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Neuropraxia (Neurapraxia)

Definition

"The nerve conducts normally distal to but not across the lesion, resulting in conduction block, which is the electrophysiological correlate of neuropraxia." — Bradley and Daroff's Neurology in Clinical Practice

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Pathophysiology

• Temporary mechanical distortion of myelin → segmental demyelination
• No Wallerian degeneration (axons and endoneurial tubes are intact)
• Results in a local conduction block at the injury site
• Nerve conduction is normal distal to the lesion but blocked across it
• Motor function is typically lost, but some sensation and autonomic function may be preserved (larger myelinated fibres are more susceptible than small unmyelinated fibres)

• Compression (e.g., "Saturday night palsy" — radial nerve compression causing wrist drop)
• Displaced fracture or dislocation (stretch/pressure on adjacent nerve)
• Tourniquet palsy
• Iatrogenic: needle trauma, injection pressure (e.g., mental nerve block)

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Classification Context: Seddon & Sunderland

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Clinical Features

• Complete motor paralysis of muscles innervated by the affected nerve
• Preserved or partially preserved sensation and autonomic function (compared to axonotmesis)
• No signs of Wallerian degeneration on nerve conduction studies
• Tinel's sign is absent (no axonal degeneration)

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Electrodiagnostic Findings (EMG/NCS)

• Nerve conduction: Normal distal to the lesion; conduction block across the lesion
• EMG: No fibrillation potentials or positive sharp waves (no denervation, since axons are intact)
• Distal motor latency and conduction velocity: Normal distal to injury for 72–96 hours after injury (axons remain electrically viable distally before degeneration — but in neuropraxia, they remain permanently viable since there is no axonal injury)

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Prognosis & Recovery

• Recovery is always complete if the causative compression/stretch is removed
• Timeline: days to 12 weeks (typically 6–12 weeks for full remyelination)
• Recovery pattern is not proximal-to-distal (unlike axonal regeneration in axonotmesis) — improvement can occur at any point along the nerve simultaneously, because axons are intact

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Management

• Non-operative in most cases — remove the offending cause (reduce fracture, release compression, remove tourniquet)
• Physiotherapy to maintain joint mobility and prevent contractures during recovery
• Serial clinical and electrophysiological monitoring
• If no recovery by 3 months, re-evaluate (may indicate a worse injury grade than initially suspected)
• Surgical exploration is not indicated for isolated neuropraxia

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Clinical Examples

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Cervical Cord Neuropraxia (Special Context)

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Axonotmesis

Definition

"Axonotmesis is the term used for a more severe blunt injury to a nerve, which causes disruption of the axons and myelin sheaths with consequent axonal degeneration." — Rockwood and Green's Fractures in Adults, 10e

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Pathophysiology

Immediate events (at the injury site)

• Axonal continuity is lost at the injury site
• The severed axon undergoes retrograde die-back to the nearest node of Ranvier proximally
• The cell body (perikaryon) undergoes chromatolysis: nuclear peripheralization, rough ER fragmentation, and perikaryal swelling — a proregenerative response

Wallerian Degeneration (distal to injury)

• Occurs within 72–96 hours of axonal disruption
• The distal axon and myelin sheath fragment and degenerate
• Schwann cells and macrophages phagocytose myelin and cellular debris
• Distal end organs (muscle fibers, sensory receptors) become denervated
• The nerve does not conduct at or distal to the injury zone

Distal nerve tube changes

• Schwann cells proliferate and switch to a pro-regenerative phenotype (upregulate neurotrophic factors, form Bands of Büngner)
• However, if the distal stump remains denervated >1 month, Schwann cells begin to atrophy and lose this phenotype
• Endoneurial tube fibrosis and shrinkage progressively obstruct axonal regrowth

Denervated muscle changes

• ~30% of muscle weight lost in the first month

• 60% loss by the fourth month

• Muscle fiber atrophy, sarcoplasm loss, and denervation hypersensitivity (upregulation of nicotinic ACh receptors beyond the motor endplate)

