Brown-Séquard syndrome results from lateral hemisection of the spinal cord, producing a characteristic pattern of ipsilateral motor paralysis and proprioception loss below the lesion, with contralateral loss of pain and temperature sensation beginning one to two segments below the level of injury. This pattern occurs because the corticospinal tract (motor) and dorsal columns (proprioception, vibration, fine touch) cross at the medulla, so they are affected on the same side as the lesion. The spinothalamic tract (pain, temperature, crude touch) crosses at the spinal cord level of entry, so it is affected on the opposite side. Common causes include penetrating trauma, spinal tumors, and multiple sclerosis.