Key Cervical Dermatomes

C2: posterior scalp (occipital neuralgia). C3: posterior neck. C4: cape/shoulder region (shoulder tip sensation — tested in diaphragm irritation from peritoneal blood → referred pain to shoulder = Kehr's sign). C5: lateral arm/deltoid region. C6: lateral forearm, thumb and index finger — tested by biceps reflex (C5-C6). C7: middle finger; tested by triceps reflex (C7-C8). C8: ring + little finger, medial forearm. T1: medial arm. Mnemonic for finger sensory levels: '1 thumb, 2 index/middle, 3 middle, 4 ring/little' maps to C6-C8. Clinical: C6-C7 radiculopathy most common from cervical disc disease (C5-6 and C6-7 disc levels).

Key Thoracic and Lumbar Dermatomes

T4: nipple line. T10: umbilicus (key landmark — used to test level of spinal anesthesia for C-section). T12/L1: inguinal ligament/groin. L2-L3: anterior thigh; L2-L3-L4 = femoral nerve territory. L3-L4: medial knee and leg; tested by patellar reflex (L3-L4). L4: medial lower leg and medial foot. L5: dorsum of foot, big toe, lateral lower leg; tested by great toe extension (EHL); NO reliable reflex for L5 (absence is normal). S1: lateral foot and little toe, heel; tested by Achilles reflex (S1-S2). S2-S4: perineum/saddle area — crucial for identifying cauda equina syndrome. Mnemonic: 'C3-4-5 keeps the diaphragm alive'; 'L3-4 reflex patellar floor'; 'S1-2 reflex Achilles too.'

Deep Tendon Reflexes and Their Levels

Deep tendon reflexes (DTRs) localize cord/root lesions: Biceps: C5-C6 (musculocutaneous nerve). Brachioradialis: C6 (radial nerve). Triceps: C7-C8 (radial nerve). Patellar (knee jerk): L3-L4 (femoral nerve). Achilles (ankle jerk): S1-S2 (tibial nerve). Reflexes are ABSENT (hyporeflexia) with LMN lesions (peripheral nerve injury, nerve root compression). Reflexes are INCREASED (hyperreflexia) with UMN lesions (above the cord level). Inverted reflexes: tapping C5-C6 area causes finger flexion instead of elbow flexion → suggests C5-C6 cord compression with LMN damage at that level and UMN damage below (classic for cervical myelopathy).

Cauda Equina Syndrome

Cauda equina syndrome (CES) = compression of lumbosacral nerve roots (L2-S5) below the conus medullaris (which ends at L1-L2 in adults). Causes: large central disc herniation (L4-5 or L5-S1), epidural abscess or hematoma, spinal tumor, trauma. Classic triad: (1) Saddle anesthesia — numbness over perineum, inner thighs, buttocks (S2-S4 dermatomes). (2) Bladder/bowel dysfunction — urinary retention (most common and earliest sign), fecal incontinence. (3) Lower extremity weakness — bilateral LMN pattern (flaccid, hyporeflexia). EMERGENCY: MRI spine (non-contrast acceptable, contrast preferred to rule out tumor/abscess) → urgent surgical decompression within 24-48 hours. Delayed surgery → permanent incontinence and paralysis. Key distinguisher from conus medullaris syndrome: conus injury has mixed UMN/LMN signs; cauda equina is pure LMN.