Rather than relying on the patient’s report to distinguish between fatigability and weakness, it is more informative to observe the patient during the performance of certain common activities such as walking, climbing stairs, and arising from a sitting, kneeling, squatting, or reclining position or using the arms over the head. Difficulty in performing these tasks signifies weakness rather than fatigue. Sometimes, the weakness of a group of muscles becomes manifest only after a period of activity; for example, the feet and legs may “drag” only after the patient has walked a long distance. The physician, upon being told this by the patient, should attempt to conduct the examination under circumstances that duplicate the complaints. Of course, these impairments of muscle function may be caused by a neuropathic or central nervous system (CNS) disturbance rather than of a myopathic one, but usually these conditions can be separated by the basic methods indicated further on in this chapter and in Chaps. 3 and 23.
Reduced strength of muscle contraction—manifest by diminished power of single contractions against resistance (peak power) and during the sustained performance of prolonged or repetitive movements (i.e., endurance)—are the indubitable signs of muscle or neuromuscular disease. In such testing, the physician may encounter difficulty in enlisting the patient’s cooperation. The tentative, hesitant performance of the asthenic or suggestible individual, or the hysteric or malingerer, poses difficulties that can be surmounted by experience and by the techniques described in Chap. 3. In infants and small children, who cannot follow commands, one assesses muscle power by the resistance to passive manipulation or by observing performance while the child is engaged in natural activities. The patient may be reluctant to fully contract the muscles in a painful limb; indeed, pain itself causes a reflex diminution in the power of contraction (algesic paresis). Estimating the strength of isometric contractions that do not require the painful part to be moved is a way around this difficulty.
Ascertaining the extent and severity of muscle weakness requires a systematic examination of the main groups of muscles. The patient is asked to contract each group with as much force as possible, while the examiner opposes the movement and offers a graded resistance in accordance with the degree of residual power (isokinetic contraction). Alternatively, the patient is asked to produce a maximal contraction and the examiner estimates power by the force needed to overcome or “break” it (isometric contraction or maximum voluntary isometric contraction). If the weakness is unilateral, one has the advantage of being able to compare it with the strength on the normal side. If it is bilateral, the physician must refer to his concept of what constitutes normalcy based on experience in muscle testing. As mentioned, one can distinguish true weakness from unwillingness to cooperate, feigned or neurasthenic weakness, and inhibition of movement by pain.
To quantitate the degree of weakness, a rating scale may be required. Widely used is the one proposed by the Medical Research Council (MRC) of Great Britain, which recognizes 6 grades of muscle strength as follows: 2—Active movement only with gravity eliminated 3—Full movement against gravity but cannot offer resistance to manual muscle opposition
Further gradations may be added, specified as 4+ for barely detectable weakness and 4for easily detected weakness, 3+ and 3-, and so on.
The ocular, facial, lingual, pharyngeal, laryngeal, cervical, shoulder, upper arm, lower arm and hand, truncal, pelvic, thigh, and lower leg and foot muscles are examined sequentially. It is most convenient to compare power generated by the same muscle from each side. To fully and properly use tools such as the MRC scale and to detect mild weakness, muscles such as the neck flexors and extensors must be tested with the patient in the prone and supine positions. The anatomic significance of each of the actions tested, that is, what roots, nerves, and muscles are involved, can be determined by referring to Table 45-1. A practiced examiner can survey the strength of these muscle groups in 2 to 3 min.