Introduction:
The term ischemic optic neuropathy was first used by Miller and Smith in the year 1966 and later on the term anterior was added to it by Hayreh. Ischemic optic neuropathy is usually of two types: anterior ischemic optic neuropathy (AION) and posterior ischemic optic neuropathy (PION). Anterior ischemic optic neuropathy is considered to be one of the most commonly occurring causes of vision loss in older people. The vision loss is either sudden or develops within a few days. The vision loss is usually limited to one eye, although the second eye may be involved later in a few patients.
What Is Anterior Ischemic Optic Neuropathy?
Anterior ischemic optic neuropathy (AION) is the sudden loss of vision caused due to the disrupted blood flow to the anterior (front) part of the optic nerve which is also known as the optic nerve head. Anterior ischemic optic neuropathy is of two types namely arteritic anterior ischemic optic neuropathy (A-AION) and nonarteritic anterior ischemic optic neuropathy (NA-AION). Arteritic AION is caused due to the inflammation (swelling) of arteries (blood vessels that supply oxygen and nutrient-rich blood to the body organs) that provide blood supply to the optic nerve. Nonarteritic AION occurs due to decreased blood flow to the optic nerve and is not caused by the inflammation of arteries supplying the optic nerve.
What Is an Optic Nerve?
The optic nerve is situated behind the eye and carries light signals from the eyes to the brain which is converted into visual images. The optic nerve comprises more than one million nerve fibers that help in carrying visual messages. These nerve fibers depend on the nutrients and oxygen supplied by the neighboring blood vessels. Disturbances in the blood supply can cause damage to the optic nerve which in turn leads to loss of vision. The severity of vision loss depends on the extent of optic nerve damage.
What Are the Causes of Anterior Ischemic Optic Neuropathy?
The definitive cause of anterior ischemic optic neuropathy is the deficient blood flow to the optic nerve. The factors which cause the deficient blood supply to the optic nerve leading to both arteritic and nonarteritic anterior ischemic optic neuropathy are as follows:
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Arteritic Anterior Ischemic Optic Neuropathy (A-AION): A-AION is caused due to inflammation of the arteries that provide blood supply to the optic nerve. This inflammation occurs due to a condition called temporal arteritis or giant cell arteritis (GCA) which causes inflammation of the lining of arteries and affects the arteries of the head and neck. Severe cases of giant cell arteritis can lead to permanent vision loss.
- Nonarteritic Anterior Ischemic Optic Neuropathy (NA-AION): It is caused by the following factors:
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High pressure inside the eyeball.
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Narrowing of the arteries that supply the optic nerve.
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Decreased blood flow to the region of the optic nerve where it leaves the eye.
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Increased thickness of the blood that is circulated by the blood vessels around the eye.
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A decrease in the pressure of the circulating blood resulting in a reduced blood supply to the optic nerve.
What Are the Risk Factors of Nonarteritic Anterior Ischemic Optic Neuropathy?
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Increased cholesterol levels.
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High blood pressure or a sudden drop in blood pressure.
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Anemia.
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Diabetes mellitus.
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Smoking.
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Heart diseases.
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Sleep apnea (sleeping disorder in which breathing stops and restarts multiple times during sleep).
What Are the Symptoms of Anterior Ischemic Optic Neuropathy?
The symptoms observed before vision loss are as follows:
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Pain while chewing food.
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Tingling sensation or pain in the scalp.
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Tiredness.
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Pain in the area located at the side of the forehead behind the eyes.
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Loss of appetite (decreased desire to eat).
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Sudden weight loss without any underlying reason.
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Neck pain.
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Muscle pain in the arms and upper legs.
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Loss of color vision.
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Sudden painless blurring or cloudiness of vision in one eye is noticed after waking up from sleep (mainly seen in NA-AION).
How Is Anterior Ischemic Optic Neuropathy Diagnosed?
The following tests are conducted to diagnose ischemic optic neuropathy:
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Medical History and Physical Examination: The doctor takes a detailed history of the patient and asks about the symptoms experienced by the patient. Physical examination of the patient with A-AION may show double vision or repeated and uncontrolled eye movements.
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Blood Tests: A sample of blood is taken to assess the erythrocyte sedimentation rate (ESR), C-reactive protein levels (CRP), and platelet count. Most of the patients with A-AION show an elevated ESR while patients with NA-AION show a normal ESR. Patients with giant cell arteritis mostly show an elevated CRP level. Platelet count along with ESR and CRP levels are elevated in patients with arteritic anterior ischemic optic neuropathy. Blood tests may also be performed to detect conditions like anemia, high cholesterol level, or diabetes.
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Magnetic Resonance Imaging (MRI): MRI has been useful for detecting abnormalities related to the optic nerve. MRI also is useful in differentiating between the two types of anterior ischemic optic neuropathy. In patients having temporal arteritis-related anterior ischemic optic neuropathy, MRI shows a central bright spot which may be the indicator of enlargement of the front part of the optic nerve. In patients with NA-AION having severe pain, an MRI of the brain and orbits (space in the skull which contains the eye and its related nerves, blood vessels, and muscles) with a contrast agent gadolinium is useful.
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Optical Coherence Tomography (OCT): OCT is an imaging test that has been carried out in patients with A-AION and NA-AION. Optical coherence tomography is used to diagnose and assess the severity of NA-AION based on blood flow to the front part of the optic nerve. In patients with A-AION, the test is used to assess the retinal nerve fiber thickening and swelling of the front part of the optic nerve.
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Fluorescein Angiography: This test involves the injection of dye into the arteries of the head and obtaining pictures of blood vessels. The pictures help in finding out the location where the blood flow has been interrupted.
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Biopsy: In cases of A-AION associated with giant cell arteritis, a biopsy of the arteries of the temple (area located at the side of the forehead behind the eyes) helps to detect arterial inflammation.
How Is Anterior Ischemic Optic Neuropathy Treated?
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Arteritic Anterior Ischemic Optic Neuropathy Treatment: Corticosteroid therapy is initiated once the condition has been diagnosed. High doses of steroids are given in the first two to three weeks which is then reduced to low doses. In severe cases, intravenous administration of high-dose steroids like Methylprednisolone may be advised for the first three days. However, most of the patients are asked to continue the low-dose steroids for a lifetime to prevent blindness. Methotrexate, which is a chemotherapy agent has been used as an alternative to steroids. Even though steroids are effective, long-term use shows adverse effects like diabetes or cataract. Hence Methotrexate is very beneficial.
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Nonarteritic Anterior Ischemic Optic Neuropathy Treatment: Medical and surgical therapies have not shown much improvement in treating nonarteritic anterior ischemic optic neuropathy. Hence treating the underlying disease or risk factors that may aggravate NA-AION may help to prevent further vision loss and manage the condition.
Conclusion:
Anterior ischemic optic neuropathy is a serious condition that might lead to vision loss. However early treatment may cause a reduction in the symptoms. Hence early diagnosis followed by an efficient treatment plan would help the patient overcome the severity of symptoms and lead a better life.
