Myasthenia gravis typically targets skeletal muscles, where the nicotinic acetylcholine receptors are located. The muscles innervated by the cranial nerves are often the first and worst affected. The most typically affected muscles include the ocular, bulbar, and facial muscles, as well as the muscles of the limbs, neck, shoulders, hips and trunk. In rare cases, there is weakness of the diaphragm and thoracic chest muscles, which can result in respiratory impairment. Weak bulbar and upper airway muscles can, in rare cases, lead to upper airway obstruction. At times MG may cause pain in the back of the neck, if the muscles that hold the head upright (neck extensors) are involved. It is thought that the reason particular muscles are affected more frequently and severely than others is due to the differential temperature of various muscles in the body.
Myasthenia gravis that effects the eye muscles in particular is typically referred to as ocular myasthenia gravis. This is a common form of the disorder, and it is characterized by droopy eyelids, and observable problems with eye coordination and object tracking. Generalized myasthenia gravis often includes ocular muscles, but in this case the symptoms have also been extended to muscles throughout the body. In particular, shoulder, limb, and muscles used for postural corrections are affected.
Myasthenic symptoms tend to fluctuate over the course of the hour and throughout the day. They are often worst at night. Many physical and environmental conditions can either exacerbate or diminish the symptoms of myasthenia gravis. The muscle fatigue associated with myasthenia gravis is made worse by repetitive activity, excessive heat, overexertion, and emotional stress. Physical conditions can also irritate the condition, including any kind of physical stress: such as surgery, radiation therapy, infection, or fever. Also known to make symptoms worse are abnormal thyroid activity and low potassium levels. Drugs that interfere with muscle activity, such as muscle relaxants, anesthetics, anticonvulsants, antiarrhythmics, some antibiotics, curariform agents and other drugs that interfere with neuromuscular transmission will also greatly increase the severity of the symptoms. Myasthenic symptoms are improved most obviously by rest. It has also been found that they are improved with cold packs and in cooler environmental temperatures.
Myasthenia gravis has been classified into four different grade levels, according to the range of severity. Grade I is the focal disease, typically affecting a specific muscle group, such as in ocular myasthenia gravis. Grade II is the generalized condition, which is classified as either mild (IIa) or moderate (IIb). Grade III is described as the severe generalized disease, which is either unmanageable or managed with great difficulty. A patient is classified as having grade IV myasthenia gravis when the condition is said to be in crisis, that is when there is a life threatening impairment of respiration due to weakness of the diaphragm, thoracic chest muscles, or bulbar and upper airway muscles. The patients who reach this point are most likely those who have had trouble swallowing and/or talking. With more advanced treatments for myasthenia gravis, grade IV myasthenia is now very rare.
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