Muscular Tone and Gait Disturbances

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Chapter 5 Muscular Tone and Gait Disturbances

Muscular tone is conventionally separated into phasic and postural types. Phasic tone is the result of rapid stretching of a tendon, attached muscle, and most importantly, the muscle spindle. The response is rapid and short-lived. Postural tone is the result of a steady, restrained stretch on tendons and attached muscles, with resultant protracted contraction of the involved muscle. Gravity is the most common stimulus for this response. Postural tone is the topic primarily discussed in this chapter, and it is referred to simply as tone.

Tone is functionally defined as resistance to passive movement (i.e., resistance experienced by the examiner while the patient’s relaxed limbs are moved about the joints). Hypotonia is decreased resistance to passive movement. Hyperextensibility is an abnormally increased range-of-joint movement. Hyperextensibility of the elbows, wrists, knees, and ankles usually accompanies hypotonia but is not pathognomonic. The combination of hypotonia and hyperextensibility allows an infant to adopt unusual and awkward-appearing postures.

The term “floppy” is frequently used to describe hypotonic infants. This term is useful only as a shorthand description of clinical manifestations and is not a formal diagnosis.

Hypotonia must not be equated with hyporeflexia or muscle weakness. For example, patients with Down syndrome commonly have normal deep tendon reflexes and normal strength but are usually hypotonic. Conversely, patients with anterior horn cell disease are weak and manifest hypotonia and hyporeflexia.

Pathology

The central and peripheral nervous systems modify tone, but intrinsic physical characteristics of the tendons, joints, and muscles and the anatomic interrelationships of these structures also contribute significantly to tone. In childhood central nervous system (CNS) dysfunction, upper motor neuron (unit) disease may cause increased or decreased muscle tone [Teddy et al., 1984]. Disease involving the lower motor neuron (unit) results in hypotonia and weakness.

The final common pathway of upper or lower motor unit modification of tone is through the gamma loop (fusimotor) system [Gordon and Ghez, 1991; Granit, 1975]. Intimately involved with monitoring and effecting tone are the two stretch-sensitive muscle receptors – the muscle spindles and the Golgi tendon organs (Figure 5-1). It also has become evident that nonreflex, mechanical mechanisms are involved in the maintenance of resting muscle tone. Spinal cord reflex responses depend on ongoing activity in interneurons [Davidoff, 1992].

Fig. 5-1 Muscle spindles and Golgi tendon organs are encapsulated structures found in skeletal muscle.

The main skeletal muscle fibers, or extrafusal fibers, are innervated by large-diameter alpha motor axons. The muscle spindle has a fusiform shape and is arranged in parallel with extrafusal fibers. It is innervated by afferent and efferent fibers. The Golgi tendon organ is found at the junction between a group of extrafusal fibers and the tendon; it is therefore in series with extrafusal fibers. Each tendon organ is innervated by a single afferent axon.

(Adapted from Houck JC, Crago PE, Rymer WZ. Functional properties of the Golgi tendon organs. In: Desmedt JE, ed. Spinal and supraspinal mechanisms of voluntary motor control and locomotion. Progress in Clinical Neurophysiology, vol. 8. Basel: Karger, 1980:33.)

Stationed in all areas of the skeletal muscle is the muscle spindle, a fusiform-shaped receptor structure (Figure 5-2). The spindle is composed of contractile fibers at each end and a capsule covering a central fluid-filled dilatation. Sensory endings wrap around the central sections of the intrafusal fibers and monitor the stretch of these fibers; they communicate through the afferent axons that are described later in this chapter. Through efferent axons, gamma neurons within the anterior horn of the spinal cord innervate the contractile muscle portions on each end of the intrafusal fiber and enhance the sensitivity of the sensory endings to stretch [Gordon and Ghez, 1991]. Gamma motor neurons that innervate muscle spindles comprise the fusimotor system.

Fig. 5-2 The main components of the muscle spindle are intrafusal fibers, sensory endings, and motor axons.

