MYASTHENIA GRAVIS

SIGNS AND SYMPTOMS
Patients with myasthenia gravis typically present with symptoms of variable ocular fatigue and weakness. The common ocular findings include ptosis, droopy eyelids that appear worse at the end of the day, orbicularis weakness, limitation of ocular motility, paradoxical lid retraction, Cogan's lid twitch (transient eyelid retraction following an extended period of down gaze), exposure keratitis and intermittent diplopia.

A small percentage of patients possess a form of the disease known as ocular myasthenia. Here, the signs and symptoms remain strictly confined to the extraocular muscles. The pupil is never involved in MG.

Systemic symptoms include intermittent fatigue of the limbs, weakness of the facial muscles and difficulty breathing, chewing, talking and swallowing. In a small percentage of patients, dysthyroidism may also be present, resulting in the mixture of ptosis and exophthalmos. Thymic neoplasia (thymoma) is an associated finding in patients over 65. Associated disorders such as diabetes mellitus, lupus erythematosus and rheumatoid arthritis occur in 20 percent of MG patients.

PATHOPHYSIOLOGY
Myasthenia gravis (MG) is an autoimmune disease that destroys key components of the neuromuscular system responsible for governing muscular activity. It has a prevalence of approximately 1 in 20,000 persons. Females are more likely to experience early onset disease (under age 50) by a ratio of 7:3. Females reach their peak incidence by the third decade while males reach their peak incidence in the fifth decade.

Up to 75 percent of patients with myasthenia present with some type of ocular symptom. Ninety percent of patients with myasthenia gravis will develop ocular signs or symptoms, and 80 percent of patients who present with ocular involvement progress to have involvement of additional muscle groups within two years of the initial presentation.

Signs and symptoms may be initiated, exacerbated or mimicked by medications such as D-penicillamine, antibiotics (polymyxn B, neomycin, gentamicin, streptomycin), beta blockers and anticonvulsants.

MANAGEMENT
Patients who report neurological signs or symptoms require a through review of family history, illnesses, systems, medications and behaviors. Inspect and measure the palpebral apertures of patients with positional lid anomalies.

In office tests for the optometrist include:

1. asking the pertinent history

2. testing the pupils

3. assessing the orbicularis function by asking the patient to squeeze their eyelids shut while you attempt to open them forcibly

4. attempt to elicit diplopia by eliminating the occlusive effects of ptosis

5. attempt to elicit superior rectus or levator fatigue by asking the patient to sustain upgaze while you observe for unexpected eyelid droop

6. attempt to elicit the Cogan's lid twitch sign by asking the patient to look into downgaze for an extended period, then to gaze up

7. apply an ice pack to the eyelid for five to 15 minutes, reevaluating the ptosis and/or ocular motility for improved position following the ice packs removal

8. ask the patient to close their eyes for 30 minutes (sleep test), reevaluating the ocular motility and/or ptosis for improved position upon awakening. Diagnosis may also be assisted by evaluating old photographs for appearance.

The quintessential method of diagnosing MG is the endrophonium hydrochloride (Tensilon) injection test. An additional laboratory test used for diagnosing MG is the acetylcholine antibody receptor test. Medical management of MG should be handled by the neurologist or neuro-ophthalmologist.

CLINICAL PEARLS

MG should always be a consideration in cases of non-restrictive, pupil sparing CN III, IV and VI nerve palsy as well as unilateral and bilateral internuclear ophthalmoplegia.

Laboratory testing is an important consideration for patients diagnosed with MG because of its association with other systemic autoimmune diseases. Pertinent studies include fasting blood sugar (FBS: diabetes), thyroid function tests (ASH, T3, T4), antinuclear antibody (ANA: lupus), rheumatoid factor (RF: arthritis) and in suspicious cases, radiological testing of the thymus gland. A purified protein derivative (PPD: tuberculosis) should be completed because steroid regimens, used to treat MG have the potential to activate or worsen dormant disease.

Patients should always be educated to report difficulties with breathing or swallowing.