Peripheral Nerves and Clinical Patterns: Dermatomes, Myotomes, and Entrapments

1) Peripheral nerve basics: roots, plexuses, and named nerves

From spinal cord to skin and muscle: the “wiring diagram”

Clinical neuroanatomy becomes usable when you track symptoms along the pathway: nerve root (spinal level) → plexus (mixing of roots) → named peripheral nerve (final distribution) → end organ (skin, muscle, joint, vessel). Symptoms can arise from any point along this chain, and the exam aims to localize the most likely level.

• Nerve root (radiculopathy): often follows a dermatome/myotome pattern; may show reflex change; provoked by spinal loading/position; can include neck/back pain.
• Plexopathy: broader, “non-dermatomal” sensory loss and multi-nerve weakness; often traumatic, inflammatory, or compressive in the thoracic outlet/axilla/groin regions.
• Peripheral mononeuropathy (entrapment/irritation): sensory loss in a named nerve territory; weakness in muscles supplied by that nerve; reflexes often spared unless the nerve carries the reflex arc.

Key plexuses and what “mixing” means clinically

Plexuses redistribute fibers from multiple roots. This is why a single root lesion can affect several named nerves, and why a named nerve lesion does not always map cleanly to a single root.

Named nerves: “signature” territories (high-yield)

Use these as quick pattern-recognition anchors. They are not exhaustive maps; overlap is normal.

• Median nerve: palmar thumb–index–middle and radial half of ring finger; thenar muscles; common entrapment at carpal tunnel and pronator region.
• Ulnar nerve: small finger and ulnar half of ring finger; interossei and ulnar hand intrinsic muscles; common entrapment at cubital tunnel and Guyon’s canal.
• Radial nerve: dorsal radial hand sensation; wrist/finger extensors; entrapment at radial tunnel/posterior interosseous (motor) and superficial radial nerve (sensory).
• Femoral nerve: anterior thigh sensation; quadriceps strength; patellar reflex involvement via L3–L4.
• Obturator nerve: medial thigh sensation; hip adductors.
• Sciatic nerve: posterior thigh; splits into tibial and common fibular; deep gluteal region can irritate.
• Common fibular (peroneal) nerve: dorsum of foot (via superficial fibular), first web space (via deep fibular); dorsiflexion/eversion weakness; vulnerable at fibular neck.
• Tibial nerve: plantar foot sensation; plantarflexors/intrinsics; entrapment at tarsal tunnel.

2) Dermatomes and myotomes for quick screening

How to use dermatomes and myotomes safely

Dermatomes and myotomes are screening tools to estimate root involvement. They are most useful when you test a small set of “sentinel” points and movements consistently, then interpret patterns (single level vs multi-level vs peripheral nerve).

Practical rule: If sensory change is “glove/stocking,” patchy, or inconsistent with a dermatome, consider peripheral nerve, central sensitization, systemic neuropathy, or non-neurogenic causes. If weakness is pain-limited without clear myotomal pattern, re-test after symptom modulation and compare to functional tasks.

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Quick dermatome screen (sentinel points)

Quick myotome screen (key movements)

Test bilaterally, grade effort, and note pain inhibition. Use short lever positions when pain is high, then progress.

Reflexes: fast localization clues

Reflexes add objectivity when pain and effort confound strength testing.

• Biceps: C5–C6
• Brachioradialis: C5–C6
• Triceps: C7
• Patellar: L3–L4
• Achilles: S1

3) Neural tissue mechanics: tension vs compression and symptom behavior

Why symptoms change with position and movement

Neural tissue is sensitive to both mechanical deformation and ischemia/inflammation. Two broad mechanical contributors are:

• Tension (increased neural mechanosensitivity): symptoms reproduced by movements that lengthen the nerve bed; often shows a “chain” behavior (e.g., ankle dorsiflexion changes posterior thigh symptoms during a straight leg raise).
• Compression (entrapment/space-occupying): symptoms provoked by sustained positions, external pressure, or narrowing of anatomical tunnels; may worsen at night or with repetitive use; may show local tenderness over the nerve.

Symptom behavior cues you can ask and test

• Time course: night symptoms and shaking the hand suggests carpal tunnel; symptoms after prolonged elbow flexion suggests cubital tunnel; symptoms after crossing legs suggests fibular nerve compression.
• Distribution: named nerve territory vs dermatomal; remember overlap and “double crush” (proximal irritation increases distal vulnerability).
• Mechanical sensitivity: symptoms reproduced by neurodynamic testing and altered by sensitizers (adding/removing a joint movement remote from the symptomatic area).
• Motor vs sensory: pure motor deficits (e.g., posterior interosseous nerve) can mimic tendon issues; pure sensory (e.g., superficial radial nerve) can mimic local skin pain.

