Muscle tone disorder

Muscle tone is defined as the residual tension of muscles during their relaxation or as resistance to passive movements during voluntary muscle relaxation ("voluntary denervation"). Muscle tone depends on factors such as the elasticity of muscle tissue, the state of the neuromuscular synapse, peripheral nerve, alpha and gamma motor neurons and interneurons of the spinal cord, as well as supraspinal influences from the cortical motor centers, basal ganglia, facilitatory and inhibitory systems of the midbrain, reticular formation of the brainstem, cerebellum and vestibular apparatus.

Tonus is thus a reflex phenomenon, which is provided by both afferent and efferent components. Muscle tone also has an involuntary component of regulation, which takes part in postural reactions, physiological synkinesis and coordination of movements.

Muscle tone may change with diseases and injuries at different levels of the nervous system. Interruption of the peripheral reflex arc leads to atony. Reduction of supraspinal influences, which usually inhibit spinal reflex systems, leads to its increase. An imbalance of descending facilitating and inhibitory influences can either decrease or increase muscle tone. It is influenced, but to a lesser extent, by mental state and voluntary regulation.

During a clinical examination, it is important to remember that muscle tone is sometimes difficult to assess, since the only reliable instrument for measuring it is the doctor's impressions during passive movement testing. It is affected by the ambient temperature (cold increases and heat decreases muscle tone), the speed of passive movements, and changing emotional states. Much also depends on the doctor's experience, which can also vary. Difficult cases require repeated muscle tone testing with the patient lying down, and the use of special tests (shoulder shake test, head drop test, leg swing test, pronation-supination, and others). It is useful not to rush into categorical tone assessments in unclear, diagnostically difficult cases.

The main types of muscle tone disorders:

I. Hypotension

II. Hypertension

1. Spasticity.
2. Extrapyramidal rigidity.
3. The phenomenon of countercontinence (gegenhalten).
4. Catatonic rigidity.
5. Decorticate and decerebrate rigidity. Hormetonia.
6. Myotonia.
7. Muscle tension (Stiffness).
8. Reflex hypertension: muscular-tonic syndromes in diseases of the joints, muscles and spine; rigidity of the neck muscles in meningitis; increased muscle tone in peripheral trauma.
9. Other types of muscle hypertension.
10. Psychogenic muscular hypertension.

I. Hypotension

Hypotonia is manifested by a decrease in muscle tone below the normal physiological level and is most typical for damage at the spinal-muscular level, but can also be observed in diseases of the cerebellum and some extrapyramidal disorders, primarily in chorea. The range of motion in the joints (their hyperextension) and the amplitude of passive excursions (especially in children) increase. With atony, the specified position of the limb is not maintained.

Diseases affecting the segmental level of the nervous system include poliomyelitis, progressive spinal amyotrophy, syringomyelia, neuropathies and polyneuropathies, as well as other diseases involving the anterior horns, posterior columns, roots and peripheral nerves. In the acute phase of transverse spinal cord injury, spinal shock develops, in which the activity of the cells of the anterior horns of the spinal cord and spinal reflexes is temporarily inhibited below the level of injury. The upper level of the spinal axis, the dysfunction of which can lead to atony, is the caudal parts of the brainstem, the involvement of which in deep coma is accompanied by complete atony and foretells a poor outcome of the coma.

Muscle tone may be reduced in various types of cerebellar damage, chorea, akinetic epileptic seizures, deep sleep, during fainting, states of impaired consciousness (fainting, metabolic coma) and immediately after death.

In attacks of cataplexy, usually associated with narcolepsy, muscle atony develops in addition to weakness. Attacks are often provoked by emotional stimuli and are usually accompanied by other manifestations of polysymptomatic narcolepsy. Rarely, cataplexy is a manifestation of a midbrain tumor. In the acute ("shock") phase of a stroke, the paralyzed limb sometimes exhibits hypotension.

A separate problem is hypotonia in infants (“floppy baby”), the causes of which are very diverse (stroke, Down syndrome, Prader-Willi syndrome, birth trauma, spinal muscular atrophy, congenital neuropathy with hypomyelination, congenital myasthenic syndromes, infant botulism, congenital myopathy, benign congenital hypotonia).

Rarely, post-stroke hemiparesis (with isolated damage to the lentiform nucleus) is accompanied by a decrease in muscle tone.

