Ophthalmologic Manifestations of Myasthenia Gravis 

Ninety percent of patients with myasthenia gravis (MG) develop ophthalmologic manifestations of the disease, a disorder of neuromuscular transmission characterized by weakness and fatigability of skeletal muscles. The basic pathology of MG is a reduced number of acetylcholine receptors (AChR) at the postsynaptic muscle membrane brought about by an acquired autoimmune reaction producing anti-AChR antibodies.

Two major clinical forms of MG are distinguished: ocular MG, in which the patient has strictly ocular symptoms, and generalized MG, in which the patient develops generalized weakness. One study found that, over a mean follow-up period of 17 years, approximately 15-17% of patients with MG had strictly ocular symptoms.

Some evidence shows that steroids or azathioprine might prevent the conversion to generalized MG in 75% of patients with ocular MG.^[1]

Bever and coworkers reported that 82% of patients who later developed generalized weakness did so in the first 2 years after diagnosis.^[2] Hence, patients who keep having strictly ocular symptoms for 3 or more years are unlikely to revert to the generalized aspect of the disease.

The juvenile form of the disease usually has a much more favorable prognosis than the adult form, with a better chance of spontaneous remission.

Ophthalmic symptoms

Among patients with myasthenia gravis (MG), 75% initially complain of ocular disturbance, mainly ptosis and diplopia. Eventually, 90% of patients with MG develop ocular symptoms. About 50% of patients will present solely with ocular symptoms, and about 50-60% of these patients will progress to develop generalized disease. Ptosis may be unilateral or bilateral, and it may shift from eye to eye.

Nonophthalmic symptoms

Oropharyngeal muscle disturbances come second in presentation, with 15% of patients first experiencing difficulty in swallowing, talking, and chewing. Limb and trunk weakness is the initial complaint of 10% of patients, and 85% of patients with MG develop a generalized weakness also affecting the limb muscles.

MG can involve the respiratory muscles and lead to respiratory failure. This can sometimes be the first presentation of the disease. Qureshi et al showed that out of 51 patients with MG and respiratory failure, 7 (14%) had no previous diagnosis of MG.^[3]

Ptosis

As previously stated, ptosis can be unilateral or bilateral. The ptosis is usually most prominent on sustained upward gaze, as shown in the image below, or on repeated eyelid closure. A subtle lid flutter is occasionally encountered.

In cases of unilateral ptosis, the contralateral lid may assume a ptotic position upon occluding the eye with the ptosis or lifting the ptotic lid with a finger (Herring phenomenon).

The lid twitch sign described by Cogan can be elicited by having the patient change gaze from the downward position to the primary position.^[4] The lid will be seen to overshoot in a twitch before gaining its initial ptotic position; this is shown in the image below. A study, however, questioned the validity of such a sign, as its sensitivity and specificity were shown to be relatively low.^[5]

Extraocular muscle involvement

Extraocular muscle involvement does not follow a certain pattern. However, the superior rectus and/or the medial rectus are commonly involved. Isolated inferior rectus palsy is rare, but when it is present, it raises the suspicion of myasthenia gravis (MG), since it is extremely rare to be encountered otherwise.

The spectrum of extraocular muscle conditions stretches from isolated muscle palsies to total external ophthalmoplegia. Any acquired ocular motility disturbance, with or without ptosis but with normally reacting pupils, should raise the clinical suspicion of MG.

Orbicularis involvement

Orbicularis involvement consistently is present when ocular symptoms are reported. Weakness in forceful closure of the eyes against resistance is present.

Pupils and ciliary muscles

Traditionally, pupils and ciliary muscles were believed not to be involved, although studies now suggest the contrary, notably in a case analysis by Cooper and coworkers.^[6]

A wide armamentarium of medical therapy exists for patients with myasthenia gravis (MG). No general consensus exists about which to start first or which to combine. Medical treatment is administered best by a neurologist. The role of the ophthalmologist is to follow up on the ophthalmic symptoms, notably diplopia and ptosis. The neurologist orchestrates the pharmacologic treatment.

Ptosis, when it becomes bothersome, is treated best by lid crutches, since surgery is contraindicated because of the fluctuation of the disease, producing lagophthalmus with all of its complications. Diplopia is eliminated by Fresnel prisms unless the deviation is large and variable, in which case an opaque lens in front of the involved eye is preferred.

Strabismus surgery

Successful muscle surgery for selected patients with a stable course of MG and persistent diplopia has been reported.^{[7, 8]}

Blepharoptosis surgery

Ptosis surgery for patients with a stable ptosis that has failed to respond to medical therapy for MG has gained popularity. The surgical technique can include external levator advancement, frontalis suspension sling, or tarsomyectomy.^{[9, 10]}

Consultations and follow-up

Consult neurologists after diagnosing patients with ocular MG. Treatment is initiated best by them rather than by the ophthalmologist. Follow-up care of the patient with MG is conducted mainly by the neurologist, who will orchestrate the treatment. The ophthalmologist is consulted accordingly.