Extrapyramidal disorders

Extrapyramidal paralysis and tone alteration

The pathogenetic basis of extrapyramidal paralysis (or paresis) is damage to the neurons of the extrapyramidal system. As a result, a specific complex of motor disorders is formed, primarily affecting the patient's tone and posture.

The main clinical manifestations include:

• Increase in muscle tone of a rigid type. Tone increases evenly in almost all muscle groups: flexors and extensors, pronators and supinators are equally tense.
• Muscle rigidity. Classic rigid paralysis develops.
• Catalepsy. The term comes from the Ancient Greek word κατάληψις (seizing, holding). This is a condition in which the patient's body or limb freezes for a long time in the position artificially given to it. Catalepsy is always combined with slowed pace and impaired coordination of movements.
• Postural reflexes. They occur when the body's position in space changes. A characteristic example is the appearance of nystagmus of the eyes or head when the patient's body is rotated.

Important differential sign: in extrapyramidal lesions, pathological reflexes are absent and there is no marked hyperreflexia. This is a key criterion that allows distinguishing this pathology from central (pyramidal) paralysis.

Hyperkinesias, convulsions, and tics

Hyperkinesia (from Ancient Greek ὑπέρ — strongly, extremely and κίνησις — movement) is a typical form of neurogenic disorder. It is expressed in excessive, violent movements resulting from impaired regulation of motor acts. Both the volume and the number of involuntary movements increase.

The pathogenesis is based on damage to various structures of the extrapyramidal system, which normally provides control of movements, muscle tone, and posture (outside the responsibility of the pyramidal tract). This same group of extrapyramidal disorders includes athetosis and spastic torticollis.

The causes of hyperkinesias are diverse and include both organic and functional disorders:

1. Neurodegenerative diseases (e.g., hereditary autosomal dominant Huntington's chorea).
2. Multiple sclerosis and encephalitis.
3. Intoxications, including side effects of neuroleptic drugs.
4. Brain tumors and traumatic brain injuries.
5. Cerebrovascular accidents.
6. Cerebral palsy.

Convulsions are singled out separately as a typical form of movement pathology. These are sudden, involuntary paroxysmal or constant muscle contractions that vary in intensity, duration, and distribution.

Tics also develop when the extrapyramidal system is damaged. The most frequent etiological factors for tics are stress, psychological trauma, hypovitaminosis, as well as organic brain damage (trauma, encephalitis), vascular disorders, and intoxications (including antipsychotics and neuroleptics).

Myasthenic hypokinesia

Myasthenic (synonyms: neuromuscular, synaptic) hypokinesia unites a group of pathologies, among which the most well-known is myasthenia gravis (myasthenia gravis), as well as other myasthenic syndromes.

The pathogenetic basis of the disease is impaired excitation impulse transmission in cholinergic neuromuscular synapses (at the stage of signal transition from motor axon terminals to skeletal muscle fibers). Transmission is disrupted due to problems with acetylcholine.

Two main development mechanisms are distinguished:

1. Blockade of postsynaptic cholinoceptors by specific antibodies (Ig). Immunoglobulins firmly anchor to the postsynaptic membrane of the muscle fiber and physically prevent the interaction of acetylcholine molecules with the cholinoceptor.
2. Decreased muscle fiber response to acetylcholine. This can be caused by hyposensitization of cholinoceptors to the neurotransmitter or the dominance of low-affinity cholinoceptors on muscle fibers.

Clinically, myasthenic hypokinesia manifests as muscle weakness of varying severity and extremely rapid muscle fatigue upon physical exertion.

Ataxia as a coordination disorder

Ataxia (from Greek ataxia — disorder, movement disturbance) is a typical locomotor disorder characterized by impaired spatial and temporal coordination of voluntary movements. It is strictly unrelated to impaired muscle strength (not accompanied by paresis or paralysis).

A whole complex of neural structures is responsible for coordination in the body: the cerebellum, spinal cord, frontal and temporal lobes of the cerebral cortex, midbrain, thalamus, and labyrinth.

Depending on the localization of the pathological process, the following forms of ataxia are distinguished:

• Cerebellar ataxia — develops with direct damage to cerebellar structures.
• Cortical ataxia — occurs with pathology of the frontal and temporal areas of the cerebral cortex.
• Vestibular ataxia — is associated with damage to the vestibular apparatus.
• Psychogenic ataxia — a specific form observed in severe distress syndrome.

A special place is occupied by sensory ataxia. Its pathogenetic basis is damage to the proprioceptive sensitivity pathways, causing the brain to stop receiving correct information about the body's position in space.

Class and exam questions

What else to study on this topic

• Mechanisms of development of myasthenia gravis
• Pathogenesis and manifestations of autosomal dominant Huntington's chorea
• Classification of hyperkinesias in neurodegenerative diseases
• Differential diagnosis of cortical, cerebellar, and vestibular ataxia
• The role of neuroleptics and antipsychotics in the development of drug-induced tics

Other topics in "Chapter 28. Standard forms of pathology of the nervous system"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.