Muscle Tone

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CHAPTER

Muscle Tone

Muscle tone has been defined as the tension in the relaxed muscle or the resistance to passive movement when voluntary contraction is absent. Because of resting tone, normal muscles have slight resistance to passive movement even in the relaxed state. The inherent attributes of muscle tissue—such as viscosity, elasticity, and extensibility—contribute to resting tone. Even apparently relaxed muscle fibers have a constant slight fixed tension by which they hold their resting position, resist changes in length, prevent undue mobility at joints, and are in position to contract when necessary. Resting muscle tone is greatest in the antigravity muscles that maintain the body in an erect position.

The resting level of tone in a muscle is dependent on activity in the spinal cord segment that innervates it, primarily the gamma motor neuron. Efferent impulses from the gamma motor neuron set the level of contraction of the intrafusal fibers of the muscle spindles. Spindle afferents in turn convey impulses to the spinal cord segment to complete the gamma loop. Descending influences from higher motor centers regulate and modulate the activity at the local spinal cord segment. All of these factors interact to determine the level of resting tone. When a muscle with normal segmental innervation is passively stretched, reflex shortening may occur; this is the stretch reflex.

The background level of muscle tone maintains normal resting limb positions and attitudes. Active muscle contraction takes place on the background of the resting level of muscle tone, and normal background tone is important for proper coordination of movement. Activity mediated by the reticular formation, the otolith organs, the vestibular apparatus, and other higher centers is important in maintaining the steady contraction of the antigravity muscles that is necessary to the standing position, as well as to other postural and righting reflexes.

Tone may be affected by disease at different levels of the nervous system. Interruption of the local spinal reflex arc abolishes resting muscle tone. Most types of hypertonicity can be abolished by interrupting either the gamma efferent impulses to the intrafusal fibers or the afferent impulses from the muscle spindles. Denervated muscle is flaccid and behaves as noncontractile tissue. Loss of impulses from the supraspinal pathways that normally inhibit lower reflex centers usually causes an increase in tone. Loss of the normal balance between higher facilitatory and inhibitory centers may either decrease or increase tone.

EXAMINATION OF TONE

Tone is difficult to assess. The determination of tone is subjective and prone to interexaminer variability. There are no methods that can measure tone quantitatively. The determination is based solely on the clinical judgment of the examiner; accurate assessment of tone requires clinical experience. It is difficult to separate slightly increased tone from poor relaxation in a tense or apprehensive patient. Tone is especially difficult to evaluate in infants, where there may be wide variations in apparent tone on different examinations, in either health or disease.

The examination of tone requires a relaxed and cooperative patient. Small talk may help the patient relax. Simple observation may reveal an abnormality of posture or resting position that indicates an underlying change in tone. Muscle palpation is sometimes useful, but well-muscled individuals may have firm muscles despite normal resting tone, while in other individuals, the muscles may feel flabby despite an underlying hypertonicity. Muscles may have a firm consistency to palpation because of edema, inflammation, spasm due to pain, or pseudohypertrophy.

The most important part of the examination of tone is determination of the resistance of relaxed muscles to passive manipulation as well as the extensibility, flexibility, and range of motion. Abnormalities of tone are more easily detected in extremity than in trunk muscles. The limb is moved passively, first slowly and through a complete range of motion and then at varying speeds. The examiner may shake the forearm to and fro and note the excursions of the patient’s hand, brace a limb and then suddenly remove the support, or note the range of movement of a part in response to a slight blow. Bilateral examination of homologous parts helps compare for differences in tone on the two sides of the body.

Tone should be assessed by both slow and rapid motion and through partial and full range of motion, documenting the distribution, type, and severity of any abnormality. Certain specific maneuvers may be helpful in evaluation of abnormal tone.

The Babinski Tonus Test

The arms are abducted at the shoulders, and the forearms are passively flexed at the elbows. With hypotonicity, there is increased flexibility and mobility, and the elbows can be bent to an angle more acute than normal. With hypertonicity, there is reduced flexibility, and passive flexion cannot be carried out beyond an obtuse angle.

The Head-Dropping Test

The patient lies supine without a pillow, completely relaxed, eyes closed, and attention diverted. The examiner places one hand under the patient’s occiput and with the other hand briskly raises the head, and then allows it to drop. Normally the head drops rapidly into the examiner’s protecting hand, but in patients with extrapyramidal rigidity there is delayed, slow, gentle dropping of the head because of rigidity affecting the flexor muscles of the neck. When meningismus is present, there is resistance to and pain on flexion of the neck.

Pendulousness of the Legs

The patient sits on the edge of a table, relaxed with legs hanging freely. The examiner either extends both legs to the same horizontal level and then releases them (Wartenberg pendulum test) or gives both legs a brisk, equal backward push. If the patient is completely relaxed and cooperative, there will normally be a swinging of the legs that progressively diminishes in range and usually disappears after six or seven oscillations. In extrapyramidal rigidity, there is a decrease in swing time but usually no qualitative change in the response. In spasticity, there may be little or no decrease in swing time, but the movements are jerky and irregular, the forward movement may be greater and more brisk than the backward, and the movement may assume a zigzag pattern. In hypotonia, the response is increased in range and prolonged beyond the normal. In all of these maneuvers, a unilateral abnormality will be more apparent.