A, The intrafusal fibers are specialized fibers; their central regions are not contractile. The sensory endings spiral around the central regions of the intrafusal fibers and are responsive to stretch of these fibers. Gamma motor neurons innervate the contractile polar regions of the intrafusal fibers. Contraction of the intrafusal fibers pulls on the central regions from both ends and increases the sensitivity of the sensory endings to stretch. B, The muscle spindle contains three types of intrafusal fibers: dynamic nuclear bag, static nuclear bag, and nuclear chain fibers. A single group Ia afferent fiber innervates all three types of intrafusal fiber, forming a primary ending. A group II afferent fiber innervates chain and static bag fibers, forming a secondary ending. Two types of efferent axons innervate different intrafusal fibers. Dynamic gamma motor axons innervate only dynamic bag fibers; static gamma motor axons innervate various combinations of chain and static bag fibers.

(A, Adapted from Hullinger M. The mammalian muscle spindle and its central control. Rev Physiol Biochem Pharmacol 1984;101:1; B, Adapted from Boyd IA. The isolated mammalian muscle spindle. Trends Neurosci 1980;3:258. Reproduced with permission from Elsevier.)

The intrafusal muscle fibers are divided into three types: nuclear chain fibers, dynamic nuclear bag fibers, and static nuclear bag fibers. These fibers derive their names from the configuration of their nuclei in the fiber center. Chain fibers have nuclei arranged in a single column, whereas bag fibers have nuclei aligned in rows of two or three. A solitary Ia afferent fiber provides primary sensory innervation for all three types of intrafusal fibers. A group II afferent fiber innervates chain and static bag fibers providing secondary sensory endings. The various sensory endings on the different types of intrafusal fibers have different sensitivities to rate of change of length. Dynamic gamma motor axons innervate the contractile portions of dynamic nuclear bag fibers, and static gamma motor axons innervate the contractile portions of the static bag fibers [Gordon and Ghez, 1991]. This intricate system of muscle spindle innervation allows the muscle stretch receptors to monitor muscle tension, length, and velocity of stretch, and provide input for maintenance of tone [Carew, 1985].

It is through their effect on the gamma motor neuron that portions of the CNS (i.e., motor cortex, thalamus, basal ganglia, vestibular nuclei, reticular formation, and cerebellum) modify tone, with ensuing hypotonia or hypertonia (i.e., spasticity) [Alexander and Delong, 1985; Brooks and Stoney, 1971; Carew, 1985; Ghez, 1985].

The Golgi tendon organs, unlike the muscle spindles, are found in series with the skeletal muscle fibers (Figure 5-3), and are attached at one end to the muscle and at the other to the tendon. A number of individual skeletal muscle fibers enter a Golgi tendon organ through a constricted collar. The muscle fibers are attached to collagen fibers within the Golgi tendon organ. A single Ib axon enters each capsule and forms branches that are interlaced among the collagen fibers. The afferent axon branches are compressed when muscle contraction occurs and impulses are transmitted. Tendon organs are much more sensitive to muscle contraction than muscle spindles. Conversely, tendon organs are much less sensitive to stretch than muscle spindles. Each of these relative sensitivities plays a specific role during the performance of various motor tasks [Gordon and Ghez, 1991].

Fig. 5-3 Golgi tendon organs are specialized structures found at the junctions between muscle and tendon.

Collagen fibers in the tendon organ attach to the muscle fibers. A single Ib afferent axon enters the capsule and branches into many unmyelinated endings that wrap around and between the collagen fibers. When the tendon organ is stretched (usually because of contraction of the muscle), the afferent axon is compressed by the collagen fibers (inset) and increases its rate of firing.

(Adapted from Schmidt RF. Motor systems. In: Schmidt RF, Thews G, eds. Human physiology [Biederman-Thorson MA, translator]. Berlin: Springer, 1983:81; inset, adapted from Swett JE, Schoultz TW. Mechanical transduction in the Golgi tendon organ: a hypothesis. Arch Ital Biol 1975;113:374.)

Evaluation of the Patient

History

The age at which hypotonia is first evident may be diagnostically crucial. The presence of hypotonia at birth or shortly thereafter can help differentiate among a number of conditions [Brooke et al., 1979; Dubowitz, 1985]. The presence or absence of weak fetal movements or the change from apparently normal fetal movements to those of decreased amplitude and vigor should be determined. Hypotonia in the setting of polyhydramnios signals prenatal interference with swallowing.