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Nerve Regeneration

• The proximal axon forms multiple growth cones (regenerative sprouting)
• Axons regrow through intact endoneurial tubes at 1–2 mm/day (~1 inch/month)
• Recovery proceeds proximal to distal
• Regenerating axons follow the same endoneurial pathway and reconnect with the same end organ (unlike neurotmesis)
• Full recovery, though slow, can approach near-normal

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Sunderland Subclassification

"If the sprouting axons are unable to reach the distal endoneurium, then a neuroma in continuity will develop, and no spontaneous recovery will occur (Sunderland class IV)." — Current Surgical Therapy, 14e

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Clinical Features

• Complete loss of motor, sensory, and autonomic function (unlike neuropraxia, where sensation/autonomic may be partially preserved)
• No conduction at or distal to the injury on nerve conduction studies
• Progressive denervation atrophy of target muscles
• Tinel's sign may be present and advance distally as regeneration proceeds — a positive prognostic sign

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Electrodiagnostic Findings (EMG/NCS)

Studies are not diagnostically reliable for 3–6 weeks after injury due to the delay in Wallerian degeneration completion.

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Prognosis

• Grade II: Excellent — complete recovery expected, axons follow intact endoneurial tubes to original targets
• Grade III: Fair — variable quality due to misdirected axons; some functional recovery but may be incomplete
• Grade IV: Poor — neuroma-in-continuity forms; surgical intervention needed

• Distance from injury to target organ (longer = worse)
• Age (younger patients regenerate better)
• Timing of decompression/repair
• Severity of Schwann cell and endoneurial tube atrophy

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Management

• Peripheral nerve repair is best performed early (≤14 days), tension-free, in a clean wound bed
• Nerve gaps addressed with nerve conduit, decellularized nerve allograft, or autograft
• Nerve transfers considered for irreparable injuries (provide shorter reinnervation distance)

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Classic Examples

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Key Distinction: Axonotmesis vs. Neuropraxia vs. Neurotmesis

"The clinical features and neurophysiologic findings may be the same for axonotmesis and neurotmesis, yet there is a major difference in prognosis and management." — Rockwood and Green's Fractures in Adults, 10e

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Guillain-Barré Syndrome (GBS)

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Definition

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Epidemiology

• Incidence: 1–2 per 100,000/year (mean 1.8/100,000)
• Affects all ages; incidence rises with age (0.8/100,000 <18 yrs → 3.2/100,000 ≥60 yrs)
• Males slightly more affected (M:F ratio 1.5:1)
• Nonseasonal (except AMAN, which has summer epidemics in China)
• Highest attack rates in persons aged 50–74 years

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Antecedent Events (Triggers)

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Pathophysiology & Immunology

AIDP (classic demyelinating form)

• T-cell mediated autoimmune attack + humoral amplification
• Earliest event: complement deposition on the inner myelin surface
• Macrophages strip myelin lamellae from the axon (macrophage-mediated demyelination)
• Perivascular mononuclear infiltration (monocytes, macrophages, T lymphocytes) in the endoneurium
• Schwann cells upregulate class II MHC, potentially presenting antigen to autoreactive T cells
• Intense inflammation can cause secondary axonal degeneration (bystander effect)

Key autoantibodies

Pathological progression (AIDP):

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Classification of Subtypes

Common Subtypes

Rare Variants

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Clinical Features

Motor

• Ascending, symmetric weakness beginning in lower limbs, spreading to upper limbs, facial, oropharyngeal, and (in severe cases) respiratory muscles
• Severity: mild difficulty walking → complete quadriplegia
• Areflexia/hyporeflexia — invariable feature (may be absent very early)
• Cranial nerve involvement in 45–75% of cases; bilateral facial paresis in ≥50%

Sensory

• Paresthesias (often worse in hands and fingers) are common at onset
• Objective sensory loss is not prominent — typically limited to distal vibration sense impairment
• Pain (limb, back, interscapular) in ~70% during acute phase; persists ≥1 year in one-third