Neurodynamic testing: practical step-by-step principles

Neural provocation tests are not “positive/negative” in isolation. They are interpreted by reproduction of familiar symptoms, side-to-side difference, and structural differentiation (changing a remote segment alters symptoms).

• Step 1: Baseline — Ask the patient to describe their typical symptoms (location, quality, intensity).
• Step 2: Position gradually — Move slowly into the test sequence until symptoms appear or resistance increases.
• Step 3: Confirm familiar symptom — “Is this the same symptom you get?”
• Step 4: Structural differentiation — Add a remote movement that should change neural load without stressing the local tissue (e.g., cervical side-bending during ULNT; ankle movement during SLR).
• Step 5: Compare sides — Note range, symptom onset, and intensity differences.
• Step 6: Stop criteria — Stop if severe pain, progressive neurological deficit signs, marked irritability, or patient distress.

Common upper limb neurodynamic tests (ULNT) targets: median-biased, ulnar-biased, radial-biased sequences. Common lower limb: straight leg raise (SLR) and slump test for sciatic/tibial bias; prone knee bend for femoral bias.

4) Common entrapment patterns and differential considerations

Carpal tunnel syndrome (median nerve at wrist) vs cervical referral/radiculopathy

Carpal tunnel pattern often presents with paresthesia in thumb–index–middle (sometimes radial half of ring), worse at night, with gripping/repetitive wrist use. Thenar weakness may appear later. Symptoms may be relieved by shaking the hand.

Cervical radiculopathy (often C6/C7) may present with neck pain or scapular region symptoms, dermatomal sensory change (thumb for C6, middle finger for C7), myotomal weakness, and reflex changes. Symptoms often change with neck position and loading.

Practical differential steps (in clinic sequence):

• Screen cervical ROM and symptom response (does neck position change hand symptoms?).
• Check dermatomes C6–C8 and myotomes C6–T1; include reflexes.
• Test median nerve territory sensation and thenar strength (abductor pollicis brevis).
• Use neural provocation: median-biased ULNT with structural differentiation (cervical side-bending).
• Use local wrist provocation (e.g., sustained wrist flexion/extension or compression) and compare with proximal provocation.
• Consider double crush if both neck and wrist tests are contributory.

Ulnar neuropathy: cubital tunnel vs Guyon’s canal

Cubital tunnel (elbow) often worsens with prolonged elbow flexion (phone use, sleeping with bent elbow). Sensory symptoms in small finger and ulnar ring finger; intrinsic weakness may appear.

Guyon’s canal (wrist) may be linked to cycling/handlebar pressure or wrist masses; sensory changes may spare the dorsal ulnar hand (because dorsal ulnar cutaneous branch leaves proximal to the canal).

• Key clue: dorsal ulnar hand sensation involved suggests lesion proximal to Guyon’s canal.
• Motor focus: interossei weakness (finger ab/adduction) and Froment-type pinch compensation can indicate ulnar motor involvement.

Radial nerve patterns: radial tunnel vs posterior interosseous vs superficial radial nerve

• Radial tunnel: aching lateral forearm pain, often tender; may mimic lateral elbow tendinopathy; usually minimal sensory loss.
• Posterior interosseous nerve (PIN): motor deficit (finger/thumb extension weakness) with little/no sensory change.
• Superficial radial nerve: sensory symptoms over dorsoradial hand; provoked by tight straps/watches or repetitive pronation/supination.

Differential tip: if pain is the main complaint near the lateral elbow, test resisted middle finger extension and forearm supination (can provoke radial tunnel), but also compare with local tendon palpation and loading; add radial-biased ULNT with structural differentiation.

Peroneal (common fibular) nerve irritation at fibular neck vs L5 radiculopathy

Common fibular nerve irritation is classically associated with compression at the fibular neck (leg crossing, prolonged kneeling/squatting, tight casts/boots). It can cause dorsiflexion and eversion weakness and sensory change over the dorsum of the foot and lateral shin (via superficial fibular), with possible first web space involvement (deep fibular).

L5 radiculopathy may present with back/buttock pain, symptoms down the lateral leg to dorsum of foot, weakness in L5 myotome (dorsiflexion, great toe extension, hip abduction), and may show positive slump/SLR behavior. Reflex changes are less consistent for L5 (no single classic deep tendon reflex), so strength/sensation patterns and provocation become more important.

