peripheral nerve injury

**Anatomy (inside to outside):** Axon → endoneurium (surrounds individual fibers) → perineurium (surrounds fascicles; blood-nerve barrier) → epineurium (surrounds the whole nerve). Mnemonic: "Endo = Inside, Peri = Partition (fascicles), Epi = Exterior."

**Classification** - **Seddon – Neurapraxia (Sunderland I):** Focal conduction block from demyelination or ischemia (e.g., compression, "Saturday night palsy"). The axon and all connective tissue are intact. There is no Wallerian degeneration. Recovery is complete in days to weeks, as remyelination occurs. - **Seddon – Axonotmesis (Sunderland II–IV):** Axon and myelin are disrupted. Wallerian degeneration follows. - II: endoneurium intact → good recovery. - III: endoneurium disrupted; perineurium intact → variable recovery with possible misdirection. - IV: perineurium disrupted; only the epineurium remains → poor recovery, often needs surgery (scar/neuroma). - **Seddon – Neurotmesis (Sunderland V):** Complete transection of the nerve, including the epineurium. There is no spontaneous recovery. Surgical repair (primary end-to-end repair or nerve graft) is needed. Neuroma formation is possible.

**Pathophysiology** - **Wallerian degeneration (distal stump):** Axon fragmentation, then myelin breakdown. Schwann cells and macrophages clear debris. Schwann cells proliferate to form bands of Büngner, which guide regrowth. - **Proximal changes:** Chromatolysis of the cell body (swelling, peripheral displacement of the nucleus, dispersed Nissl substance). - **Regeneration:** Growth cone advances about 1 mm/day, guided by Schwann cell tubes. Regeneration does not occur in the CNS because oligodendrocytes inhibit regrowth (Nogo) and a glial scar forms. Regeneration requires an intact endoneurial tube. - **Denervated muscle:** Atrophy and fibrosis if reinnervation is delayed (> 12–18 months).

**Mechanisms/Etiologies** - Compression/entrapment (carpal tunnel, cubital tunnel) - Traction/stretch (Erb palsy) - Laceration/transection - Fracture/dislocation (e.g., humeral shaft → radial nerve) - Ischemia (compartment syndrome, diabetic vasa nervorum) - Toxic/metabolic (diabetes, alcohol, B12 deficiency, lead, vincristine) - Iatrogenic (injection, surgery)

**Classic injury–deficit associations (high-yield)** - **Axillary (C5–6):** Surgical neck humerus fracture/anterior shoulder dislocation → deltoid weakness, loss of sensation over the lateral shoulder ("regimental badge"). - **Radial (C5–T1):** Midshaft humerus fracture, "Saturday night palsy," crutches → wrist drop, loss of triceps reflex (if proximal). - **Musculocutaneous (C5–7):** Weak elbow flexion/supination; loss of lateral forearm sensation. - **Median (C5–T1):** Supracondylar humerus fracture → "hand of benediction" (when attempting to make a fist), ape hand, loss of thumb opposition. Carpal tunnel (wrist) → thenar atrophy and weak LOAF muscles (lumbricals 1–2, opponens pollicis, abductor pollicis brevis, flexor pollicis brevis). Anterior interosseous branch → weak "OK sign." - **Ulnar (C8–T1):** Medial epicondyle fracture/cubital tunnel (funny bone) → "claw hand" (more severe with distal lesions), Froment sign, loss of sensation over the medial 1.5 digits. Hook of hamate fracture → distal ulnar injury. - **Long thoracic (C5–7):** Axillary node dissection/mastectomy → winged scapula (serratus anterior). - **Upper trunk (Erb–Duchenne, C5–6):** Shoulder dystocia/fall on shoulder → "waiter's tip" (arm adducted, medially rotated, extended). - **Lower trunk (Klumpke, C8–T1):** Upward traction on the arm/cervical rib/Pancoast tumor → claw hand ± Horner syndrome. - **Common peroneal (L4–S2):** Fibular head fracture/tight cast → foot drop, steppage gait, loss of dorsiflexion/eversion, loss of sensation over the dorsum of the foot and lateral leg. - **Tibial (L4–S3):** Cannot plantarflex or invert; loss of sensation over the sole. - **Superior gluteal (L4–S1):** Intramuscular injection in the wrong gluteal quadrant, hip surgery → Trendelenburg sign (pelvis drops on the contralateral side when standing on the affected leg). - **Femoral (L2–4):** Pelvic fracture, hematoma, surgery → weak knee extension, loss of patellar reflex. - **Sciatic (L4–S3):** Posterior hip dislocation, gluteal injection → weak knee flexion, and loss of all function below the knee. - **Recurrent laryngeal nerve (CN X branch):** Thyroidectomy → hoarseness. - **Facial nerve (CN VII):** Temporal bone fracture, Bell palsy → ipsilateral facial paralysis involving the forehead (LMN pattern).

**Clinical findings** - LMN signs: Weakness, flaccidity, atrophy, fasciculations, decreased or absent reflexes, and sensory loss in a dermatomal or peripheral nerve distribution. - Tinel sign (percussion over the nerve produces tingling) suggests regeneration or neuroma. - Positive Phalen/Tinel signs → carpal tunnel.

**Diagnosis** - Clinical exam is the most important initial step. - EMG/NCS: Best done 2–3 weeks after injury, because Wallerian degeneration takes time. Findings in axonotmesis include reduced CMAP/SNAP amplitude and fibrillation potentials/positive sharp waves on EMG. Neurapraxia shows a conduction block with normal distal conduction. - MRI or ultrasound can localize lesions or identify neuromas.

**Management** - Neurapraxia and mild axonotmesis: Observation, protection, PT, and splinting to prevent contractures. - Open or sharp transection: Early surgical exploration and repair. - Closed injuries without recovery at about 3 months: Surgical exploration. - Neuropathic pain: Gabapentinoids, TCAs, SNRIs. - Late reconstruction: Tendon transfers. - Prevent contractures and pressure sores in the denervated limb.

**Prognosis factors:** Younger age, distal lesions, clean/sharp injuries, early repair, and preservation of connective tissue layers all predict better recovery.