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We control the muscles of our body by stimulating their contraction via a motor nerve. If this nervous transmission is interrupted by nerve or muscle disease then weakness of the involved muscles will result. If generalized weakness affects the muscles which control respiration then the disease can be life threatening. ICU care is generally required in these situations to monitor and treat respiratory failure. Specialized treatments to directly treat the disease process and care from experienced physicians and nurses in a neuro-ICU can reduce complication rates and duration of illness. The two common causes of severe neuromuscular weakness are myasthenia gravis and the Guillain-Barré syndrome.
Myasthenia Gravis causes weakness that fluctuates from hour to hour and day to day. The muscles usually affected by this weakness control the eyes, face, neck, and throat. In half of patients weakness affects the arms or legs as well. The disease results from an auto-immune attack of the patient's immune system against receptors that stimulate muscle contraction.
Myasthenia Gravis causes weakness that fluctuates from hour to hour and day to day. The muscles usually affected by this weakness control the eyes, face, neck, and throat. In half of patients weakness affects the arms or legs as well. The disease results from an auto-immune attack of the patient's immune system against receptors that stimulate muscle contraction.
A motor neuron innervating a muscle ends on that muscle at a neuromuscular junction. At this microscopic junction a chemical substance known as acetylcholine is released from the nerve terminal. Under normal circumstances acetylcholine binds its receptor on the muscle surface. This binding triggers an electrical endplate potential in muscle cells. If this electrical potential rises above a threshold value then a second electrical potential or action potential is generated and the muscle contracts. In an individual with myasthenia gravis the body's antibodies are directed against its own acetylcholine receptors on the muscle surface. Because of this the number of functional receptors is reduced. As a result, the endplate potentials fail to reach threshold and the muscle fails to contract, resulting in weakness and fatigability.
Myasthenic crisis occurs when myasthenic weakness affects the respiratory muscles and breathing is dangerously impaired. In this situation the volume of air in the lung decreases, a situation known as atelectasis. By definition, a patient in myasthenic crisis has respiratory failure requiring intubation (the passage of a tube into the patient's airway to keep it patent) and mechanical ventilation. Crisis can be produced by infection (40%) or can occur spontaneously (30%). Other causes of myasthenic crisis include aspiration, pregnancy, medications, and surgery. About one quarter of patients can be taken off the respirator within a week, 50% within two weeks, and 75% within a month. About one third of the patients experiencing a myasthenic crisis will experience a second crisis.
With proper ventilatory support, the prognosis of myasthenic crisis is good, with a mortality rate of about 5%. The treatment is plasma phoresis involves the process of removing blood from the body and "cleansing" if of antibodies directed against acetylcholine receptors. It results in short-term improvement in 75% of patients, and can reduce the duration of intubation and mechanical ventilation. Intravenous immunoglobulin (IVIG) is another treatment option, but has not been adequately tested yet. Treatment by an experienced team of physicians focuses minimizing the amount of time on the respirator, limiting complications, and reducing the duration of illness.
In Guillain-Barré syndrome the body synthesizes antibodies which are directed against peripheral nerves innervating muscles. Normally these nerves are wrapped in a myelin sheath which serves to increase the speed of conduction of nerve impulses along their length (like plastic around a copper wire). Under an auto-immune attack the nerves become inflamed and demyelinated, and the myelin sheath is stripped off. This has the effect of slowing or completely interrupting nerve impulses and preventing contraction of the muscles they innervate.
The symptoms usually begin with burning or tingling sensations (paresthesia) in the distal extremities. This is followed by a rapidly progressive and symmetrical paralysis or weakness which begins in the legs and ascends to the head (called ascending paralysis). In more severe cases the disease progresses to affect the muscles of swallowing and respiration. In this situation the volume of air in the lung decreases. About 20% of all patients with Guillain-Barré syndrome experience respiratory failure in which intubation (the passage of a tube into the patient's airway to keep it patent) and mechanical ventilation are required. It is in these patients that the treatment offered by a neuro-ICU is particularly helpful.
Guillain-Barré syndrome generally has an excellent prognosis, with most patients making a full recovery. About 20% of patients will have respiratory failure and require treatment in an intensive care unit. Overall, about 5% of all patients have a form of permanent disabling weakness, and 5% will die from medical complications. Features correlated with a poor prognosis include: (1) advanced age of the patient; (2) a weakness which is rapidly progressive over the first week; and (3) respiratory failure requiring intubation and mechanical ventilation.
There are two specific treatment approaches employed to combat Guillain-Barré syndrome:
Disturbances in autonomic function associated with the condition can cause rapid heart rate and high blood pressure. These complications are treated with continuous infusions of short-acting beta-blockers such as esmolol and labetalol. The experience if Guillain-Barré is extremely stressful, and can lead to severe depression. This may require treatment.
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