The optic nerve is mainly made up of the axons of the retinal ganglion cells from the retina. Nerve fibers travel from the optic nerve head through a sieve-like structure called the lamina cribrosa into the extraocular space. Just posterior to sclera optic nerve gets myelin sheath covering. From there, it traverses through the apex of orbit into intracranial space where right and left optic nerve crosses each other forming chiasm and finally ending its journey at the visual cortex in the occipital lobe. The optic nerve may be damaged by ischemia anywhere along its path, starting from its origin. It is mainly divided into anterior and posterior. A portion of the optic nerve between retina and sclera is supplied by a network of 6 to 12 short posterior ciliary arteries at the back of the globe. These vessels form a circumferential network called the circle of Zinn-Haller. The perfusion of the optic nerve head is in a centripetal and segmental fashion, which explains the characteristic pattern of visual field loss. Branches from pial plexus supply the retrolaminar portion of the optic nerve.
Anterior ischemic optic neuropathy [also known as AION] is far more common than posterior ischemic optic neuropathy [also known as PION]
Anterior ischemic is further divided into arteritic and non-arteritic.
Arteritic anterior ischemic optic neuropathy [AAION]: It occurs in patients age more than 70 years. It is caused by inflammatory and thrombotic occlusion of short posterior ciliary arteries. It is associated with systemic vasculitis, Giant cell arteritis (GCA) being the most common cause.
Nonarteritic ischemic optic neuropathy [NAION]: The cause of nonarteritic anterior ischemic optic neuropathy is uncertain. The variance in the anatomical structure i.e., structural crowding of the optic disc, is a major causative factor so far discussed. This type of disc is commonly mentioned as "disc at risk." It is believed that the blood supply of such nerves is tenuous, and there is impaired autoregulation in the presence of any stress factor leading to capillary filling pressure falling below a critical level leading to damage. The associated risk factors include the following:
Posterior ischemic optic neuropathy [PION]: It is far much less common than anterior ischemic optic neuropathy. It mainly occurs under three circumstances:
Patients present with sudden painless loss of vision and physical examination vary depending on the part of the nerve involved and its mechanism.
It usually occurs in old patients with the mean age being 70 years, and the majority of cases are linked to vasculitis, Giant cell arteritis (GCA) being the most common. Vision loss is severe being less than 20/200 in 70% of cases, and it goes to no perception of light in 20% of patients.
It is very important to inquire about symptoms of GCA as, without treatment, about half of patients will have vision loss in their fellow eye, sometimes within days, and there are other life-threatening complications related to this disease, including stroke, aortic dissection, and myocardial infarction which could be prevented.
Systemic symptoms of GCA include temporal headache, scalp tenderness, jaw claudication due to ischemia of masseter muscle. Other less specific findings include weight loss, fever, night sweats, malaise, and depression. The majority of patients with GCA have polymyalgia rheumatica at diagnosis. It is characterized by pain and stiffness in proximal muscle groups like shoulders and thigh muscles.
American College of Rheumatology Criteria for Giant Cell Arteritis (1990)
At least three of the following:
On examination, pupil reaction is sluggish, and there is a relative afferent pupillary deficit except when both eyes are involved simultaneously. The fundus examination shows chalky white optic disc edema, cotton wool spots (a sign of ischemia of nerve fiber layer) away from optic edema.
Vision loss can be acute, subacute, or stepwise and is not associated with pain usually. Vision loss is less severe than AAION, and about half of patients see 20/60 or better. Patients usually report vision loss after awakening in the morning. Vision loss may remain static, progressive over weeks to months before the final stabilization of vision.
Fundus examination shows:
Gradually optic edema settles, crowding of nerve fibers decreases, and atrophy sets in with enlargement of the optic cup. The involvement of the second eye over some time is common, and it gives the clinical appearance of pseudo-Foster Kennedy syndrome in which previously involved eye is atrophic in contrast to hyperemia and swelling in the acute eye. In true Foster Kennedy syndrome, there is optic disc atrophy in one eye due to compression of the optic nerve by intracranial mass and optic disc edema in the contralateral eye due to raised intracranial pressure.
Vision loss can be unilateral or bilateral with an absence of optic disc edema in acute stages; however, optic atrophy settles after a few months. It is a rare disease and mainly considered a diagnosis of exclusion.
