posterior spinal artery

ANATOMY - Origin: Two posterior spinal arteries (right and left) typically arise from the vertebral arteries (~60%) or from the posterior inferior cerebellar arteries (PICA). They descend along the posterolateral sulci, medial to the dorsal nerve root entry zones. - Reinforcement: Fed by segmental radicular (posterior radicular) arteries from the posterior intercostal, lumbar, and sacral branches of the aorta, and by cervical branches (ascending/deep cervical, vertebral). Because of this plexiform anastomotic network with multiple feeders, the posterior circulation is rich in collateral flow. - Territory: Posterior one-third of the spinal cord. This includes the dorsal columns (fasciculus gracilis and cuneatus), the dorsal horns, and portions of the posterior lateral corticospinal tract in some segments. - Contrast with the anterior spinal artery (ASA): The single ASA arises from the union of branches of both vertebral arteries and supplies the anterior two-thirds (anterior horns, spinothalamic tracts, lateral corticospinal tracts, and autonomic fibers). The ASA is reinforced by the artery of Adamkiewicz (arteria radicularis magna, usually arising at T9–L2, left-sided in most people). The thoracic watershed zone (T4–T6) is the most vulnerable region.

CLINICAL SIGNIFICANCE - Posterior spinal artery syndrome (rare): Infarction or occlusion produces ipsilateral loss of dorsal column functions below the level of the lesion: proprioception, vibration, and fine/discriminative touch. This causes sensory ataxia and a positive Romberg sign. Because the corticospinal and spinothalamic tracts are mostly supplied by the ASA, motor strength and pain/temperature sensation are generally preserved. Segmental dorsal horn involvement may produce local sensory loss and loss of reflexes at the level of the lesion. - Why it is rare: The paired vessels, abundant collaterals, and plexiform pattern make ischemia uncommon. Anterior spinal artery infarction is far more common. - Causes: Atherosclerosis, vertebral artery dissection, vasculitis (for example, polyarteritis nodosa), aortic surgery or dissection, hypotension, embolism, and compression.

BOARD-RELEVANT COMPARISON - Anterior spinal artery syndrome: bilateral loss of motor function (initially flaccid, later spastic), loss of pain/temperature below the lesion, and autonomic dysfunction (bowel/bladder), with preserved dorsal column function (vibration, proprioception, light touch). - Posterior spinal artery syndrome: loss of vibration, proprioception, and fine touch, with preserved motor function and pain/temperature. - Other dorsal column lesions to differentiate: tabes dorsalis (tertiary syphilis, with lightning pains, Argyll Robertson pupils, and Charcot joints), subacute combined degeneration (vitamin B12 deficiency, which affects dorsal columns plus lateral corticospinal tracts), and Friedreich ataxia (GAA repeat expansion in frataxin, with dorsal column, spinocerebellar, and corticospinal degeneration plus hypertrophic cardiomyopathy). - Brown-Séquard syndrome (hemisection): ipsilateral loss of motor function and dorsal column function below the lesion, with contralateral loss of pain/temperature beginning 1–2 levels below.

HIGH-YIELD PEARLS - Dorsal columns are uncrossed in the cord and decussate in the medulla (medial lemniscus), so the deficit is ipsilateral. - Anterior spinal artery syndrome is the classic spinal cord infarct on exams. Posterior spinal artery involvement is the "spared" territory in that setting. - Venous drainage and vascular supply of the spinal cord are frequently tested together with the artery of Adamkiewicz in the context of aortic aneurysm repair, which carries a risk of anterior spinal artery syndrome.