II. Hypertension

Spasticity

Spasticity develops with any lesions of the cortical (upper) motor neuron and (mainly) the corticospinal (pyramidal) tract. In the genesis of spasticity, an imbalance of inhibitory and facilitatory influences from the reticular formation of the midbrain and brainstem with a subsequent imbalance of alpha and gamma motor neurons of the spinal cord is important. The phenomenon of the "jackknife" is often detected. The degree of hypertonicity can vary from mild to extremely pronounced, when the doctor is unable to overcome spasticity. Spasticity is accompanied by tendon hyperreflexia and pathological reflexes, clonus and, sometimes, protective reflexes and pathological synkinesis, as well as a decrease in superficial reflexes.

In hemiparesis or hemiplegia of cerebral origin, spasticity is most pronounced in the flexor muscles of the arms and extensors of the legs. In bilateral cerebral (and some spinal) injuries, spasticity in the adductor muscles of the thigh leads to characteristic dysbasia. In relatively severe spinal injuries, flexor muscle spasm, spinal automatism reflexes, and flexor paraplegia are more often formed in the legs.

Extrapyramidal rigidity

Extrapyramidal rigidity is observed in diseases and injuries affecting the basal ganglia or their connections with the midbrain and reticular formation of the brainstem. Increased tone affects both flexors and extensors (increased muscle tone of the plastic type); resistance to passive movements is noted with limb movements in all directions. The severity of rigidity may vary in the proximal and distal parts of the limbs, in the upper or lower part of the body, and on the right or left half. At the same time, the "cogwheel" phenomenon is often observed.

The main causes of extrapyramidal rigidity: rigidity of this type is most often observed in Parkinson's disease and other parkinsonian syndromes (vascular, toxic, hypoxic, postencephalitic, posttraumatic and others). In this case, there is a tendency for gradual involvement of all muscles, but the muscles of the neck, trunk and flexors are affected more severely. Muscle rigidity is combined here with symptoms of hypokinesia and (or) low-frequency resting tremor (4-6 Hz). Postural disorders of varying severity are also characteristic. Rigidity on one side of the body increases with active movements of the contralateral limbs.

Less frequently, plastic hypertonus is observed in tonic forms of dystonic syndromes (the debut of generalized dystonia, the tonic form of spasmodic torticollis, foot dystonia, etc.). This type of hypertonus sometimes causes serious difficulties in conducting a syndromic differential diagnosis (Parkinsonism syndrome, dystonic syndrome, pyramidal syndrome). The most reliable way to recognize dystonia is to analyze its dynamics.

Dystonia (a term not intended to describe muscle tone but a specific type of hyperkinesis) is characterized by muscle contractions that lead to characteristic postural (dystonic) phenomena.

The phenomenon of counter-continuity

The phenomenon of counter-continence or gegenhalten is manifested by increasing resistance to any passive movements in all directions. The doctor makes ever greater efforts to overcome the resistance.

Main causes: the phenomenon is observed with damage to the corticospinal or mixed (corticospinal and extrapyramidal) pathways in the anterior (frontal) parts of the brain. The predominance of this symptom (as well as the grasping reflex) on one hand indicates bilateral damage to the frontal lobes with a predominance of damage in the contralateral hemisphere (metabolic, vascular, degenerative and other pathological processes).

Catatonic rigidity

There is no generally accepted definition of catatonia. This form of increased muscle tone is similar in many respects to extrapyramidal rigidity and probably has partially coinciding pathophysiological mechanisms. The phenomenon of "wax flexibility", given "freezing poses" (catalepsy), "strange motor skills" against the background of gross mental disorders in the picture of schizophrenia are characteristic. Catatonia is a syndrome that has not yet received a clear conceptual design. It is unusual in that it erases the boundary between psychiatric and neurological disorders.

Main causes: catatonia syndrome has been described in non-convulsive forms of epileptic status, as well as in some severe organic lesions of the brain (brain tumor, diabetic ketoacidosis, hepatic encephalopathy), which, however, requires further clarification. It is usually characteristic of schizophrenia. In schizophrenia, catatonia is manifested by a complex of symptoms, including mutism, psychosis and unusual motor activity, varying from outbursts of agitation to stupor. Associated manifestations: negativism, echolalia, echopraxia, stereotypies, mannerisms, automatic obedience.

