What is Optic Neuritis (ON)?
Optic neuritis (ON) is an inflammation of the optic nerve. It is also known as retrobulbar neuritis. The retina, in the back of the eye, senses light and converts this into a nerve signal. The optic nerve carries the nerve signal from the retina to the brain where it is interpreted into a visual image. If inflammation affects the optic nerve the signal between the retina and the brain is affected.
What are the symptoms of ON?
The most common symptom is loss of vision. This can be in one eye or both. This can affect color vision, particularly red color, so that colors are not as vivid. It can also affect non-color vision so that the vision becomes grey or dim. If one eye is involved this can affect depth perception.
Sometimes nerve signals can be generated in the absence of a light stimulus. This causes flashes of light which are called phosphenes. However, phosphenes can occur in normal people, particularly in the dark, with pressure on the eye, or when trying to fall asleep.
Pain behind the eye can sometimes occur with ON. This may get worse with eye movements.
Occasionally, increases in body temperature can worsen the conduction of signals in the optic nerve. This can occur with increases in body temperature due to exercise, hot baths or showers, or fevers. This can lead to abrupt worsening or loss of vision. However, when body temperature returns to normal then vision also returns. This is named Uhthoff’s phenomenon.
What diseases cause ON?
The most common cause of ON is MS. However, many other diseases must be considered. These include neuromyelitis optica, compression by masses in the orbit, infections (Lyme disease, syphilis, zoster, HIV), blockage of the blood supply to the eye (ischemic optic neuropathy), vitamin deficiencies (B12),tobacco/alcohol amblyopia, other inflammatory diseases (lupus, sarcoid), toxins (methanol, some medications) and inherited diseases of mitochondria (Leber’s hereditary optic neuropathy).
How often does ON lead to MS?
ON may be the first attack of MS. Overall, 60-70% of people with ON go on to have MS, if there is no other explanation for the ON. The risk of having MS is best determined by the MRI. If the MRI is normal, then the risk of MS is 25% after 15 years. If the MRI is abnormal, the risk of MS is 78% after 15 years. This is discussed in more detail in the section on clinically isolated syndrome (CIS).
What diagnostic tests are needed for ON?
If ON occurs in a patient for the first time, evaluation by an ophthalmologist or neuro- ophthalmologist is recommended. Optometrists usually prefer that these patients be evaluated by an ophthalmologist. Our MS eye clinic is particularly skilled at evaluating these patients.
Evaluations include a careful eye examination. In addition, testing of the visual field may be performed. This involves testing to see whether there are blind spots in the vision or if the area of vision is constricted. In addition, ocular coherence tomography (OCT) is often done to measure the condition of the retina.
Blood tests for other causes of optic nerve disease may be needed. These are determined by the findings on the ophthalmologic examination.
If no other explanation for the ON is found, an MRI of the brain is usually done to help determine the risk of MS.
How is ON treated?
Acute attacks of ON are treated like MS attacks. This generally involves corticosteroids if the symptoms are severe enough to warrant it. If symptoms are severe and if they fail to respond to corticosteroids, then plasma exchange may be used. These treatments are discussed further in the section on treating acute attacks of MS.
If patients have an abnormal MRI, and thus have a high risk of having MS, treatment with medications to decrease the frequency of MS attacks may be warranted. These are discussed further in the section on treating MS.