Myasthenia gravis is a chronic neuromuscular disorder that results in muscle weakness. Patients often first notice weakness in the muscles that control the movement of their eyes and eyelids. Many later develop muscular weakness elsewhere.
For about 15 percent of patients with myasthenia gravis, the muscles controlling eye and eyelid movement remain the only muscles affected. In these cases, the condition is known as ocular myasthenia gravis.
People with symptoms of ocular myasthenia gravis for five years that have not affected other muscles in their body are unlikely to develop weakness elsewhere.
What causes ocular myasthenia gravis?
Nerve cells send signals to muscle cells by releasing a chemical called acetylcholine. The chemical binds to acetylcholine receptors on muscle cells. In myasthenia gravis, which is an autoimmune disorder, the body’s immune system mistakenly attacks these acetylcholine receptors, disrupting the signals from nerve cells to muscle cells that tell a muscle to contract.
There are several possible explanations for why myasthenia gravis affects eye muscles as it does. Among reasons suggested is that eye muscle weakness is more noticeable than mild muscle weakness in other parts of the body. Eye muscles are also structurally different from limb or truck muscles, and contract frequently so they might tire more quickly.
Another possibility is that the eye muscles are more sensitive to immune system attacks in myasthenia gravis.
What are the symptoms of ocular myasthenia gravis?
Common symptoms of ocular myasthenia gravis include double vision, also known as diplopia, because of an inability of the eyes to align properly. Patients with ocular myasthenia gravis may also have drooping in one or both eyelids, a condition called ptosis, or have trouble focusing their vision.
Muscle weakness in ocular myasthenia gravis often affects upward eye movement first. Weakness worsens with continual use of eye muscles, so that it is typically more severe at the end of the day.
How is ocular myasthenia gravis diagnosed?
Diagnosis usually begins with a physical examination, especially of the eyes and eye movement.
Several different tests may be used to diagnose ocular myasthenia gravis. One is known as the ice test, where an ice pack is placed over a patient’s closed eyelids for several minutes. If the ice pack relieves symptoms such as eyelid drooping or trouble with eye movement, it is an indication of ocular myasthenia gravis. Reducing muscle temperature is thought to reduce the activity of an enzyme that breaks down acetylcholine, thereby allowing better signaling between nerve cells and muscle.
A similar test, called the sleep test, looks at whether resting the eyes by keeping them closed for about 30 minutes relieves symptoms.
Another test involves administering small doses of a compound called edrophonium over a short period of time to observe reactions. Edrophonium increases levels of acetylcholine at the junction of nerve and muscle cells, temporarily alleviating symptoms of ocular myasthenia gravis.
The patient may also be tested for blood levels of antibodies against the acetylcholine receptor, and additional bloodwork may be requested.
Finally, nerve stimulation studies may also be conducted to see if muscles respond normally to electrical stimulation.
How is ocular myasthenia gravis treated?
Double vision may be treated through simple measures, such as applying a patch to one eye or wearing a single opaque contact lens. The treatment should be alternated between eyes to reduce eye strain.
Specialized eyelid tape can be used to improve vision blocked by drooping eyelids, although it is recommended that one eye be taped at a time and alternate the taped eyelid. Glasses can also be equipped with thin wires called eyelid crutches to help people with ptosis.
Myasthenia Gravis News is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.