Autonomic (65% of hospitalized patients)

• Most significant in the first 2–4 weeks (peak paralysis)
• Orthostatic hypotension, urinary retention, GI atony (ileus), iridoplegia
• Sinus tachycardia, hypertension, anhidrosis or diaphoresis
• Bradycardia, heart block, asystole — "vagal spells" triggered by tracheal suctioning
• ECG changes: T-wave abnormalities, ST depression, QT prolongation, heart block
• Up to two-thirds of deaths during recovery result from cardiovascular/respiratory dysfunction

Respiratory

• Respiratory failure requiring ventilation: 9–30% of hospitalized patients
• Higher risk with older age and rapid progression
• Monitor with bedside FVC and NIF (negative inspiratory force); intubate if FVC <20 mL/kg or rapid decline ("20-30-40 rule")

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Diagnostic Criteria (Asbury & Cornblath)

Required

1. Progressive weakness of both legs and arms
2. Areflexia or hyporeflexia

Supportive — Clinical

• Progression over days to 4 weeks
• Relative symmetry
• Mild sensory symptoms/signs
• Bifacial palsies
• Autonomic dysfunction
• Absence of fever at onset
• Recovery beginning 2–4 weeks after progression ceases

Supportive — Laboratory

• Albuminocytological dissociation: elevated CSF protein with <10 cells/μL (normal cell count)
• Electrodiagnostic features: conduction slowing or block

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Investigations

CSF Analysis

• Classic finding: high protein, normal cells (albuminocytological dissociation)
• Protein often >45 mg/dL; may be >200 mg/dL in severe cases
• CSF pleocytosis >50 cells/μL should cast doubt on GBS diagnosis (consider Lyme, HIV, CMV)

Electrodiagnostics (EMG/NCS)

• May be normal in the first week — characteristic changes emerge at 1–2 weeks
• AIDP: reduced conduction velocity, prolonged distal latencies, conduction block, absent/prolonged F-waves, temporal dispersion
• AMAN: reduced/absent CMAPs with normal SNAPs, normal conduction velocity (no demyelination)
• AMSAN: markedly reduced/absent CMAPs and SNAPs; abundant fibrillations on needle EMG

Antibodies

• Anti-GQ1b in Miller-Fisher syndrome
• Anti-GM1 in AMAN (especially post-C. jejuni)

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Treatment

Immunotherapy (equally effective options)

• Combination of IVIG + PE is not superior to either alone
• Corticosteroids are NOT beneficial (no evidence of benefit; may worsen outcome)
• Neither treatment alters the extent of nerve damage or disease progression — they accelerate recovery

Supportive Care (crucial)

• Respiratory monitoring: serial FVC, NIF; prophylactic intubation if FVC <20 mL/kg
• Cardiac monitoring in ICU (continuous ECG, arterial line)
• DVT prophylaxis (high risk due to immobility)
• Pain management (gabapentin, opioids, carbamazepine)
• Physiotherapy and rehabilitation
• Nutritional support; nasogastric feeding if bulbar involvement

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Prognosis

• Mortality: ~1–5% (down from 33% before positive-pressure ventilation)
• ~80% recover independent ambulation by 6 months
• ~10–15% have persistent severe disability
• ~5–10% relapse (consider CIDP if >2 relapses)
• Poor prognostic factors: older age, rapid progression, preceding C. jejuni infection, AMSAN subtype, need for ventilation, absent CMAPs on EMG

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Differential Diagnosis

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Common Peroneal Nerve (CPN)

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Anatomy & Course

Origin

• Arises from the sciatic nerve in the popliteal fossa (dorsal divisions of L4, L5, S1, S2)
• The sciatic nerve divides into the CPN and the tibial nerve close to the popliteal fossa

Course

1. Runs along the lateral border of the popliteal fossa adjacent to the biceps femoris tendon
2. Winds around the neck of the fibula (lateral aspect) — most vulnerable point
3. Passes through the origin of peroneus longus muscle ("fibular tunnel")
4. Divides into its two terminal branches: the deep and superficial peroneal nerves

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Branches & Their Functions

Before Division (in popliteal fossa)

Deep Peroneal Nerve (Anterior Tibial Nerve)

Superficial Peroneal Nerve

Anatomical Variant

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Why Is the CPN So Vulnerable?

