Ocular myasthenia gravis

Ocular myasthenia gravis

Ocular myasthenia gravis (MG) is a disease of the neuromuscular junction resulting in hallmark variability in muscle weakness and fatigability. MG is an autoimmune disease where anomalous antibodies are produced against the naturally occurring acetylcholine receptors in voluntary muscles. MG is an autoimmune disease where anomalous antibodies are produced against the naturally occurring acetylcholine receptors in voluntary muscles. MG may be limited to the muscles of the eye (ocular MG), leading to abrupt onset of weakness/fatigability of the eyelids or eye movement. MG may also involve other muscle groups.

The most common symptoms seen in patients with ocular MG are diplopia (double vision), ptosis (droopy eyelids), and incomplete eye closure. Compared to other involved skeletal muscles, only slight weakness of the extra ocular muscle may cause diplopia and visual disturbances to occur. These symptoms occur due to weakness of the muscle that control eyeball and eyelid movement. Light sensitivity due to sluggish pupils may occur in some patients. Symptoms are frequently influenced by environmental, emotional, and physical factors. Some of these factors include bright sunlight, extreme temperature, emotional stress, illness, surgery, menstruation, and pregnancy, among others. Symptoms tend to be worse at the end of the day.

Symptoms

 Diplopia or double vision results when the eyes cannot be focused as desired due to weakness of one or more of the extraocular muscles which control eye movement. This most often occurs when looking up or to the side. To compensate for the weakness, the patient may tilt his/her head or turn their face to allow the stronger eye to work. For example, if the muscle which allows the eye to look upward is weak, the patient could tilt their head back to look up.

 Ptosis is the drooping of one or both eyelids, also due to muscle weakness. Fluttering or twitching of the eyelid may occasionally be seen. If both eyelids are droopy, one may be worse than the other.

Nystagmus  or constant involuntary repeated movement of the eyeball in any direction may also occur in one or both eyes.

Treatment

Treatment of ocular MG is aimed at relieving the symptoms of ptosis and diplopia, as well as preventing the development of generalized MG symptoms. Immune suppression with steroids is often the main therapy. Steroid doses must be increased slowly because of a risk of precipitating myasthenic crisis. After achieving the highest target dose, steroids are then slowly tapered down to the lowest effective dose. Often, acetylcholinesterase inhibitors such as pyridostigmine and neostigmine are also employed to help control symptoms. When steroids are contraindicated, acetylcholinesterase inhibitors can be tried as the primary therapy. Steroid-sparing agents such as azathioprine and mycophenolate may also have a role in treating ocular MG. Other treatments for MG include plasmapheresis, intravenous immunoglobulin, and other immunosuppressive agents, but these are rarely required for ocular MG. Patients should also be evaluated for thymoma. Thymoma should be resected surgically. Ocular MG without thymoma is not usually treated with thymectomy. Topical agents may be useful as additional therapy for mild or moderate ptosis. Nonpharmacologic treatments include occlusive devices, prisms, eyelid supports, contact lenses, and (in long-standing, stable cases) strabismus surgery or eyelid elevation surgery.