Carpal Tunnel Syndrome (CTS)

• Most common entrapment neuropathy; median nerve compressed under transverse carpal ligament (flexor retinaculum)
• Symptoms: numbness/tingling in digits 1–3 and radial half of digit 4; nocturnal pain; shake sign (flicking hands for relief)
• Provocative tests: Phalen's (wrist flexion 60 s), Tinel's (tapping at carpal tunnel), Durkan's (direct compression)
• Thenar weakness: APB (abductor pollicis brevis) — late finding; opponens pollicis affected
• Spared: palmar cutaneous branch exits proximal to carpal tunnel → thenar palm sensation preserved in CTS

NCS/EMG in CTS

• Sensory latencies are more sensitive than motor latencies for early CTS
• Focal slowing of median sensory and/or motor conduction at the wrist
• Compare median to ulnar or radial sensory latencies (ring finger comparison study is most sensitive)
• Severe CTS → absent median SNAP → thenar denervation (fibs/PSWs in APB)

Pronator Teres Syndrome

• Median nerve compressed at pronator teres or fibrous arch of FDS
• Key distinguisher from CTS: sensory loss includes thenar palm (palmar cutaneous branch involved)
• Aching forearm pain worsened by pronation; no nocturnal symptoms (unlike CTS)
• NCS: normal at wrist; EMG may show denervation in pronator teres, FCR, FDS

Anterior Interosseous Nerve (AIN) Syndrome

• Pure motor branch of the median nerve — no sensory loss
• Innervates: FPL (flexor pollicis longus), FDP to digits 2–3, pronator quadratus
• Cannot make OK sign: unable to flex DIP of thumb and index finger → pinch is pad-to-pad instead of tip-to-tip
• Causes: Parsonage-Turner syndrome (most common), fibrous bands, accessory muscles

Anatomy & Entrapment Sites

Clinical Signs

• Froment's sign: patient grips paper between thumb and index finger; weak adductor pollicis → compensatory FPL flexion (thumb IP flexion) — median nerve substitution
• Ulnar claw hand: loss of medial lumbricals and interossei → unopposed EDC causes MCP hyperextension; unopposed FDP causes PIP/DIP flexion of digits 4–5
• Ulnar paradox: higher lesions → LESS clawing (FDP to digits 4/5 also denervated, so no unopposed flexion at DIP)
• Wartenberg sign: weak 3rd palmar interosseous with unopposed extensor digiti minimi (abductor moment) → small finger abducted at rest
• Tardy ulnar palsy: delayed-onset ulnar neuropathy at elbow from remote fracture/deformity (cubitus valgus)

Martin-Gruber Anastomosis

• Present in 20–25% of the population; median-to-ulnar nerve crossover in the forearm
• Motor fibers from median nerve cross to ulnar nerve in the forearm → innervate ulnar hand muscles via ulnar nerve distally
• NCS pitfall: On routine ulnar motor study (recording ADM/FDI): proximal (above-elbow or elbow) CMAP is smaller than wrist CMAP, mimicking conduction block. On median motor study recording thenar, stimulation proximally can produce an initial positive deflection (volume conduction from ulnar-innervated muscles activated by crossover fibers).
• Can cause diagnostic confusion in CTS and ulnar neuropathy — apparent conduction block or unusual CMAP drop

Radial Nerve Anatomy & Compression Sites

Key Clinical Points

• Saturday night palsy: wrist drop + finger drop but triceps and brachioradialis usually spared (branches exit above spiral groove)
• Finger extensors extend at MCP joints — interossei extend at IP joints; patient with radial palsy can still extend IP joints via interossei
• PIN vs. radial nerve: PIN spares ECRL → wrist can extend but deviates radially; no sensory loss (pure motor)
• PIN compression at arcade of Frohse; causes include lipoma, synovitis (RA), fracture

Common Peroneal Neuropathy

• Most common compressive neuropathy in the lower extremity; at fibular head (neck of fibula)
• Causes: leg crossing, prolonged bed rest, casts, weight loss, fibular head fracture
• Footdrop (weak ankle dorsiflexion) + weak eversion + weak toe extension
• Sensory loss: dorsal foot + lateral leg (superficial peroneal territory)

Deep vs. Superficial Peroneal

Differential Diagnosis of Footdrop

Femoral Neuropathy

• Motor: quadriceps weakness (knee extension) + iliopsoas may be weak if lesion is proximal
• Sensory: anterior thigh + medial leg (saphenous nerve)
• Reflex: absent/reduced patellar (knee jerk)
• Causes: psoas/iliacus hematoma (anticoagulation!), diabetes, pelvic surgery, hip hyperextension (lithotomy position)
• Hip hyperextension worsens symptoms (stretches femoral nerve over inguinal ligament)
• CT abdomen/pelvis to evaluate for retroperitoneal hematoma

Meralgia Paresthetica

• Lateral femoral cutaneous nerve (L2/L3) — pure sensory
• Compressed under inguinal ligament near ASIS
• Symptoms: burning pain/numbness of anterolateral thigh; no motor deficit, no reflex change
• Risk factors: obesity, tight belts/clothing, pregnancy, diabetes
• Treatment: remove offending compression, weight loss; local nerve block; usually self-limited

