Ocular Myasthenia Gravis

Ocular myasthenia gravis, a type of myasthenia gravis( MG) is an autoimmune disease, where some antibodies are produced that disrupt the communication between the muscle and nerves at the neuromuscular junction which in turn causes weakness and muscle fatigue. When this unusual condition affects the eye muscles, it is known as ocular myasthenia gravis (OCG).

However, ocular myasthenia gravis in many instances can be a common symptom of the generalized myasthenia gravis. The term myasthenia gravis is said to have Latin and Greek origin, which means grave muscle weakness. The disease usually causes muscle fatigue, however, sometimes may also lead to paralysis.

Causes
Normally muscle contraction in the body occurs when the central nervous system generates and transmits electrical signals to the muscle fibers by means of nerve impulses, which are converted into chemical message at the neuromuscular junction (place where nerves connect with the muscles). At neuromuscular junction, the nerve endings release a chemical acetylcholine (ACh), a neurotransmitter, which binds to the corresponding ACh receptors present in the muscle fibers. Now, in case of myasthenia gravis, the immune system automatically produces some antibodies against ACh receptors, that prevent the ACh from binding to the receptors. This in turn disrupts the communication between the nervous system and the voluntary muscles.

Symptoms
Patients of ocular myasthenia gravis usually suffer from double vision, also known as diplopia, and drooping of eyelid or ptosis. Some other symptoms include sensitivity to light and weakness in closing the eyelids. Generally when myasthenia gravis (MG) affects a single extraocular muscle of the eye, it causes double vision whenever the eye is moved towards that particular muscle. On the other hand, ptosis occurs when the levator palpebrae superioris, the eye muscle that is responsible for elevating the eyelids is affected. However, the disease generally does not affect the pupils. Some conditions like exposure to bright light, emotional stress, viral illness, pregnancy, surgery, etc are found to aggravate the symptoms.

Diagnosis and Treatment
The diagnosis of ocular myasthenia gravis usually begins with the study of patients' history and results of various examinations, mainly neurological examinations. Eyelids and eye muscles are also examined to detect the disease. Since it is an autoimmune disease, therefore blood test is also carried out to determine the presence of antibodies that block the Ach receptors. Antibodies have been found in almost 70% of the patients of ocular MG. Some other tests to detect the disease include repetitive nerve stimulation and single-fiber electromyography (EMC) of the frontalis muscle. Tensilon test is another important test for confirming the disease, in which edrophonium is intravenously administered into the body, that can temporarily prevent the breakdown of ACh. If the patient is suffering from MG, then there will be a marked reduction of the weakness experienced in the muscles of the eyes.

Patients of MG, in general are treated with anticholinesterase agents that generally include mestinon. Sometimes, immunosuppressive therapy is also carried out. For the patients suffering from double vision, eye patches frosted lens, occluding contact lens can be used, while for the ptosis patients ptosis tape are used. Besides, crutches can also be placed on eyeglasses of the patients of ptosis.

Thymus gland is supposed to be triggering factor of MG disease in general, as it plays a significant role in the development of the immune system in the early life. In the patients of MG, clusters of immune cells have been discovered, which are generally found in spleen and lymph nodes when the body's immune system responses. Sometimes thymomus or tumors are also found in the patients of MG. These tumors are generally benign, though sometimes can also be malignant and may require surgical removal. But, till now the exact relationship between the thymus gland and myasthenia gravis has not been established. Besides, this disease is also thought to be related with thyroid disease and other autoimmune diseases like scleroderma, rheumatoid arthritis, Hashimoto’s thyroiditis, multiple sclerosis, and thyroid ophthalmopathy.