Practical differential steps:

• Ask about external compression and recent weight loss/immobility (risk for fibular nerve palsy).
• Test L5 myotome broadly: great toe extension and hip abduction in addition to ankle dorsiflexion.
• Check sensation: first web space (deep fibular) vs broader dorsum (superficial fibular) vs L5 dermatome.
• Use slump/SLR with structural differentiation (neck flexion/ankle dorsiflexion changes symptoms suggests neural mechanosensitivity).
• Palpate/percuss at fibular neck for local reproduction (Tinel-type response).

Tarsal tunnel (tibial nerve) vs plantar fasciopathy vs S1 referral

• Tarsal tunnel: burning/tingling plantar foot, worse with prolonged standing/walking; may be provoked by sustained ankle eversion/dorsiflexion or local compression posterior to medial malleolus.
• Plantar fasciopathy: focal heel pain with first steps, local tenderness at medial calcaneal tubercle; typically not tingling.
• S1 referral: may include calf symptoms, lateral foot sensory changes, Achilles reflex change, and neurodynamic sensitivity.

Structured neuro screen template (sensation, strength, reflexes, provocation, red flags)

When to run a full screen

Perform a structured neuro screen when symptoms include numbness/tingling, burning pain, radiating pain, unexplained weakness, clumsiness, night symptoms, or when the distribution is unclear. Re-screen if symptoms change rapidly or after an intervention that could alter neural load.

Template you can copy into notes

NEURO SCREEN (Upper / Lower limb)  Date: ___  Side: R / L  Irritability: low / mod / high  Dominant hand/foot: ___  Key complaint: ___  Onset/mechanism: ___  1) SENSATION (light touch / pinprick as appropriate)  - Dermatomes (sentinel points): C5 ___ C6 ___ C7 ___ C8 ___ T1 ___ / L2 ___ L3 ___ L4 ___ L5 ___ S1 ___ S2 ___  - Named nerve territory check (if indicated): median ___ ulnar ___ radial ___ tibial ___ fibular ___  - Notes: hypoesthesia / hyperesthesia / allodynia / normal; map area: ___  2) STRENGTH (myotomes + key peripheral muscles)  - Myotomes: C5 abd ___ C6 flex/ext ___ C7 ext ___ C8 grip/flex ___ T1 interossei ___ / L2 hip flex ___ L3 knee ext ___ L4 DF ___ L5 EHL ___ S1 PF ___  - Peripheral “signature” muscles (if indicated): APB (median) ___ FDI (ulnar) ___ wrist/finger extensors (radial/PIN) ___ peroneals ___ tibialis anterior ___  - Notes: pain-limited? true weakness? fatigability? ___  3) REFLEXES  - Biceps (C5-6) ___  Brachioradialis (C5-6) ___  Triceps (C7) ___  Patellar (L3-4) ___  Achilles (S1) ___  - Notes: 0/1+/2+/3+/4+, asymmetry, clonus: ___  4) NEURAL PROVOCATION / MECHANOSENSITIVITY  - ULNT median ___ ulnar ___ radial ___ (symptom reproduction? structural differentiation?)  - SLR ___ Slump ___ Prone knee bend ___  - Local entrapment provocation (as indicated): wrist compression/flexion ___ elbow flexion ___ fibular neck Tinel ___ tarsal tunnel compression ___  - Notes: familiar symptoms Y/N; onset angle; side difference; easing factors: ___  5) RED FLAGS / URGENT INDICATORS (screen and document)  - Progressive motor weakness (new/worsening foot drop, hand intrinsic wasting): Y/N  - Bilateral neuro symptoms or gait disturbance not explained by MSK: Y/N  - Bowel/bladder changes or saddle anesthesia: Y/N  - Systemic red flags (fever, unexplained weight loss, cancer history, infection risk): Y/N  - Severe unremitting night pain not mechanical: Y/N  - Suspected fracture/major trauma with neuro deficit: Y/N  - If any Y: escalate/urgent referral per local policy.

Interpretation shortcuts (pattern logic)

• Dermatome + myotome + reflex align → higher suspicion of root involvement.
• Named nerve sensory + named nerve motor with minimal reflex change → higher suspicion of mononeuropathy/entrapment.
• Neurodynamic test reproduces familiar symptoms and changes with structural differentiation → neural mechanosensitivity likely contributing (does not specify level alone).
• Disproportionate pain, widespread non-anatomical sensory change, or inconsistent findings → broaden differential (central sensitization, systemic neuropathy, vascular, metabolic, functional overlay) and screen red flags.