Episodic Weakness Syndrome

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There are a number of unusual syndromes in which weakness is intermittent or fleeting. These are listed in Table 15-15 . Because of the phenomenon of fatigability and recovery after rest the patient with myasthenia gravis may complain of episodic weakness. Myasthenia gravis is seldom confused with these other disorders because of the features mentioned earlier and the predilection of the weakness for the ocular and cranial muscles. An episodic hemiparesis is more apt to lead to confusion. The most common cause of this is transient ischemic attack but the potential for confusion is greater with partial motor seizures or hemiplegic migraine. These entities can be differentiated clinically by paying careful attention to the mode of onset and the course of the weakness. Transient ischemic attack produces an abrupt and simultaneous onset of weakness in all the muscles that will be affected during the attack. A partial motor seizure may become manifest as an inhibition of motor function, producing a stepwise progression of weakness from one body part to the next ("march") over many seconds to a few minutes as the wave of excitation or inhibition spreads along the cortical motor strip. The more common clonic jerking of the limbs may be followed by a postictal weakness ( Todd's paresis) that lasts minutes to hours. The weakness of hemiplegic migraine takes even longer to develop than the inhibitory seizure, usually requiring several minutes to reach its completion.

Psychogenic paralyses can usually be readily diagnosed by the normal state of the reflexes and the nonanatomical distribution of sensory loss. When they are distracted or think they are unobserved, patients with this form of paralysis can usually be observed to move or use the affected body part in a way that should not be possible. The cataleptic attack is sudden and brief and is usually triggered by emotion. The history reveals other features of the narcolepsy syndrome. Another feature of narcolepsy is sleep paralysis. The attack generally occurs when the patient is beginning to arouse and awaken early in the morning. As the patient becomes aware of his surroundings it is apparent that he is paralyzed and cannot move or talk, although respiration is unaffected. this attack can be aborted by an external stimulus, usually touch; sometimes the patient himself can abort the paralysis by imagining a touch. This phenomenon differentiates the attack of sleep paralysis from periodic paralysis (see later discussion). An attack of sleep paralysis may also be accompanied by frightening hallucinations ( hypnogogic hallucinations).

The term drop attack refers to sudden falling spells without warning, loss of consciousness, or postictal symptoms. These attacks are very brief and are presumed to result from a sudden loss of muscular strength due to dysfunction of the corticospinal tracts at the level of the pyramidal tracts in the medulla or the high cervical cord. The attacks may result from brief ischemia to this region as in a vertebrobasilar ischemic attack or transient compression by excessive movement of the odontoid in a patient with an unstable atlantoaxial articulation. The ligament holding the odontoid in place may be destroyed by rheumatoid arthritis or trauma, and certain movements of the head, especially extension, can cause the odontoid to transiently compress the cervicomedullary junction. Similar attacks of tetraparesis can also occur in the chronic cerebellar tonsillar herniation characteristic of the Chiari malformation or in patients with severe congenital cervical spinal stenosis during Valsalva maneuvers or after falls. In the latter examples of mechanical compression, the weakness is generally more prolonged than it is in the classic drop attack, in which the person is immediately able to get to his or her feet after hitting the ground. There is also a syndrome of idiopathic drop attacks in elderly women that has a benign prognosis.

By definition, drop attacks occur without loss of consciousness,

TABLE 15-15 -- EPISODIC WEAKNESS-DIFFERENTIAL DIAGNOSIS

Disorders

Key Features

Diagnostic Tests

COMMON

Transient ischemic attack

All symptoms begin at once

Carotid ultrasound

LESS COMMON

Partial motor seizure with

Gradual "march" of symptoms in several seconds to a few

EEG

Todd's paresis

minutes

Hemiplegie migraine Myasthenia gravis

Sleep paralysis Drop attacks

Gradual development over several minutes; family history

Fatigability, recovery with rest; predilection for ocular and cranial muscles

Normal reflexes

Triggered by emotion; association with other features of narcolepsy; episodes very brief

Narcolepsy; terminated by touch

Sudden loss of postural tone without loss of consciousness

Tensilon test, repetitive stimulation test

Sleep study

MRI, magnetic resonance angiography (MRA), x-ray of eervieal spine with flexion extension, EEG

Sleep study

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