conus medullaris

ANATOMY: The conus medullaris is the terminal portion of the spinal cord containing sacral (S2-S5) and coccygeal spinal segments. It ends at the L1-L2 vertebral level in adults (about L3 in neonates, because the vertebral column grows faster than the cord, the "ascent" of the cord). Below it, the filum terminale (a pial extension) anchors the cord to the coccyx, and the cauda equina (lumbar, sacral, and coccygeal nerve roots) continues within the lumbar cistern (subarachnoid space). The lumbar cistern, below L2, is where lumbar puncture is performed (L3-L4 or L4-L5, at the level of the iliac crests) to avoid cord injury.

CONUS MEDULLARIS SYNDROME (CMS): Caused by compression or injury of the sacral cord segments and lumbar roots at about T12-L2. Causes: disc herniation, vertebral fracture/burst fracture (T12-L1 trauma), tumors (ependymoma, especially myxopapillary ependymoma of the conus/filum terminale; metastases), epidural abscess or hematoma, ischemia (anterior spinal artery or artery of Adamkiewicz occlusion), infections, and demyelination. Clinical features: - Onset: often sudden, bilateral, and symmetric. - Pain: usually mild and bilateral (not severe radicular). - Saddle (perineal) anesthesia: symmetric. - Early and severe bowel/bladder dysfunction: urinary retention with overflow incontinence, fecal incontinence, loss of anal sphincter tone, and loss of the bulbocavernosus and anal wink reflexes. Erectile dysfunction. - Motor: mild or variable leg weakness; mixed UMN and LMN signs. Hyperreflexia and an upgoing Babinski can occur if upper conus involvement is present, but the Achilles reflex is often lost. - Sensory: sensory dissociation may occur.

CONUS MEDULLARIS vs CAUDA EQUINA SYNDROME: - Lesion: conus = spinal cord (S2-S5 segments); cauda equina = nerve roots below L2 (peripheral nerves). - Onset: conus = sudden, bilateral; cauda equina = gradual, often unilateral or asymmetric. - Pain: conus = less severe, bilateral; cauda equina = severe radicular pain. - Sensory: conus = symmetric saddle anesthesia; cauda equina = asymmetric saddle anesthesia. - Motor: conus = mild, may have UMN and LMN signs; cauda equina = LMN signs (flaccid paralysis, marked weakness, hyporeflexia/areflexia). - Bladder/bowel: both involved, but conus is early and prominent; cauda equina is later and variable. Both lesions can coexist because the conus and cauda equina are anatomically adjacent.

WORKUP AND MANAGEMENT: This is a neurosurgical emergency. Obtain an urgent MRI of the lumbosacral spine (with and without contrast) and check a post-void residual bladder scan. Treatment is urgent surgical decompression for compressive causes, high-dose steroids for tumor-related edema, antibiotics plus drainage for abscess, and radiation or chemotherapy for malignancy. Outcomes depend on how quickly decompression occurs.

HIGH-YIELD BOARD PEARLS: - Cord ends at L1-L2 (adult); dural sac ends at S2. - Bowel/bladder dysfunction and saddle anesthesia are red flags for CMS or cauda equina syndrome and call for emergent MRI. - Myxopapillary ependymoma arises in the conus/filum terminale. - Spinal cord ascent explains why spinal levels do not match vertebral levels in the lower spine. - Conus lesions involve S2-S4 ("S2, 3, 4 keeps the poop and pee off the floor"), affecting the micturition reflex, anal sphincter, and sexual function.