Brown-Séquard syndrome is a set of symptoms that arise from a hemisection (section on the right or left side) of the spinal cord (spinal hemisection).
Since the spinal cord is composed of both descending motor neurons and ascending sensory neurons, which partially cross to the other side of the cord, a simple hemilateral spinal cord injury results in a clinical picture with both sides of the affected body:
On the injured side, there is loss of motor function, proprioception (perception of the position of limbs and joints) and epicritic sensitivity (perception of vibration and fine touch).
On the opposite side there is loss of protopathic sensitivity (gross perception of pain, temperature and pressure).
The syndrome most often occurs with a rupture or injury to the spinal cord, and it also occurs, less frequently, with spinal canal tumors.
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To understand the symptoms present in the syndrome, it is necessary to understand the anatomy and function of the nervous tracts that pass through the spinal cord.
The ascending tracts perform the functions of protopathic sensitivity, epicritic sensitivity, and proprioceptive sensitivity.
Afferent nerves for protopathic sensitivity (coarse perception of pressure, temperature and pain) penetrate the spinal cord through the dorsal nerve roots and cross, still in the same segment, to the other side (contralateral). Thereafter, they form the spinothalamic tract (composed of the lateral spinothalamic tract and the anterior spinothalamic tract), which runs to the thalamus and then connects with the somatosensory cortex of the brain.
Afferent nerves for epicritic sensitivity (fine perception of touch and vibration) and for conscious proprioceptive sensitivity (perception of the position of limbs and joints) ascend posteriorly through the dorsal column-medial lemniscus system in the posterior funiculus (composed of the gracilis and cuneiform fasciculus) on the same side (ipsilateral) from the spinal cord to the brain, crossing to the other side only in the spinal bulb.
Afferent nerves for unconscious proprioceptive sensitivity are part of the spinocerebellar tract (composed of the anterior spinocerebellar tract and the posterior spinocerebellar tract). Both the anterior and posterior tracts ultimately transmit information to the cerebellum ipsilaterally. The anterior spinocerebellar tract performs a double crossing, whereas the posterior spinocerebellar tract does not cross.
The descending nervous tracts perform the functions of motricity (fine and gross movement) and the autonomic nervous system.
Between 70 and 90% of the nerve fibers of the pyramidal pathways (responsible for fine motor skills) cross before the spinal cord in the medulla oblongata and descend through the lateral funiculus as the lateral corticospinal tract (1a). The remaining nerve fibers descend ipsilaterally through the anterior funiculus as the anterior corticospinal tract (1b) and do not cross or cross only in the terminal portion.
The extrapyramidal pathways (2a-d), responsible for gross movement, and the autonomic nervous system pathways also descend ipsilaterally.
Consequences of a hemilion
A spinal cord hemisection causes damage to many nervous tracts, both in tracts that cross it and in tracts that do not cross it, resulting in the occurrence of neurological deficits on both sides. Brown-Séquard syndrome is then characterized by a lesion in each of the three main neural systems: in the corticospinal tract, in the gracilis and cuneiform fasciculus of the dorsal column-medial lemniscus system, in the spinothalamic tract and in the spinocerebellar tract. The bundles of the autonomic nervous system can also be damaged.
Contralateral symptoms are caused by lesions in the tracts that cross the spinal cord. The tracts that cross the spinal cord are the spinothalamic tract and the anterior spinocerebellar tract. As the anterior spinocerebellar tract performs a double crossing, first in the spinal cord and then in the pons, the contralateral symptoms are the exclusive consequence of the spinothalamic tract injury.
Spinothalamic tract injury causes loss of protopathic sensation on the opposite side of the body: gross sensations of pain, temperature, and pressure are impaired.
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