticosteroids even on suspicion, in order to immediately arrest the inflammatory process and prevent further ischemic damage to the optic nerve, which is responsible for the vision of the affected eye. The non-arteritic AION and PION are also known to resolve faster and better if treated with corticosteroids at high doses within 2 weeks of the start of symptoms. Both visual acuity and the visual field are preserved to a greater extent with this form of therapy. Besides, it is necessary to adequately treat risk factors such as a high blood pressure which are important in the pathogenesis of the non-arteritic form of AION. About 40% of patients with non-arteritic AION will recover spontaneously, however.
Surgical PION is still an enigma at present with respect to its treatment, and care should therefore be taken to prevent its occurrence.