| Disease Name | sporadic | | NMD Category | Disease of the Neuromuscular Junction | | Clinical Onset | childhood to adulthood | | Clinical Findings | The muscles innervated by the cranial nerves are often the first and worst affected.The muscles around the eyes (extraocular) and those used in swallowing are most frequently affected. Eyelid weakness results in ptosis, and eyebrows are furrowed to compensate for ptosis. A sustained upgaze exacerbates the ptosis, while closing the eyes for a short period improves it. Typically, extraocular muscle weakness is asymmetric. Swallowing may become difficult and aspiration may occur with fluids, giving rise to coughing or choking while drinking. Weak pharyngeal muscles may collapse the upper airway. Such weakness may produce acute respiratory failure.
However, any muscle may be affected by this disorder. Weakness in other muscle groups is usually proximal and symmetric. Sensory examination and deep tendon reflexes are normal. Facial muscle weakness is common, and bilateral facial muscle weakness produces a mask like face with ptosis and a horizontal smile. Weakness of palatal muscles can result in a nasal twang to the voice and nasal regurgitation of food, especially liquids. Severe jaw weakness may cause the jaw to hang open. Weakness of neck muscles is common, and neck flexors usually are affected more severely than neck extensors. Upper limb muscles are more likely to be involved than lower limb muscles. In the upper limbs, deltoids and extensors of the wrist and fingers are affected most. Triceps are more likely to be affected than biceps. In the lower extremities, commonly involved muscles include hip flexors, quadriceps, and hamstrings, with involvement of foot dorsiflexors or plantar flexors less common.
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| | Diagnostic Work-up | An important aspect of the physical examination is to recognize an MG patient in whom respiratory failure is imminent. Difficulty breathing requires prompt treatment. Benefits of treatment are monitored by strength testing, which must be carefully determined in various muscles to assess the severity and extent of the disease. Strength rarely returns to normal, and can vary considerably throughout the day.
For comparison to congenital myasthenic syndromes, see Diagnostic Criteria for Neuromuscular Diseases http://www.enmc.org/nmd/diagnostic.html.
| | Treatment Notes | Treatment goals must be individualized according to the severity of disease, the patient's age and sex, and the degree of functional impairment. Drug therapy and/or removal of the thymus gland is often, but not always, effective. Cholinesterase inhibitors such as pyridostigmine- and neostigmine-bromide retard the enzymatic hydrolysis of acetylcholine at cholinergic synapses, so that acetylcholine accumulates at the neuromuscular junction and its effect is prolonged. Use of corticosteroids such as presnisone show marked improvement or complete relief of symptoms in more than 75% of patients, and some improvement occurs in most of the rest. Plasma exchange is used as a short-term intervention for patients with sudden worsening of myasthenic symptoms, to rapidly improve strength before surgery, or as a chronic intermittent treatment for patients who are refractory to all other treatments. Several research groups have reported a favorable response to high-dose (2 grams/kg infused over 2 to 5 days) immuneglobulin.
Progression: Disease progression is variable, from mild to more severe disease over weeks to months. Maximum weakness occurs during the first year in two-thirds of patients.
| | Protein ID | 37 | | Gene Location | | | Abstract | Myasthenia gravis (MG) is the most common primary disorder of neuromuscular transmission, eg, normal communication between the nerve and muscle is disrupted. The cause is thought to be an acquired autoimmune response that produces antibodies to acetylcholine receptors (AChR). However, genetic abnormalities (congenital myasthenic syndromes) at the neuromuscular junction related to AChR have now been identified but are not as yet well characterized. For now, the so-called acquired type is a chronic disorder characterized by weakness and abnormally rapid fatigue of various voluntary muscles, particularly those controlled by the brain stem (bulbar-innervated).
When impulses travel down the nerve, normally the nerve endings release a neurotransmitter substance called acetylcholine, which travels through the short neuromuscular junction and results in the activation of muscle contraction. In MG, the receptors for acetylcholine at the muscle surface are destroyed or modulated by antibodies that prevent the normal reaction from occurring. The antibodies are produced by the patient's own immune system, which is believed to generate an abnormal autoimmune reaction that results in an attack on the patient's own neuromuscular junction. Myasthenia gravis typically targets skeletal muscles where the nicotinic acetylcholine receptors are located. The muscles most affected include the ocular, bulbar, and facial muscles, as well as the muscles of the limbs, neck, shoulders, hips and trunk. Myasthenia gravis that effects the eye muscles in particular is referred to as ocular myasthenia gravis, a common form characterized by droopy eyelids and observable problems with eye coordination and object tracking.
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