Tarsal Tunnel Syndrome

• Tibial nerve compressed behind the medial malleolus under the flexor retinaculum
• Symptoms: burning pain/numbness of the sole of the foot (medial and lateral plantar nerves)
• Tinel's sign at medial malleolus; worsened by prolonged standing/walking
• NCS: prolonged distal latency or reduced amplitude of medial/lateral plantar nerves

Sciatic Neuropathy

• Two divisions within one sheath: peroneal (lateral) and tibial (medial)
• Peroneal division more vulnerable (lateral position, tethered at fibular head, less connective tissue protection)
• Short head of biceps femoris: only muscle above the knee innervated by the peroneal division of the sciatic nerve — if denervated on EMG, localizes to sciatic (not peroneal at fibular head)
• Causes: hip arthroplasty, posterior hip dislocation, deep IM injection (gluteal), piriformis syndrome

Brachial Plexus Anatomy

Classic Brachial Plexus Syndromes

Parsonage-Turner Syndrome (Neuralgic Amyotrophy)

• Acute onset severe shoulder/arm pain → followed by flaccid weakness days to weeks later
• Upper plexus/trunk most commonly affected (C5/C6); patchy, multifocal distribution possible
• Antecedents: viral illness, vaccination, surgery, postpartum
• Long thoracic nerve (serratus anterior → winged scapula) is classically involved
• EMG: multifocal denervation of upper trunk muscles; may see involvement outside single trunk/cord
• Prognosis: self-limited; ~80–90% recover good function over 1–3 years
• Treatment: pain management (acute phase); PT for range of motion

Neoplastic vs. Radiation Plexopathy

Neurogenic Thoracic Outlet Syndrome (nTOS)

• Mechanism: compression of the lower trunk (C8/T1) by a cervical rib, anomalous fibrous band, or hypertrophied scalene muscle.
• Demographics: typically young women; insidious onset of medial forearm/hand pain & paresthesia, thenar > hypothenar atrophy ("Gilliatt-Sumner hand"), C8/T1 sensorimotor pattern.
• NCS: the medial antebrachial cutaneous (MAC) SNAP is the earliest abnormality (low or absent); median CMAP recording over APB is reduced; ulnar SNAP often low; ulnar CMAP relatively preserved.
• EMG: chronic denervation/reinnervation of T1-innervated thenar muscles (APB out of proportion to ulnar intrinsics), with median > ulnar involvement in the hand.
• Imaging: cervical-rib X-ray, MRI/MRA brachial plexus with provocative arm positioning.
• Treatment: physical therapy first-line; surgical decompression (first-rib resection ± scalenectomy) for refractory or progressive deficits.

Axillary & Musculocutaneous Nerves

• Axillary nerve (C5/C6, posterior cord): classic injury in anterior shoulder dislocation or surgical-neck humeral fracture → deltoid + teres minor weakness (loss of arm abduction 15–90°) with sensory loss over the lateral deltoid (“regimental badge”). Always assess deltoid before AND after shoulder reduction.
• Musculocutaneous nerve (C5–C7, lateral cord): rare isolated injury (heavy weight-lifting, surgical retraction, anterior shoulder dislocation) → weak elbow flexion + supination (biceps, brachialis), absent biceps reflex, sensory loss along the lateral forearm via the lateral antebrachial cutaneous nerve.
• Suprascapular nerve (C5/C6, off upper trunk): notched at the suprascapular foramen by repetitive overhead activity (volleyball, swimming) or ganglion cyst → weakness of supraspinatus (initial abduction) and infraspinatus (external rotation), often painless wasting of the posterior shoulder.

Localization Pearls — Ulnar vs. T1 vs. Median

Anatomy

Diabetic Amyotrophy (Diabetic Lumbosacral Radiculoplexus Neuropathy)

• Acute/subacute severe unilateral thigh pain → proximal leg weakness + atrophy; may spread contralaterally
• Predominantly lumbar plexus (L2–L4): quadriceps weakness, absent knee jerk, anterior thigh sensory loss; distal and sacral involvement may occur with progression
• Pathology: ischemic injury from microvasculitis of vasa nervorum (not metabolic)
• Can occur with mild or newly diagnosed diabetes; HbA1c may be only mildly elevated
• Often associated with weight loss
• Self-limited over 6–18 months; some advocate IV methylprednisolone or IVIG (limited evidence)

Other Causes of LS Plexopathy

• Pelvic masses/tumors: colorectal, cervical, prostate, lymphoma — insidious onset, progressive
• Retroperitoneal hemorrhage: anticoagulation, hemophilia — acute lumbar plexus involvement, femoral neuropathy overlap
• Radiation: typically 1–5 years after pelvic radiation; painless; myokymic discharges on EMG (same principle as brachial)
• Surgical injury: lithotomy position (lumbosacral trunk compression), cesarean section, hip replacement
• Idiopathic lumbosacral plexitis: LS equivalent of Parsonage-Turner syndrome