The Shoulder-Shaking Test

The examiner places her hands on the patient’s shoulders and shakes them briskly back and forth, observing the reciprocal motion of the arms. With extrapyramidal disease, there will be a decreased range of arm swing on the affected side. With hypotonia, especially that associated with cerebellar disease, the excursions of the arm swing will be greater than normal.

The Arm-Dropping Test

The patient’s arms are briskly raised to shoulder level and then dropped. In spasticity, there is a delay in the downward movement of the affected arm, causing it to hang up briefly on the affected side (Bechterew’s or Bekhterew’s sign); with hypotonicity, the dropping is more abrupt than normal. A similar maneuver may be carried out by lifting and then dropping the extended legs of the recumbent patient.

Hand Position

Hypotonicity, especially that associated with cerebellar disease or Sydenham’s chorea, may cause the hands to assume a characteristic posture. With the arms and hands outstretched, there is flexion at the wrists and hyperextension of the fingers (“spooning”) accompanied by moderate overpronation. With the arms raised overhead, the overpronation is exaggerated with the palms turned outward. This overpronation phenomenon differs from the pronator drift sign, in which the overpronation is due to weakness of corticospinal innervated muscles or increased tone in the pronator muscles.

MYOTATIC IRRITABILITY, MYOEDEMA, AND TENDERNESS

In addition to the inspection, palpation, and resistance to passive motion used in the assessment of tone, it is sometimes useful to observe the reaction to direct percussion of the muscle belly. The idiomuscular contraction is the brief and feeble contraction of a muscle belly after it is tapped with a percussion hammer, causing a slight depression even when the deep tendon reflex (DTR) is absent. Myotatic irritability has been defined as both the response to direct percussion as well as the ability of a muscle to contract in response to sudden stretch.

The response to direct muscle percussion in normal muscle is very slight and, in most muscles, is seen or felt with difficulty. The reaction may be more pronounced in wasting diseases, such as cachexia and emaciation, and in some diseases of the lower motor neuron. Hyperexcitability to such stimulation occurs in tetanus, tetany, and certain electrolyte disturbances. Occasionally, after a muscle is percussed with a reflex hammer, a wave of contraction radiates along the muscle away from the point of percussion. A small ridge or temporary swelling may persist for several seconds at the point of stimulation. This stationary muscle mounding is known as myoedema. There is no accompanying electrical muscle activity. The idiomuscular contraction causes a slight depression, myoedema a rounding up. The mechanism of myoedema is poorly understood, but it is probably a normal physiologic phenomenon. Its presence alone does not indicate a neuromuscular disorder, but the response may be exaggerated in some circumstances, most notably hypothyroid myopathy and cachexia. Myoedema is electrically silent on electromyography. Hypothyroidism may also cause an electrically active muscle mounding and spreading contraction, manifest by a burst of normal motor unit action potentials upon percussion (for video, see Loomis et al.). Myotonia is a persisting contraction following mechanical stimulation of muscle that is quite different from myoedema (see below). In rippling muscle disease, there are wave-like muscle contractions evoked by muscle stretch that move laterally along muscle over 5 to 20 seconds. The phenomenon is especially prominent in large proximal muscles (see http://www.youtube.com/watch?v=vKgFtIbCzcg).

During muscle palpation, muscle tenderness may sometimes be elicited. Muscle tenderness on squeezing the muscle belly, or even with very slight pressure, may cause exquisite pain. Widespread muscle tenderness to palpation may occur with inflammatory myopathy, especially polymyositis and dermatomyositis, in some neuropathies, and in acute poliomyelitis. Focal muscle tenderness occurs with trauma or overexertion of muscles.

ABNORMALITIES OF TONE

Pathologic conditions may cause an increase or decrease in tone. In addition, there are different varieties of hypotonicity and hypertonicity. Hypotonicity may develop from disease of the motor unit, the proprioceptive pathways, cerebellar lesions, and in the choreas. The muscle may be flaccid, flabby, and soft to palpation. The involved joints offer decreased resistance to passive movement. The excursion of the joint may be increased with an absence of the normal “checking” action on extreme passive motion. If the involved extremity is lifted and allowed to drop, it falls abruptly. A slight blow causes it to sway through an excessive excursion. The DTRs are usually decreased or absent when hypotonia is due to a lesion involving the motor unit or proprioceptive pathways.

Hypotonia

When hypotonia is due to disease of the motor unit, there is invariably some degree of accompanying weakness. The hypotonia that results from central processes (e.g., cerebellar disease) does not cause weakness; muscle power is preserved even though hypotonia is demonstrable on examination. Infantile hypotonia (floppy baby syndrome) is a common clinical condition in which there is a generalized decrease in muscle tone, typically affecting a neonate (for video, see http://www.youtube.com/watch?v=PZFtQSe5Pd8

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