Peripheral Neuropathy

1. Clinical Overview

Peripheral Neuropathy (PN) refers to any disorder of the peripheral nervous system (PNS), encompassing damage to the axon, myelin sheath, or cell body of sensory, motor, or autonomic nerves. It is an exceedingly common condition, affecting 2-8% of the adult population and up to 50% of individuals with diabetes. The clinical spectrum ranges from the ubiquitous "length-dependent" diabetic sensorimotor polyneuropathy (painful paresthesias in feet) to life-threatening acute demyelinating conditions like Guillain-Barré Syndrome (GBS).

The hallmark of successful management lies in accurate phenotyping:

1. Is it Axonal or Demyelinating? (NCS/EMG distinguishes).
2. Is it Symmetric or Asymmetric? (Systemic vs Local/Vasculitis).
3. Is it Large Fiber or Small Fiber? (EMG detects Large, Skin Biopsy detects Small).

Clinical Pearl:

The "Length-Dependent" Rule: Standard polyneuropathies (Diabetes, Alcohol, B12) affect the longest nerves first. Symptoms start in the toes, ascend to the knees, then appear in the fingers. If symptoms start in the hands before the knees, or are asymmetric, STOP. This is NOT a typical metabolic neuropathy. Think Vasculitis, CIDP, or Lead Poisoning.

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2. Epidemiology

• Prevalence: 2.4% of the general population; rises to 8% in those > 55 years.
• Diabetes: The single most common cause globally. 50% of diabetics develop neuropathy within 25 years.
• Idiopathic: Despite extensive workup, ~25-30% of cases remain "Cryptogenic Sensory Polyneuropathy" (CSPN), predominantly in the elderly.
• Chemotherapy (CIPN): Affects 30-40% of patients treated with taxanes, platinums, or vinca alkaloids.
• HIV: Affects ~30% of HIV+ individuals (disease or drug-related).

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3. Pathophysiology

The Two Primary Mechanisms

1. Axonal Degeneration ("Dying Back"):
   • Metabolic insult (Glucose, Alcohol) disrupts axonal transport.
   • The distal axon degenerates first because nutrient transport fails over long distances.
   • Clinical: Slow progression, muscle wasting (late), reduced amplitude on NCS.
   • Examples: Diabetes, Alcohol, B12, Toxins.
2. Segmental Demyelination:
   • Autoimmune attack or genetic defect in Schwann cells/Myelin.
   • Saltatory conduction fails $\rightarrow$ Conduction Block or Slowing.
   • Clinical: Weakness > Sensory loss, Areflexia (early), Slowed velocity on NCS.
   • Examples: GBS, CIDP, CMT Type 1.

Small Fiber vs Large Fiber

• Large Fibers (A-alpha, A-beta): Carry Vibration, Proprioception, Motor.
  • Damage: Balance loss, Ataxia, Weakness.
• Small Fibers (Unmyelinated C-fibers, A-delta): Carry Pain, Temperature, Autonomic function.
  • Damage: Burning pain ("on fire"), Allodynia, Orthostasis. NCS is NORMAL.

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4. Clinical Assessment

History: The "7 Questions"

1. Time Course: Acute (days = GBS/Vasculitis) vs Chronic (years = Diabetes/Genetic).
2. Distribution: Symmetric (Stocking-Glove) vs Asymmetric (Mononeuritis Multiplex).
3. Modality: Sensory (Diabetes) vs Motor (GBS/Lead/CMT).
4. Autonomic Symptoms: Fainting, erectile dysfunction, gastroparesis (Suggests Diabetes or Amyloid).
5. Toxin History: Alcohol intake? Chemotherapy? Occupational lead/arsenic?
6. Family History: High arches (Pes Cavus)? Hammer toes? (Suggests CMT).
7. Diet: Vegan (B12)? Previous gastric bypass?

Physical Examination

• Inspection: Pes Cavus (high arches), Hammer toes, Muscle wasting (distal legs/hands), Trophic changes (shiny skin, hair loss), Ulcers.
• Tone/Power: Usually normal tone. Distal weakness (Extensor Hallucis Longus first).
• Reflexes:
  • Ankle Jerks: Lost early in length-dependent neuropathy.
  • Knee Jerks: Preserved until late.
  • Red Flag: Generalized areflexia suggests demyelinating process (GBS/CIDP).
• Sensation:
  • Vibration (128Hz Tuning Fork): First modality lost in Large Fiber/Diabetic neuropathy. Test on big toe IPJ.
  • Proprioception: Joint Position Sense.
  • Pinprick/Temperature: Test for small fiber loss. Look for a "level" (stocking distribution).
• Balance: Romberg's Test (Positive = Sensory Ataxia).
• Gait: High steppage gait (foot drop)? Wide-based (ataxia)?