1. It is superficial — lies directly against bone with minimal soft tissue protection
2. It is tethered — fixed at the sciatic bifurcation proximally and at the fibular tunnel distally, making it susceptible to traction
3. The fibular tunnel (origin of peroneus longus) is a tight fibro-osseous channel

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Causes of CPN Injury

At the Fibular Head / Neck (most common site)

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Clinical Features of CPN Palsy

Motor

• Foot drop — inability to dorsiflex the foot (hallmark)
• Steppage gait — patient raises knee high to clear the plantar-flexed foot
• Weakness of toe extension (EHL, EDL)
• Weakness of foot eversion (peroneus longus/brevis) — may be spared in selective deep peroneal lesions
• Deep peroneal (anterior compartment) is more commonly affected than the whole nerve — weakness more prominent in dorsiflexion/toe extension than eversion

Sensory

• Loss over lateral lower two-thirds of leg and dorsum of foot (superficial peroneal distribution)
• First web space preserved in pure superficial peroneal lesions; lost in deep peroneal or complete CPN lesions
• Pain is rare (except with intraneural ganglia, which typically present with knee/peroneal distribution pain and fluctuating weakness with weight bearing)

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Differential Diagnosis of Foot Drop

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Investigations

Electrodiagnostics (EMG/NCS)

• Nerve conduction: Motor studies recording tibialis anterior or EDB — reduced/absent CMAP; focal conduction block or slowing across the fibular head can sometimes be demonstrated
• Contrary to ulnar nerve at the elbow, localized slowing at the fibular head is not the most common finding — axonal loss is the most frequent pathophysiology
• Axonal loss: diffusely low/absent fibular motor + superficial peroneal sensory amplitudes
• Needle EMG: Denervation (fibrillations, positive sharp waves) in CPN-innervated muscles (tibialis anterior, peronei, EDB); short head of biceps femoris normal (distinguishes from sciatic/L5); tibialis posterior and medial gastrocnemius normal (distinguishes from L5 radiculopathy)
• Normal paraspinal muscles (distinguishes from L5 root)

Imaging

• Ultrasound: Slightly more accurate than MRI for compressive fibular neuropathies; thickened nerve (cross-sectional area >8 mm²) in ~70%; identifies intraneural ganglia and soft-tissue masses
• MRI: Effective for intraneural ganglia; shows denervation edema in anterior and lateral compartment muscles (T2 hyperintensity)

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Management

Conservative

• Remove the offending cause (improve positioning, remove cast/bandage, avoid leg crossing)
• Ankle-foot orthosis (AFO) — custom-made plastic; improves steppage gait and prevents equinus contracture
• Physiotherapy: stretching, strengthening, gait training
• Most acute demyelinating (neuropraxia) lesions recover fully within days to 12 weeks

Surgical Indications

• No spontaneous improvement after 3 months of conservative management
• Pain or slowly progressive lesion (may indicate intraneural ganglion or tumour)
• Penetrating injury or known neurotmesis
• Options: neurolysis, excision of compressive lesion (ganglion, osteochondroma), nerve grafting

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Prognosis

• Uniformly good for acute demyelinating (neuropraxia) lesions
• Delayed and incomplete recovery for axonal loss lesions and stretch injuries
• The distal fibular motor amplitude (recording tibialis anterior) is the best prognostic indicator — correlates with extent of axonal loss
• Studies should be performed bilaterally and compared for accurate assessment

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ACL and MCL Injury — X-Ray Findings

Key Concept Upfront

• Associated bony avulsion fractures (pathognomonic markers of ligament injury)
• Joint effusion / lipohemarthrosis
• Joint space widening under stress
• Tibial slope and limb alignment

