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 the 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 nonneurological 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. 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, whereas 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 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 cogwheel, hence the term cogwheel rigidity. The physiological explanation for cog wheeling and rigidity is still being debated; evidence favors a disturbance in the long-loop stretch reflex (see previous discussion of muscle tone in the section, Gamma Motor Neuron). It is important to note that the phenomenon of cogwheeling 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.