Particular ocular signs are highly characteristic of myasthenia. For example, sustained upgaze (for 30 or more seconds) will usually induce or exaggerate ptosis and may uncover myasthenic ocular motor weakness, thereby marking it as “fatigable.” Another characteristic sign, not as frequently elicitable as fatigable ptosis, was described by Cogan and is a twitching of the upper eyelid that appears a moment after the patient moves the eyes from a downward to the primary position (“lid-twitch” sign). Or, after sustained upward gaze, one or more twitches may be observed upon closure of the eyelids or during horizontal movements of the eyes. Repeated ocular versions when tracking a target or by an optokinetic stimulus may disclose progressive paresis of the muscles that carry out these movements. Unilateral painless ptosis without either ophthalmoplegia or pupillary abnormality in an adult will most often prove to be due to myasthenia. Usually, there is subtle ptosis of the other eye that can be revealed by manually elevating the more affected eyelid. Attempts by the patient to overcome ptosis may impart a staring expression of the opposite eye. Bright sunlight is said to aggravate the ocular signs and cold to improve them. The application of an ice pack over the eye often relieves the ptosis for a brief period.
Muscles of facial expression, mastication, swallowing, and speech are affected in 80 percent of patients at some time in the illness, and in 5 to 10 percent, these are the first or only muscles to be involved. Less frequent is early involvement of the flexors and extensors of the neck, muscles of the shoulder girdle, and flexors of the hips. (This pattern may be associated with a special autoantibody as will be discussed later.) Of the trunk muscles, the erector spinae are the most frequently affected. In the most advanced cases, all muscles are weakened, including the diaphragmatic, abdominal, and intercostal, and even the external (skeletal muscle) sphincters of the bladder and bowel. As the disease progresses, the involvement of any group of muscles closely parallels their degree of weakness early in the disease. The clinical rule also holds that the proximal muscles are far more vulnerable than distal ones, as they are in most other forms of myopathy.
Another characteristic and understandable feature of myasthenic weakness is its tendency to increase as the day wears on or with repeated use of an affected muscle group but curiously, patients seldom volunteer this information. A few patients report paradoxical worsening on awakening, especially if they have not taken medication during the night but a larger proportion of patients are close to asymptomatic after a night’s rest. In general terms, therefore, myasthenia gravis may be conceived as a fluctuating and fatigable oculofaciobulbar palsy.
Other features result from the topography and fatigability of the disease. The natural smile becomes transformed into a snarl; the jaw may sag, so that it must be propped up by the patient’s hand; chewing tough food may be difficult and the patient may have to terminate a meal because of inability to masticate and swallow. It may be more difficult to eat after talking, and the voice fades and becomes nasal after sustained conversation. Women may complain of inability to fix their hair or makeup because of fatigue of the shoulders, or of difficulty in applying lipstick because they are unable to purse and roll their lips. Weakness of the neck muscles causes fatigue in holding up the head. In cases with generalized weakness, there is difficulty in retaining flatus because of weakness of the external rectal sphincter.