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ACL Injury — X-Ray Findings

Standard Views

1. Segond Fracture ⭐ (Pathognomonic for ACL Tear)

• Definition: Small, vertically oriented cortical avulsion fracture off the posterolateral margin of the lateral tibial plateau
• Mechanism: Avulsion by the anterolateral ligament (ALL) or lateral joint capsule during forced internal rotation + varus stress
• Location on X-ray: Lateral tibial plateau, just distal to the articular surface — AP view
• ACL association: ACL rupture present in 75–100% of Segond fractures
• Also highly associated with lateral meniscus tear and medial meniscus tear (~75% each)

2. Tibial Eminence / Intercondylar Eminence Fracture

• Avulsion of the ACL tibial insertion — more common in children (instead of midsubstance ACL tear)
• Seen on lateral view as a displaced bony fragment lifted off the tibial eminence
• Classified by Meyers & McKeever (Type I–III based on displacement)
• On AP: irregular or absent tibial eminence; on lateral: fragment displaced superiorly into joint

3. Lipohemarthrosis (Lateral View)

• Fat-fluid level in the suprapatellar bursa on lateral X-ray
• Indicates an intra-articular fracture with release of bone marrow fat
• Strongly suggests ACL tibial avulsion or tibial plateau fracture
• Specific but not exclusive to ACL injury

4. Blumensaat Line (Lateral View — Indirect Sign)

• Blumensaat line = line drawn along the roof of the intercondylar notch on lateral X-ray
• In a normal knee at ~30° flexion, the inferior pole of the patella should lie on this line
• Not an X-ray sign of ACL injury per se — used on MRI: intact ACL fibers should parallel Blumensaat line on sagittal imaging

5. Posterior Tibial Slope (Lateral View)

• Measured on lateral radiograph
• Values >12° are associated with ACL graft failure after reconstruction
• Important in surgical planning

6. Joint Effusion

• Hemarthrosis beginning within 12 hours of injury is highly suggestive of ACL tear (vs. meniscal tears which cause delayed swelling at 12–24 hours)
• Seen as suprapatellar soft tissue fullness on lateral view

Summary — ACL X-ray Findings

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MCL Injury — X-Ray Findings

1. Pellegrini-Stieda Lesion (Pellegrini-Stieda Disease)

• Definition: Calcification / ossification of the proximal MCL (at or near the femoral attachment) following injury
• Represents a chronic/healed MCL injury — a late finding (weeks to months post-injury)
• Seen on AP view as a linear or curvilinear calcification along the medial femoral condyle
• Patient presents with medial knee pain, local tenderness, and a palpable mass at the femoral MCL attachment
• Due to incomplete MCL rupture → periosteal reaction → heterotopic ossification

2. Medial Joint Space Widening (Stress Views)

• Valgus stress radiograph: widening of the medial joint space indicates MCL incompetence
• Grading:
  • Grade I (sprain): <5 mm opening
  • Grade II (partial tear): 5–9 mm opening
  • Grade III (complete tear): ≥10 mm opening (or 3–5 mm more than contralateral side)

3. Avulsion Fractures at MCL Attachment

• MCL avulsion from medial femoral epicondyle (proximal MCL)
• MCL avulsion from medial tibial flare (distal MCL)
• Small flakes of bone visible on AP view at the medial joint margin

4. Reverse Segond Fracture

• Medial counterpart of the Segond fracture
• Small cortical avulsion off the medial tibial plateau (deep MCL / medial meniscotibial ligament attachment)
• Highly associated with PCL injury and MCL injury
• Indicates severe rotational instability (anteromedial)

Summary — MCL X-ray Findings

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Combined ACL + MCL Injury ("Terrible Triad" / O'Donoghue Triad)

• Segond fracture (ACL component)
• Medial joint space widening on valgus stress (MCL component)
• Diffuse hemarthrosis / lipohemarthrosis

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Recommended X-ray Series for Knee Ligament Injury

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