Clinical meaning
Brown-Séquard syndrome results from lateral hemisection of the spinal cord, producing a characteristic pattern of ipsilateral and contralateral neurological deficits explained by the anatomy of ascending and descending spinal tracts. Three major tracts are affected: (1) the lateral corticospinal tract carries upper motor neuron fibers that have ALREADY decussated at the medullary pyramids -- damage produces ipsilateral spastic paralysis (UMN signs: hyperreflexia, positive Babinski, clonus) below the lesion level; (2) the dorsal columns (fasciculus gracilis and cuneatus) carry ipsilateral proprioception, vibration, and fine touch fibers that ascend uncrossed to the medulla before decussating -- damage produces ipsilateral loss of proprioception, vibration sense, and two-point discrimination below the lesion; (3) the lateral spinothalamic tract carries pain and temperature fibers that enter the cord, ascend 1-2 segments in Lissauer's tract, then cross through the anterior white commissure -- damage produces CONTRALATERAL loss of pain and temperature sensation beginning 1-2 dermatomes below the lesion level. Common etiologies include penetrating trauma (stab wounds -- most classic cause), spinal cord tumors (extramedullary meningiomas, schwannomas), multiple sclerosis plaques, and epidural hematomas. The ASIA (American Spinal Injury Association) Impairment Scale classifies spinal cord injury severity from A (complete) through E (normal), with Brown-Séquard typically classified as ASIA C or D due to preserved contralateral motor function.