Signs and localization of peripheral paralysis
Peripheral (flaccid) paralysis results from damage to lower motor neurons. Its classic clinical presentation includes muscle hypotonia (or atony), hyporeflexia (or areflexia), and marked muscle atrophy.
Electrophysiological studies—electromyography (EMG) and nerve conduction studies (NCS)—are used for precise instrumental lesion localization.
An essential clinical rule for localization diagnosis:
- If the lesion is located distal to the roots (in nerve plexuses or peripheral nerves), paralysis is invariably accompanied by autonomic and sensory disturbances.
- If the cerebral cortex is affected (which extremely rarely produces flaccid paralysis), tendon reflexes are preserved or hyperactive, and muscle tone is not reduced.
Anatomical features of the spinal cord
Within the spinal cord, motor, sensory, and autonomic fibers are confined to a remarkably tight space. The white matter consists of ascending and descending tracts, while the gray matter forms the horns:
- Anterior horns: clusters of motor neurons.
- Lateral horns: autonomic centers.
- Posterior horns: somatosensory neurons.
During embryogenesis, the spinal cord grows more slowly than the vertebral column. As a result, in adults it terminates at the L1–L2 intervertebral disc level. The nerve roots of the lower thoracic and lumbosacral segments must descend inferiorly, forming the cauda equina.
The spinal cord has two enlargements that innervate the limbs: the cervical (C4–Th1) for the brachial plexus and the lumbar (L2–S3) for the lumbosacral plexus.
Syndromes of gray matter and nerve root lesions
Depending on the lesion's site, specific neurological syndromes develop:
- Anterior horn syndrome: selective loss of motor neurons (e.g., in paralytic poliomyelitis or spinal muscular atrophy). Flaccid paresis develops with prominent atrophy. The pattern of weakness is characteristically "patchy", with proximal limb regions affected more severely than distal ones.
- Posterior horn syndrome: results in a specific dissociated sensory loss. Pain and temperature sensation are lost ipsilaterally, while deep (proprioceptive) and light touch sensations are preserved. This occurs due to disruption of fibers decussating in the anterior white commissure.
- Posterior root syndrome: complete lesion leads to loss of all sensory modalities in the corresponding dermatomes (with pain being most severely affected). When multiple adjacent roots are damaged, the reflex arc is disrupted, leading to hypotonia and areflexia.
- Central gray matter lesion: characteristic of syringomyelia (fluid-filled cavities within the spinal cord). It produces a bilateral "cape-like" or "jacket-like" sensory loss, flaccid paresis at the lesion level, and spastic paresis below the level of the lesion.
Combined pathway lesions
A pathological degenerative process frequently affects multiple spinal cord structures simultaneously, producing combined syndromes:
- Amyotrophic lateral sclerosis (ALS): simultaneous degeneration of upper (cortical) and lower (anterior horn) motor neurons. A paradoxical clinical picture emerges: despite marked muscle atrophy, deep tendon reflexes remain hyperactive due to spasticity. Bulbar palsy with dysarthria and dysphagia develops at later stages.
- Subacute combined degeneration of the spinal cord: a consequence of vitamin B12 deficiency. It predominantly affects the posterior columns and lateral corticospinal (pyramidal) tracts. The patient develops spastic paraparesis with a positive Babinski sign, accompanied by sensory ataxia and loss of joint position and vibration sense.
- Corticospinal tract syndrome: isolated descending degeneration of pyramidal pathways. Seen in primary lateral sclerosis and Strümpell's disease (hereditary spastic paraplegia, associated with mutations in over 40 genes).
Briefly: answers to topical questions
Which nerve fibers are affected in syringomyelia, and what motor impairments does this cause?
Syringomyelia primarily damages decussating sensory fibers; with larger lesions, anterior horn lower motor neurons and descending pyramidal tracts (tractus pyramidalis) in the lateral columns become involved.
This leads to the following motor impairments:
- Flaccid (peripheral) paresis — develops at the level of the affected segments due to destruction of motor neurons.
- Spastic (central) paresis — develops below the level of the lesion due to compression or degeneration of the corticospinal tract.
Which anatomical structures form the cauda equina of the spinal cord?
The cauda equina is composed of the nerve roots of the lower spinal cord segments.
This structure forms below the conus medullaris and includes:
- Lumbar roots — emerging from the corresponding lumbar segments.
- Sacral roots — emerging from the sacral segments.
- Coccygeal roots — emerging from the coccygeal segment.
How can you easily remember dissociated sensory loss in posterior horn lesions? Picture this: "pain and temperature burn out, but touch remains untouched." Only the decussating spinothalamic fibers are damaged, while deep sensation travels safely upwards via the intact posterior columns.
Class and exam questions
Why are deep tendon reflexes increased in amyotrophic lateral sclerosis (ALS) despite muscle atrophy?
This occurs due to simultaneous degeneration of upper motor neurons (corticospinal tract). The resulting spasticity and hyperreflexia mask the clinical signs of lower motor neuron loss until their complete degeneration.
How can a plexus lesion be clinically distinguished from anterior root damage?
Lesions situated distal to the nerve roots (in the plexuses and peripheral nerves) are always accompanied by prominent sensory and autonomic disturbances within their innervation territory.
Can motor deficits occur in herpes zoster?
Yes. The virus primarily affects dorsal root ganglia (herpes zoster), but EMG studies show that in approximately two-thirds of cases, the infection spreads to the anterior horns, causing a segmental motor deficit. Because it most commonly involves the thoracic region, patients often do not notice it.
What else to study on this topic
- Diagnostic investigations: principles of EMG and NCS
- Posterior column syndromes (tabes dorsalis)
- Clinical features and pathogenesis of syringomyelia and syringobulbia
- Hereditary spastic paraplegia (Strümpell's disease)
- Localization diagnosis of nerve plexus lesions
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