A careful genetic history must be sought because several conditions characterized by hypotonia are hereditary. In a retrospective review of hypotonic infants, Birdi et al. reported a family history of neuromuscular or neurologic disorders in almost half [Birdi et al., 2005].

Examination

Preterm infants, even when healthy, are normally hypotonic relative to a term newborn; therefore, corrected ages must be considered when assessing preterm infants during the first months of life. The finding of fixed contractures in the neonatal period suggests that hypotonia is associated with primary disorder of bone or muscle or an antenatal insult.

The infant’s tendency to assume unusual postures may indicate the presence of hypotonia – especially the “frogleg” position, in which the supine infant lies with the lower limbs externally rotated and abducted. Hypotonia is often associated with generalized weakness, with resultant poor suck, cry, and respiratory effort in addition to a paucity of spontaneous limb movements. Weakness should be suspected if the infant does not briskly withdraw a limb or cannot sustain the raised limb position in response to painful stimuli.

Tone should be assessed both in the active state and when the neonate is at rest; active tone of the extremities is normally higher than passive tone. Passive pronation, supination, flexion, and extension of the limbs and gently shaking the hands and feet are the best ways to assess tone. The hands move over a large amplitude when the arms are shaken gently at the wrists. Often, in the hypotonic infant, the elbows can be extended beyond their normal range. The scarf sign involves wrapping the infant’s arm across the chest toward the neck on the contralateral side and is positive when the elbow can be readily moved beyond the midline. While abnormal in the term infant, it can normally be seen in preterm infants. The traction maneuver is one of the best means to evaluate tone, since it allows simultaneous evaluation of head control, flexion of elbows during infant participation, and general body and back posture (see Figure 3-7). The hypotonic infant’s foot can be brought to the opposite ear, and extreme passive foot dorsiflexion may be possible when hypotonia is profound.

The hypotonic infant will slip through the hands of the examiner when held under the axillae (i.e., vertical suspension maneuver). If the hypotonic infant is supported by the trunk in an outstretched prone position (i.e., horizontal suspension maneuver), gravity causes flexion, or droop of the head and extremities (“inverted comma”). The normal response is anti-gravity with neck extension, straight back and limb flexion.

Weakness of facial muscles, weakness of muscles necessary for adequate suck and swallow, and paresis of the eyelid levators and extraocular muscles are often associated with genetic myopathies. The tongue should be carefully examined for atrophy and fasciculations. Evaluation of muscle weakness can be facilitated with the traction maneuver and by ascertaining the withdrawal response to appropriate stimuli and the ability to resist gravity. Paucity of movement signals the likely presence of concomitant weakness. If limb weakness is present, localization of the weakness to the proximal or distal extremities should be attempted. Older children may present with talipes planus, pronation at the ankles, and genu recurvatum.

The pectus excavatum deformity and a bell-shaped chest indicate relative weakness of intercostal muscles compared to better-preserved strength of the diaphragm during respiratory efforts. Skeletal deformities and fixed contractures are often present in congenital myotonic dystrophy and some congenital myopathies. Fixed contractures of the limbs may signal the presence of arthrogryposis multiplex congenita, which may result from dysfunction at a number of lower motor neuron unit sites [Lebenthal et al., 1970; Yuill and Lynch, 1974] (see Chapter 88).

Fasciculations of limb muscles are difficult to observe in infants because of abundant subcutaneous tissue. However, the experienced examiner can usually palpate the underlying muscle beneath the fat and estimate the adequacy of muscle bulk.

Deep tendon reflexes should always be elicited at all ages. The triceps reflex may be difficult to elicit in preterm and term newborns; reflexes are variably present at the biceps, easier to obtain at the pectoralis, and usually present at the patellar and Achilles tendons.

