Myasthenia gravis is a common autoimmune disorder caused by an abnormality at the neuromuscular junction. In this video, from our Clinical Neurology Essentials course, we'll cover the pathophysiology of the disease, as well as an array of clinical exams that you can use to diagnose this condition in your patients. You'll discover some tests that you can use to help you to differentiate between ocular and generalized myasthenia gravis and determine whether your patient has a neuromuscular junction problem or a structural problem (such as a stroke or nerve injury).
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[00:00:00] The neuromuscular junction is a specialized synapse allowing the conversion of nerve electrical action potentials to chemical impulses which transmit the message for a muscle to contract. There are multiple clinical syndromes with the most common being myasthenia gravis. In this autoimmune disorder which occurs in younger women or in older men, antibodies against the acetylcholine
[00:00:30] receptor are produced which block transmission of impulses as well as reduce the amount of acetylcholine that gets released. Let's look at the clinical characteristics of this class of diseases. All the disorders affecting the neuromuscular junction are characterized by fluctuating weakness and fatigability of muscle power. Weakness is often manifested in the cranial nerves such as an eye and bulbar muscles
[00:01:00] that can also be seen in gait and the limbs when repetitive muscle use occurs with occupational activities. Eye movements and ptosis, facial weakness, and speech, and chewing are often affected. Limb muscle weakness is almost always accompanied by facial, eye, or bulbar weakness. Strength loss is variable due to the fatigability of muscle. Often
[00:01:30] weakness is more pronounced as the day progresses. It often is amenable to rest with strength improving after a rest period. However, in general, as the day progresses, weakness also increases. Ocular symptoms including ptosis, diplopia, and blurring of vision are commonly seen neuromuscular junction disorders. A snarling facial appearance during laughter is seen due to weak face muscles.
[00:02:00] Speech can become slurred and annunciation altered particularly as the patient talks for an extended period of time. Dysphagia occurs as well. Neck muscles can weaken and the patient may have trouble holding the head up. Shortness of breath can occur with respiratory muscle involvement. So how should we assess the neuromuscular junction? Use of muscles repeatedly will often lead to weakness.
[00:02:30] Repetitive flexion of the biceps against resistance often will result in weakening inflection. Similarly, repetitive humoral abduction of the deltoid against resistance might show fatigue. There is a form of myasthenia gravis that is limited to the eyelids and extraocular muscles called ocular myasthenia. These generalized muscle weakness signs won't be present in an ocular myasthenic but can be useful
[00:03:00] to assess fatigability in the generalized disease form. You can test respiratory fatigue in a patient and follow their response to treatments by having them count at a stable cadence on a single breath and noting the number they consistently reach. This may then be rechecked after medications are used or a treatment regimen is instituted for the patient. Normal is to reach about 50. If you ask the patient to gaze
[00:03:30] upwards with the head straight and to hold their gaze upward, there may be weakening of the eyelid producing an increased ptosis. This can be seen in both ocular and generalized myasthenia. A bedside test one can employ to test for myasthenia produced fatigue and weakness makes use of an ice pack. This is a simple equivalent of using tensilon to reverse weakness. Application of the ice pack over the lid will improve
[00:04:00] the lid weakness or ptosis in myasthenia gravis. It might also improve an ophthalmoparesis. Similarly resting with the eyes closed for 30 minutes may improve ophthalmoparesis. The ability to reverse the ptosis suggests there is a neuromuscular junction problem and not a structural cause such as stroke or nerve injury.