The patient should be in a state of sufficient undress for the physician to be able to appreciate his or her general bodily habitus and especially the muscle bulk of the extremities. Before actual testing of muscle strength begins, simple observation may disclose asymmetries in muscle bulk of the extremities or more focal atrophy of specific muscles or muscle groups. The upper extremities should be inspected in both the pronated and supinated positions, which is especially important to appreciate differences in the extensor and flexor compartments of the forearm. The lower extremities should be observed from the front and the back while the patient is standing.
If a significant asymmetry or focal atrophy is apparent, the bulk of the muscle groups in the arm, forearm, thigh, and leg can be measured objectively by measuring the girth and comparing sides. The point at which the circumference is to be measured is marked with a ballpoint pen, usually at the point of greatest estimated girth. To find the same site on the opposite limb, the distance to the closest bony landmark is measured, and that measurement is used on the opposite limb to obtain precise symmetrical measurements. Muscle bulk is usually greater in the dominant limb; a difference of 1.0 cm is accepted as within the range of normal in the leg and thigh, and 0.5 cm is acceptable in the forearm and arm.
In most cases, a significant difference indicates hypotrophy of the smaller side. However, in some cases the difference may be the result of an unusual hypertrophy or pseudohypertrophy in patients with muscular dystrophy. Pseudohypertrophy in a patient with muscular dystrophy is generally symmetrical and most commonly involves the calves. Therefore, any disparity in muscle bulk between the upper and lower extremities should also be sought. During the inspection of muscle bulk, one should also look for fasciculations. The ease with which fasciculations may be seen depends on how much subcutaneous tissue is present. In thin elderly men with suspected amyotrophic lateral sclerosis, the shoulder girdle and pectoral muscles are often a good place to look for fasciculations. In patients with more subcutaneous adipose tissue, the first dorsal interosseous muscle of the hand is better. Forcible contraction or percussion of the muscle may increase the frequency of the fasciculations.
Fasciculations are a feature of disorders of the anterior horn cell or root compression. Therefore, fasciculations are often viewed as an ominous sign. They need not be because they are commonly experienced as a benign phenomenon in the absence of any disorder of the motor neuron or spinal nerve roots. '25' Fasciculations of a benign nature can generally be distinguished from fasciculations secondary to lower motor neuron disease by several features. They have a predilection for males and for certain muscle groups, especially the calves and thighs. When they occur in the arm muscles they tend to be seen as a repetitive twitch in the same muscle fascicle as opposed to the random nonstereotyped twitches of many parts of the muscle seen in patients with anterior horn cell disease. Benign fasciculations in the calves are more difficult to distinguish clinically because they may be frequent and multiple and have a more malignant appearance. However, there is no associated weakness or atrophy of the affected muscles with benign fasciculations. Electrophysiologically, a benign fasciculation appears like a normal motor unit; there are no other features of denervation in the muscle. A malignant fasciculation in a patient with motor neuron disease is more complex and has a longer duration and a higher amplitude.y The predilection toward benign fasciculations in males tends to be familial and increases progressively with age.
Muscle tone is assessed by asking the patient to relax completely while the examiner moves each joint through the full range of flexion and extension. Patients vary in their ability to relax. Generally it is easier for them to relax the lower extremities in the sitting position, whereas the upper limbs can be examined in either the sitting or lying position. Some patients, especially mildly demented elderly people, find it difficult not to voluntarily help move the limb in the desired direction. In a completely relaxed patient, no resistance should be felt at the wrist and elbow, and minimal resistance at the shoulder, knee, and ankle. It is very important to compare sides because a minimal but pathological increase in tone may initially be considered normal until one compares it with the normal side. The degree of relaxation varies from patient to patient, and any change from normal is necessarily somewhat subjective. In assessing tone, it is also important to recognize that patients with non-neurological disease, specifically pain or bone or joint abnormalities, may demonstrate resistance to passive movement, thus confounding the examination.
Flaccidity, in particular, may be difficult to judge. '27' Certain maneuvers may be helpful in showing a difference from one side to the other. For example, flexion of the wrist can be compared by noting the distance the thumb can be brought to the flexor aspect of the forearm. Another maneuver is to shake the forearm and observe the floppiness of the movements of the hand at the wrist, or, with
the arms raised overhead, compare the degree of flexion or limpness of the wrist on each side. The lower extremities can be tested by rapidly flexing the thigh after instructing the patient to let the leg flop. The sudden flexion of the thigh raises the knee. In a patient with normal tone, the heel may come off the bed slightly and transiently and then drag along the sheet as the thigh is flexed. The heel of the flaccid leg will be dragged across the bed from the very beginning, while the spastic leg will jerk upward and the heel may never fall back to the bed.