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5. Classification

By Etiology (The "DAMN THING" Mnemonic)

• Diabetes (Most common) / Drugs (Chemo, Isoniazid, Amiodarone)
• Alcohol
• Metabolic (B12, Folate, Thiamine, Hypothyroid, Renal failure)
• Neoplastic (Paraneoplastic, Myeloma, Amyloid) / Nutritional
• Trauma (Compression)
• Hereditary (Charcot-Marie-Tooth)
• Immune (GBS, CIDP, Vasculitis, Lupus, Sjögren's)
• Nfectious (HIV, Lyme, Leprosy, Hep C)
• Granulomatous (Sarcoid)

By Pattern

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6. Investigations

The "Screening Bundle" (For all new symmetric neuropathies)

1. FBC, ESR/CRP: Screen for infection, inflammation, malignancy.
2. HbA1c & Fasting Glucose: Diabetes is #1 cause.
3. U&E, LFT: Renal failure (uremia), Alcohol markers (GGT).
4. B12 & Folate: Treatable cause. (Check Methylmalonic Acid if B12 borderline).
5. TSH: Hypothyroidism.
6. SPEP/Immunofixation: Screen for Paraproteinemia (Myeloma, MGUS, Amyloid). Crucial in unexplained cases.

Specialized Tests (The "Second Line")

• Nerve Conduction Studies (NCS) / EMG:
  • The Gold Standard for classifying Axonal vs Demyelinating.
  • Axonal: Reduced Amplitude. Normal Velocity.
  • Demyelinating: Reduced Velocity (less than 38m/s). Prolonged Latencies. Conduction Block.
• Lumbar Puncture: High protein + Normal cells = Albuminocytologic dissociation (GBS/CIDP).
• Autoimmune Panel: ANA, ENA, ANCA (Vasculitis), Anti-Ganglioside (GBS variants).
• Genetic Testing: PMP22 duplication (CMT Type 1A).
• Skin Biopsy: For Small Fiber Neuropathy (counts Intra-Epidermal Nerve Fiber Density) when NCS is normal.

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7. Management: The Protocols

Management rests on three pillars: Treating the Cause, Symptom Management (Pain), and Preventing Complications.

Treatment Algorithm

graph TD
    A[Patient with Neuropathy] --> B{Treatable Cause Found?}
    B -->|Yes| C[Treat Underlying Disease]
    B -->|No - Idiopathic| D[Symptomatic Management Only]
    
    C --> E[Diabetes: Glycemic Control]
    C --> F[B12 Deficiency: IM Replenishment]
    C --> G[Alcohol: Cessation + Thiamine]
    C --> H[Inflammatory/CIDP: IVIG/Steroids]

    A --> I{Neuropathic Pain?}
    I -->|Yes| J[First Line Agents]
    J --> K{Response?}
    K -->|Good| L[Continue]
    K -->|Poor| M[Switch Class or Combine]
    M --> N[Second Line / Specialist Refer]

A. Disease-Modifying Therapy

• Diabetes: Strict glycemic control slows progression (especially in Type 1).
• B12 Deficiency: Load with IM Hydroxocobalamin ($1mg \times 5$ doses) then maintenance.
• Alcohol: Strict cessation. Thiamine (B1) supplements.
• Vasculitis/CIDP: Immunosuppression (Steroids, Cyclophosphamide, IVIG, Plasma Exchange).
• Drug-Induced: Stop the offending agent (change chemotherapy regimen if oncologically safe).

B. Pain Management (Neuropathic Agents)

Standard analgesics (Paracetamol/NSAIDs) are ineffective.

C. Safety & Prevention

• Foot Care: Daily inspection with a mirror. Podiatry referral.
• Shoes: Wide-fitting, seamless shoes. Orthotics for high arches.
• Falls Prevention: Remove rugs, install rails, night lights (proproception loss worsens in dark).
• Burn Prevention: Test water temperature with elbow, not hand/foot.