In patients with AAION, GCA should always be ruled out. Elevated erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), are sensitive but not specific to GCA. Temporal artery biopsy is definitive to rule out GCA. A minimum of 2 cm length of the artery is needed as a specimen. Biopsy results show evidence of granulomatous inflammation with the involvement of internal elastic lamina. Biopsy results can give false-negative results; therefore, noninvasive techniques like MRI and Doppler ultrasound are used.
Vascular risk factors, including blood pressure determination, fasting lipid profile, and assessment of blood glucose and hemoglobin A1c should be excluded as many a time, patients are not aware of it. Screening questions for sleep apnea should be asked. The patient's medications, including PDE 5 inhibitors, should be reviewed to rule out nocturnal hypotension. In patients age less than 50, additional testing includes hyperhomocysteinemia in nonarteritic anterior ischemic optic neuropathy and is easily treated with vitamin B12.
Fluorescein angiography: Normally, the choroid fills up 3 to 5 seconds before retinal arteries. In AAION, there is delayed choroidal filling.
Visual fields are usually abnormal on confrontation tests (e.g., finger counting) and always abnormal with automated perimetry. Abnormal fields usually take the form of an altitudinal defect, which is more common inferiorly.
AAION: In cases of high suspicion treatment with high dose corticosteroids should be started immediately after drawing samples for CBC, ESR, CRP without waiting for results of investigations. Immediate treatment prevents vision loss in another eye as there are high chances of involvement of the second eye within days to weeks. Temporal artery biopsy can be delayed for 7 to 10 days if immediate facilities are not available. Pathological changes in the temporal artery will not be affected as they take weeks to months to change. High dose intravenous methylprednisolone (1 g/day) for 3 to 5 days is preferred. Some studies suggest different duration for intravenous steroid use, a three-day course of high-dose IV steroids, suggesting benefit on visual recovery in the same eye and prevention of disease in the second eye is favored. The consensus approach after intravenous therapy is to start moderate to a high dose of oral prednisone (1 mg/kg/day). Steroids are slowly tapered over 3 to 12 months, depending on the response. ESR and CRP are followed closely throughout the course.
There is no proven therapy for NAION. Treatments that are thought to affect, to some extent, include corticosteroids, optic nerve sheath fenestration surgery, levodopa. Prevention of fellow eye is the main concern, and aspirin has shown good results. Adequate control of diabetes mellitus, hypertension, hyperlipidemia, and smoking, obstructive sleep apnea has proven benefit in secondary prevention of nonarteritic anterior ischemic optic neuropathy and PION.
Optic ischemia, especially NAION, can mimic optic neuritis as there is sudden vision loss, RAPD, and optic disc swelling with associated hyperemia. Points that help in differentiating two diseases are that in optic neuritis:
Contrast enhancement MRI may help in differentiating the two. The affected optic nerve appears normal in NAION, but there is an enhancement of optic nerve in optic neuritis with the use of gadolinium.
Optic nerve compression can mimic anterior ischemic optic neuropathy. Compression can occur from tumors of the optic nerve, orbital inflammatory disease, ophthalmic artery aneurysm. Contrast imaging of the full length of the optic nerve is appropriate to differentiate other causes from optic nerve ischemia.
Sometimes fundus picture of optic disc drusen can mimic optic ischemia. The optic disc in ODD appears elevated and small. Disc blurring occurs due to stasis of axoplasmic flow in axons. Most of the patients don't experience any visual symptoms. Transient visual obscurations are experienced by a few patients.
Prognosis is very poor, and visual loss is usually permanent. A very few patients show partial recovery after the immediate administration of systemic steroids.
Optic atrophy is the most serious and inevitable complication without urgent treatment. It develops over a few months after ischemia of the optic nerve.
The patient should be educated about immediate medical consultation without any delay in cases of sudden, severe vision loss.
The patient should maintain routine health checkups to control diseases like hypertension, diabetes, obstructive sleep apnea as they play a critical role in the prevention of optic ischemia in the second eye.
Since the treatment of AAION involves long term use of steroids, physician referral should be sought to provide comprehensive care and prevent long term complications of systemic steroid therapy.
Optic ischemia is a relatively common presentation. Patients often present with vision loss to primary care physicians. Thus, these clinicians must know about the gravity of disease and the need for immediate treatment. Prompt referral should be made to an ophthalmologist. Condition is best managed by an interprofessional team, including a neurologist, ophthalmologist, radiologist, pathologist, and rheumatologist. Close follow up with the interprofessional team is critical in the treatment of affected eye and prevention of disease in the fellow eye.
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