Acute onset of progressive, profound weakness and hypotonia in previously normal infants suggests the possibility of infantile botulism [Infant botulism, 2003; Kao et al., 1976; Pickett et al., 1976; Ravid et al., 2000; Thompson et al., 1980]. There is almost always accompanying constipation, poor feeding, and bulbar involvement.

In addition to most of the previously mentioned characteristics, hypotonia in the ambulatory child may manifest with a waddling gait, genu recurvatum, and talipes planus. There may be pronation of the feet at the ankles. The presence of scoliosis suggests associated weakness and neuromuscular disease.

Weakness is often readily diagnosed in the infant and younger child by observation; in the older child, more formal and discrete muscle testing is possible, as described in Chapter 2. Examination also includes deep tendon reflexes, plantar reflexes, myotonic response to percussion and scrutiny of muscles for evidence of fasciculations.

When the lower motor unit is involved, the deep tendon reflexes range from hypoactive to absent. The reflexes are uniformly absent in infantile spinal muscular atrophy [Bundey and Lovelace, 1975; Smith and Swaiman, 1983]. Reflexes tend to be increased or normal when the upper motor unit is involved, but may be decreased in the case of acute injury or concomitant involvement of the basal ganglia or their output tracts.

Further neurologic examination is necessary and should include the search for fasciculations, ptosis, squint, myotonia, and extensor plantar reflexes. The presence of squint or ptosis suggests the possibility of congenital myopathies [Clancy et al., 1980; McComb et al., 1979; Riggs et al., 2003], myotonic dystrophy, myasthenia gravis [Holmes et al., 1980; Namba et al., 1970], or mitochondrial myopathies (see Chapters 37 and 93). Knowledge about the congenital myopathies has grown considerably during the past decade, and the clinical and genetic complexities have become increasingly evident [Bruno and Minetti, 2004; Kirschner and Bonnemann, 2004].

Specific laboratory studies may be essential in establishing the diagnosis. When lower motor unit diseases are considered, serum enzyme determinations, nerve conduction velocities, electromyography, and nerve or muscle biopsies may be of importance. However, there is increasing reliance for specific diagnosis on genetic analysis, which has been replacing the need for biopsy in cases where the clinical phenotype and other laboratory evidence are convergent. When upper motor unit diseases are involved, a careful history, electroencephalography, evoked potentials, brain imaging, specific endocrine evaluations, and specific enzyme determinations may be required.

Diagnosis

For didactic purposes and simplification, the motor pathway from the motor neuron in the motor strip to the skeletal muscle fiber can be divided into upper and lower motor neuron units. The upper motor neuron (unit) includes the pyramidal neuron in the motor cortex and the myelinated nerve fiber, which traverses the corticospinal tract and eventually terminates in the internuncial pool in the spinal cord adjacent to the anterior horn cell. The lower motor neuron (unit) consists of the anterior horn cell, peripheral nerve, neuromuscular junction, and muscle. Disorders affecting muscular tone are divided into upper and lower motor unit disorders. Combined disorders also occur. It cannot be overstressed that upper motor unit disease may result in increased or diminished muscle tone in infants and young children.

It is important to distinguish whether hypotonia is derived from a central or peripheral etiology. While there can be difficulty in distinguishing the localization, one study examined sensitivity and specificity of findings predictive of primary neuromuscular disorders. These included a history of reduced fetal movements with polyhydramnios, significant impairment or absence of antigravity movements, and presence of contractures [Vasta et al., 2005]. Congenital hypotonia may be extremely difficult to categorize; however, certain characteristics may prove helpful in diagnosis (Table 5-1) [Harris, 2008].

Table 5-1 Differentiation of Central versus Peripheral Causes of Congenital Hypotonia

Characteristic	Central	Peripheral
Weakness	Mild to moderate	Significant (“paralytic”)
Deep tendon reflexes	Decreased or increased	Absent
Placing reaction	Sluggish	Absent
Motor delays	Yes	Yes
Antigravity movements in prone and supine	Some (less than normal)	Often absent
Pull to sit	Head lag (more than normal)	Marked head lag
Cognition/affect	Delayed	Typical
Ability to “build up” tone, e.g., tapping under knees with infant in supine to assist him/her in holding hips in adduction	Yes	No