What is Non-Arteritic Anterior Ischemic Optic Neuropathy (NAION)?
There are two locations in the optic nerve for ischemic optic neuropathy anterior and posterior. The anterior type involves the optic nerve head and the posterior comprises the optic nerve beginning 1mm posterior to the optic nerve head. There are two kinds of anterior ischemic optic neuropathy non-arteritic and arteritic which is nearly always secondary to temporal arteritis. Non-arteritic ischemic optic neuropathy is the loss of blood supply to the optic nerve head not caused by temporal arteritis resulting in swelling of the optic nerve head. This ischemia leads to loss of optic nerve head tissue and loss of vision.
What Causes NAION?
The vast majority of cases are diagnosed as idiopathic, which means a precise etiology was not found. There are some cases associated with certain diseases or medications.
Erectile dysfunction medications have been reported to have caused NAION. The blood flow alteration associated with these medications may cause a decrease in the blood flow in the optic nerve head.
Optic Nerve Head Drusen
Optic nerve head drusen has been associated with ANION and is felt to be due to the crowding in the optic nerve head. This is believed to alter the blood flow in the optic nerve head.
Sleep Apnea Syndrome
There have been some cases of sleep apnea being associated with NAION but further study needs to be done.
Most cases occur in people over 50 years of age. Men and women are equally affected.
ANION is thought to be the result of altered or loss of blood flow in the optic nerve head. There several related theories in regards to its cause.
Optic Nerve Head Anatomy
The vast majority of people who develop ANION have anatomically small optic nerves. It is postulated that these small optic nerves may be prone to vascular flow compression or loss.
There are many different theories about how the ischemia occurs in the optic nerve head. Some relate to venous drainage problems. Others to changes in blood flow while asleep to poor autoregulation of blood flow associated with vascular diseases and blood pressure medications.
Symptoms of Non-Arteritic Anterior Ischemic Optic Neuropathy
The normal presentation of patients with ANION is a sudden onset of unilateral loss of vision which is painless in almost all cases and usually involves the lower half of the vision.
Diagnosing Non-Arteritic Anterior Ischemic Optic Neuropathy
The visual acuity can vary greatly with 20/20 to 20/400. People with no light perception or total loss of vision are extremely rare and other etiologies should be considered.
Pupils will almost always be equal and round. There will usually be an afferent pupil defect.
The majority of patients will have an altitudinal field defect with the inferior field being the most common.
Optic Nerve Head and Retinal Appearance
In the acute phase of ANION, the optic nerve head will appear swollen. All or portions of the optic nerve head can be swollen. Peripapillary splinter hemorrhages are seen in the majority of patients. The retinal arteries can be narrow in areas around the optic nerve head.
Treatment of Non-Arteritic Anterior Ischemic Optic Neuropathy
There is no effective treatment for NAION. It is important to differentiate arteritic and non-arteritic sense treatment needs to be started immediately if it is arteritic.
The vision can worsen in the first 2 or 3 weeks but then stabilizes. The vision can improve in the first 6 months and then remains relatively stable. Younger patients usually do better with less loss of vision.