Motor System: Weakness

Michael Ronthal

▪ INTRODUCTION

The neurological examination begins with inspection.

Look for abnormal movement, fasciculations, and atrophy. Abnormal movements affecting whole limbs or parts thereof are discussed in the chapter on movement disorders. Fasciculations are spontaneous sudden, short-lived contractions of fiber groups that “flicker” under the skin. They are essentially nonspecific but are frequent in chronic degenerative diseases such as motor neuron disease. Fasciculations can be a normal phenomenon (benign fasciculation or cramp syndrome) or part of any peripheral neuropathic process.

Look for atrophy. The distribution of atrophy gives a clue to the site of pathology—it may be in the distribution of a root (myotome) or of a peripheral nerve.

▪ TONE

Hypertonicity suggests central rather than peripheral disease.

Tone is tested by evaluating resistance of the muscles to passive movement. If the patient relaxes completely, the muscles are relatively flaccid and there is no resistance to passive joint movement by the examiner. In the lower limbs, with the patient supine and relaxed, rotate the leg from side to side—the foot should “flop” from side to side at the ankle. If the foot moves en bloc with the leg, there is hypertonia. Again, with the patient relaxed and supine, suddenly jerk the lower limb upward, tugging in the popliteal fossa. If the knee bends smoothly and the heel scrapes the bed, tone is normal; if there is a jerk of the leg upward before it relaxes, there is hypertonicity.

Extrapyramidal Hypertonicity

In the upper limbs a passive rotary movement of the hand on the wrist tests hypertonicity; the patient may be distracted by tapping the other hand on the knee, which will accentuate the hypertonicity or trigger it. The hypertonia of extrapyramidal disease is described as “plastic” or “lead pipe,” meaning that the increase in tone is present throughout the range of movement. If a tremor is superimposed on the hypertonicity, the examiner records this as “cogwheeling.” Plastic rigidity and cogwheeling are characteristic of Parkinson’s disease.

Upper Motor Neuron Hypertonicity

Upper motor neuron (UMN) hypertonicity often shows a marked increase of tone at the beginning of the movement, followed by sudden release of tone. This has been likened to opening a clasp.

Hypotonicity is present in patients with peripheral paralysis and in those with cerebellar dysfunction.

▪ WEAKNESS

Weakness as a symptom may or may not represent true motor dysfunction and should be confirmed by meticulous examination and documentation of the signs.

Although it is important to record the severity or grade of weakness, its distribution will usually guide one to the correct diagnosis.

By definition, any particular muscle is weak if the examiner can overcome its action by countering that action by applying pressure close to the joint that the muscle activates and using appropriate testing strength. If testing finger movement, one should use only the fingers to test that movement. If testing a more proximal and stronger muscle, one can use stronger opposing forces.

Grading Weakness

The severity of weakness is often recorded using a somewhat artificial grading system, as follows:

GRADING SYSTEM:

Grade 0 = Paralysis

Grade 1 = Only a flicker of voluntary movement

Grade 2 = Able to move the joint with gravity eliminated

Grade 3 = Able to move the joint against gravity

Grade 4 = “Weak”

Grade 5 = Normal strength

The grading system works reasonably well except for grade 4, which hides a multitude of sins. This is usually expanded in some way—simply the addition of “mild, moderate and severe” makes it more serviceable and reproducible.

Weakness Distribution

The distribution of weakness is of paramount importance.

Lower Motor Neuron Distribution

Proximal weakness in the limb girdles is usually suggestive of a myopathic process and is often accompanied by weakness of neck flexion or neck extension.

Distal weakness in the small muscles of the hands and feet may suggest a more peripheral neuropathic process.

In between proximal is the weakness corresponding to specific nerves or nerve roots (Table 2.1).

A plexopathy is suggested if the weakness does not fit the distribution of a single nerve root or single nerve. For example, in high pelvic plexopathy there will be weakness of any two of the iliopsoas, quadriceps, and adductor magnus. If all three are involved, radiculopathy is diagnosed; if only one of the three is weak, this suggests a lesion of the nerve supplying that muscle.

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