In assessing hypertonus, one must differentiate between spasticity and rigidity. In spasticity the distribution of increased tone is very specific for the upper and lower extremities. The flexors of the arm (primarily the biceps) and the extensors and adductors of the leg display a greater increase in tone. This can be vividly seen in the classic hemiplegic posture in which the arm is flexed at the elbow and wrist and adducted against the chest while the leg is stiffly extended and the foot is inverted and flexed in a plantar direction. One can see spasticity in the adductors of the leg in the tendency of the hemiplegic leg to "scissor" over the good leg; this scissoring is most easily seen in a patient with spastic paraplegia. When the patient is lying down, the increase in tone in the adductors of the thigh can be felt by rapidly rotating the thigh back and forth to detect the increase in tone when the thigh is rotated externally.
The difference in tone in the flexors of the arm and the extensors of the leg can also give rise to the spastic catch, or the clasped knife phenomenon. This is a manifestation of the heightened stretch reflex that occurs in patients with lesions of the upper motor neuron pathways. By rapidly flexing and extending the elbow or knee, a sudden stretch is put on the muscle by lengthening it. The reflex contraction resists this lengthening and is sometimes referred to as the lengthening reaction. It is felt as a catch or interruption in the velocity of extension at the elbow or flexion at the knee. It is important to extend the elbow or flex the knee rapidly because the rate of stretch is important in eliciting the maximum response. The muscle spindles are maximally activated with a rapid stretch. Even with a very rapid stretch, a free interval occurs at the start of the movement, followed by the resistance. As the muscle is lengthened further, the resistance gives way, a process that has been compared to opening the blade of a clasped knife.
In contrast to spasticity, rigidity involves an equal increase in tone occurs in the flexors and extensors. The increase in tone is felt throughout the range of movement, and has been compared to bending a lead pipe. As the rigidity increases, it becomes more difficult to rapidly flex and extend the joint. In fact, it may be easier to feel the rigidity with slower movements. It is important, however, to flex and extend the joint repetitively because the rigidity may gradually build up in intensity with repeated motion. Often the rigidity is not felt as a continuous smooth change, as implied by the terms plastic or lead pipe. Instead, a rapid and rhythmical succession of catches and releases occurs that has been compared to a lever jumping from one cog to another in the turning of a cog wheel, hence the term cog wheel rigidity. The physiological explanation for cog wheeling and rigidity is still being debated; current evidence favors a disturbance in the long loop stretch reflex (see earlier discussion of muscle tone in the section, Gamma Motor Neuron). It is important to note that the phenomenon of cog wheeling can occur in patients with essential or familial tremor in the presence of normal tone and may be confused with parkinsonism. In these instances, the clinician feels the alternate activation of the flexors and extensors that produce the tremor, but does not feel the hypertonicity.
The gegenhalten phenomenon is not a true increase in muscle tone but a pseudovoluntary resistance by the patient against any passive movement of the limb. Each attempt at moving the limb by the examiner is met with an equal and opposing force. This can give rise to the appearance of increased tone. It is involuntary to the extent that the patient has great difficulty in voluntarily suppressing the urge to resist. Gegenhalten is usually associated with diffuse cerebral disease and dementia.
The strength of a movement can be measured according to certain criteria; the most commonly used scale is that devised by the Medical Research Council y ( Table. 15-1 ). The grade assigned to the muscle is the maximum force generated by an effort to move the involved body area. Judgment of the presence of mild weakness depends on making appropriate comparisons, one arm versus the other, or the arms versus the legs, or extension versus flexion. The examiner should always look for patterns of weakness and compare the weak limb or muscle against the normal one in the same individual. Of course, if the weakness is generalized, judgment must be based on an estimate of the power normally generated by the age and bulk of the muscle.
More objective measurements can be made with the use of specially designed instruments to measure force. The only one widely used in clinical practice measures grip strength. Recently an electronic strain-gauge that measures maximum isometric force has also been used in clinical research trials for the treatment of amyotrophic lateral sclerosis. y Some degree of objective measurement can also be obtained through formal assessment by a specially trained physical therapist. These semiquantitative methods (semiquantitative because they require the full cooperation of the patient being examined) are not primarily used for diagnostic purposes but rather for measuring changes that occur over time in response to some disease or treatment.
The most important purpose of strength testing is to
TABLE 15-1 -- THE MEDICAL RESEARCH COUNCIL RATING OF MUSCLE STRENGTH
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