Decorticate and decerebrate rigidity

Decerebrate rigidity is manifested by constant rigidity in all extensors (antigravity muscles), which can sometimes increase (spontaneously or with painful stimulation in a patient in a coma), manifested by forced extension of the arms and legs, their adduction, slight pronation and trismus. Decorticate rigidity is manifested by flexion of the elbow joints and wrists with extension of the legs and feet. Decerebrate rigidity in patients in a coma ("extensor pathological postures", "extensor postural reactions") has a worse prognosis compared to decorticate rigidity ("flexor pathological postures").

Similar generalized rigidity or spasticity with retraction (extension) of the neck and, sometimes, the trunk (opisthotonus) can be observed in meningitis or meningism, the tonic phase of an epileptic seizure, and in processes in the posterior cranial fossa occurring with intracranial hypertension.

A variant of extensor and flexor spasms in a patient in a coma is rapidly changing muscle tone in the limbs (hormetonia) in patients in the acute phase of hemorrhagic stroke.

Myotonia

Congenital and acquired types of myotonia, myotonic dystrophy, paramyotonia and, sometimes, myxedema are manifested by increased muscle tone, which is revealed, as a rule, not during passive movements, but after active voluntary contraction. In paramyotonia, a marked increase in muscle tone is provoked by cold. Myotonia is revealed in a test of clenching fingers into a fist, manifested by a slow relaxation of spasmodic muscles; repeated movements lead to a gradual restoration of normal movements. Electrical stimulation of muscles causes their increased contraction and slow relaxation (the so-called myotonic reaction). Percussion (hammer strike) of the tongue or thenar reveals a characteristic myotonic phenomenon - a "dimple" at the site of impact and adduction of the thumb with slow relaxation of the muscles. Muscles may be hypertrophied.

Muscle tension (stiffness)

Muscle tension is a special group of syndromes, whose pathogenesis is associated primarily with spinal (interneurons) or peripheral lesions (motor unit hyperactivity syndromes).

Isaacs syndrome (neuromyotonia, pseudomyotonia) is manifested by rigidity, which initially appears in the distal parts of the limbs and gradually spreads to the proximal, axial and other muscles (face, bulbar muscles) with difficulty of movement, dysbasia and constant myokymia in the affected muscles.

Stiff-person syndrome, on the other hand, begins with rigidity of the axial and proximal muscles (primarily the muscles of the pelvic girdle and trunk) and is accompanied by characteristic spasms of great intensity in response to external stimuli of different modalities (increased startle reaction).

Closely related to this group of muscular-tonic disorders are McArdle's disease, paroxysmal myoglobulinemia, and tetanus.

Tetanus is an infectious disease characterized by generalized muscle rigidity, although the muscles of the face and lower jaw are involved first. Muscle spasms are characteristic of this disease, occurring spontaneously or in response to tactile, auditory, visual and other stimuli. Between spasms, a pronounced, usually generalized, rigidity persists.

"Reflex" rigidity

“Reflex” rigidity unites syndromes of muscular-tonic tension in response to painful irritation in diseases of the joints, spine and muscles (for example, protective muscle tension in appendicitis; myofascial syndromes; cervicogenic headaches; other vertebrogenic syndromes; increased muscle tone in peripheral trauma).

Other types of muscle hypertonia include muscle rigidity during an epileptic seizure, tetany, and some other conditions.

High muscle tone is observed during the tonic phase of generalized seizures. Sometimes, purely tonic epileptic seizures without a clonic phase are observed. The pathophysiology of this hypertonia is not completely clear.

Tetany manifests itself as a syndrome of increased neuromuscular excitability (symptoms of Chvostek, Trousseau, Erb, etc.), carpopedal spasms, paresthesia. More common are variants of latent tetany against the background of hyperventilation and other psychovegetative disorders. A rarer cause is endocrinopathy (hypoparathyroidism).

Psychogenic hypertension

Psychogenic hypertension is most clearly manifested in the classic picture of a psychogenic (hysterical) seizure (pseudo-seizure) with the formation of a “hysterical arc”, with a pseudo-dystonic variant of psychogenic hyperkinesis, and also (less often) in the picture of lower pseudo-paraparesis with pseudo-hypertonicity in the feet.

Diagnostic studies for muscle tone disorders

EMG, determination of nerve conduction velocity, general and biochemical blood analysis, electrolytes in the blood, CT or MRI of the brain, cerebrospinal fluid analysis. It may be necessary: muscle biopsy, CPK in the blood, consultation with an endocrinologist, psychiatrist.

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