How is weakness characterized in myasthenia gravis (MG)?

Updated: Aug 27, 2018
  • Author: Abbas Jowkar, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

Weakness can be present in a variety of different muscles and is usually proximal and not symmetrical. Sensory examination and deep tendon reflexes are normal.

Distribution of weakness in a large cohort of patients with myasthenia gravis (n=609) (Open Table in a new window)

Distribution of Weakness Percentage of Patients
Localized ocular 17%
Ocular and generalized 50%
Ocular and bulbar 13%
Ocular and limb 20%

Weakness of the facial muscles is almost always present. Bilateral facial muscle weakness produces a “sagging and expressionless” face, and a horizontal smile. At rest, the corners of the mouth droop downward, giving the patient a look of sadness. Attempts to smile result in contraction of the medial portion of the upper lip and horizontal contraction of the corners of the mouth with loss of the natural upward curling, giving the patient's smile an appearance of a snarl (“myasthenic snarl”). Patients are unable to whistle, suck through a straw, or blow up a balloon. This often impedes bedside respiratory assessment because the lips form a poor seal around the mouthpiece of the measuring device. The frontalis muscle may be chronically contracted, giving a worried or surprised look to the patient. A unilateral frontalis ‘‘hypercontraction’’ is a clue that the lid elevators are weak on that side. Also, to compensate for ptosis, the sclerae below the limbi may be exposed secondary to weak lower lids. Mild proptosis attributable to extraocular muscle weakness also may be present. Bell’s phenomenon, which is upward rotation of the eyeballs during attempted eyelid closure, is appreciated on examination because of weakness of the orbicularis oculi muscle resulting in incomplete closure of the eyelids. Weakness of eyelid closure is seen in most patients with MG and should be specifically tested by asking patients to forcefully close their eyes while the examiner attempts to manually open the eyelids.


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