department of neurology  
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NEUROMUSCULAR WEAKNESS




What is meant by neuromuscular weakness and paralysis?


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
» Guillain-Barré syndrome
 

MYASTHENIA GRAVIS



What is myasthenia gravis?


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.

What are the symptoms of myasthenia gravis?


The initial symptom in 40% of cases of myasthenia gravis is weakness of the eye muscles and eyelid. This results in double vision (diplopia) and drooping of the eyelids (ptosis). These two symptoms will eventually occur in 85% of patients with myasthenia gravis.
Weakness of the lips, mouth, tongue and throat can lead to dysarthria (slurred speech) and dysphagia (difficulty in swallowing). Aspiration of food into the wind-pipe and lungs can lead to pneumonia, which can in turn precipitate respiratory failure.
Approximately 20% of patients experience an attack of myasthenic weakness so severe that the muscles of breathing fail, necessitating placement on a respirator. This condition is called myasthenic crisis.

What causes myasthenia gravis?


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.

What is myasthenic crisis?


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.

How is myasthenic crisis treated in a neuro-ICU?


With proper ventilatory support, the prognosis of myasthenic crisis is good, with a mortality rate of about 5%. Plasmapheresis 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.
 

GUILLAIN-BARRÉ



What is Guillain-Barré syndrome?


Guillain-Barré results from a rapid attack of the immunologic system against the nerves that control muscle activity, leading to rapidly progressive paralysis. It is usually precipitated by an immune response to an antecedent viral infection (cold, flu, or gastroenteritis). Although it may occur at any age its highest incidence is between the ages of 50 and 75.

What causes Guillain-Barré syndrome?


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.

What are the symptoms of Guillain-Barré Syndrome?


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.

What is the prognosis of Guillain-Barré syndrome?


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.

How is Guillain-Barré syndrome treated in a neuro-ICU?


There are two specific treatment approaches employed to combat Guillain-Barré syndrome:
  1. Plasmapheresis: This process involves removing blood from the body, separating out the cellular elements of the blood, resuspending these cellular elements in a plasma substitute, and then reinfusing this mixture into the body. The purpose of this procedure is to "cleanse" the blood of antibodies directed against the myelin sheaths of peripheral nerves.

  2. IVIG: This treatment involves the administration of high doses of intravenous immunoglobulin.
Complications associated with Guillain-Barré syndrome must also be managed. Patients with this disorder can experience severe pain which is treated with non-steroidal anti-inflammatory drugs (such as ibuprofen) and narcotics (such as morphine). 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 labetolol. The experience if Guillaine-Barré is extremely stressful, and can lead to severe depression. This may require treatment.

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Affiliated with New York-Presbyterian Hospital || Last updated: August 8, 2012 |
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