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8. Complications

1. The Diabetic Foot

• Mechanism: Neuropathy (Loss of sensation) + Ischemia (PAD) + Deformity (Motor imbalance).
• Result: Unnoticed trauma $\rightarrow$ Ulceration $\rightarrow$ Osteomyelitis $\rightarrow$ Amputation.
• Charcot Arthropathy: Neurogenic destruction of joints (mid-foot collapse/rocker-bottom foot). Presents as a hot, swollen, erythematous foot without pain.

2. Autonomic Neuropathy

• Cardiovascular: Postural hypotension (Fall risk), Resting tachycardia, Silent MI.
• Gastrointestinal: Gastroparesis (nausea, erratic glucose control), Diarrhea (bacterial overgrowth).
• Genitourinary: Erectile dysfunction, Neurogenic bladder (retention/overflow).
• Sudomotor: Anhydrosis (dry skin) or gustatory sweating.

3. Falls & Fractures

• Loss of proprioception leads to sensory ataxia. Highest risk in the dark ("Washbasin Sign"

• falling when washing face with eyes closed).

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9. Integrated Care: The Multidisciplinary Team

• Podiatry: Essential for routine nail care, callus debridement, and footwear advice.
• Physiotherapy: Gait training, balance exercises, prevention of contractures.
• Pain Specialist: For multimodal management of refractory neuropathic pain.
• Occupational Therapy: Home adaptations (rails, ramps) and aids for ADLs (button hooks).

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10. Prognosis

• Reversible Causes: B12, Hypothyroid, and some drug-induced neuropathies can improve significantly if treated early.
• Axonal Degeneration: Recovery is very slow (1mm/day) and limited by the length of the nerve. Distal axons may never reconnect.
• Diabetic Neuropathy: Generally progressive, but rate can be slowed. Pain often "burns out" after years, leaving numbness (which is higher risk for ulcers).
• GBS: 80% recover function, but fatigue can persist for years.

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11. Evidence & Guidelines

Landmark Trials

• DCCT (NEJM 1993): Intensive glycemic control in Type 1 Diabetes reduced neuropathy risk by 60%.
• OPTION Trial: Amitriptyline, Duloxetine, and Pregabalin have similar efficacy for painful diabetic neuropathy. Combination therapy gives better relief but more side effects.
• PREDICT: Genetic markers for taxane-induced neuropathy.

Guidelines (NICE CG173 / AAN)

• First Line for Pain: Offer a choice of Amitriptyline, Duloxetine, Gabapentin, or Pregabalin.
• Rescue: Tramadol only for acute rescue, not long term. Opioids are strictly avoided (ineffective for nerve pain, highly addictive).
• Cannabinoids: Evidence remains weak/mixed.

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12. Future Horizons

• NaV1.7 Blockers: Selective sodium channel blockers targeting pain fibers without causing central sedation.
• Neuroprotective Agents: Drugs to prevent Chemotherapy-Induced Peripheral Neuropathy (many trials, limited success so far).
• Gene Therapy: For CMT (CRISPR/Cas9 targeting PMP22).

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13. Special Populations

A. The Elderly ("Idiopathic Axonal")

• Most common diagnosis in > 75s.
• Slowly progressive sensory loss + mild ataxia.
• Management: Reassurance. Fall prevention. Avoid heavy immunosuppression unless rapidly progressive.

B. Cancer Patients

• Paraneoplastic Subacute Sensory Neuronopathy: Anti-Hu antibodies. Rapid, severe sensory loss (cell body death in dorsal root ganglion).
• Chemotherapy: Taxanes (Paclitaxel) cause "Coasting"

• symptoms worsen for weeks after stopping the drug.

C. HIV

• Distal Sensory Polyneuropathy (DSP): Due to the virus itself or older ART drugs (Stavudine, Didanosine - "d-drugs"). Modern ART (Tenofovir/TAF) is much safer.

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14. Patient Education

Top 3 Things to Tell Patients

1. "Pain does not mean damage": Neuropathic pain is a "false alarm" from broken wires. Walking on it does not cause harm (unless you have a numb ulcer).
2. "Look at your feet every day": Because you can't feel a stone in your shoe or a blister forming. Use a mirror to check the soles.
3. "Drugs take time": Painkillers like Amitriptyline take weeks to work. They don't work "on demand" like ibuprofen. You must take them every night.

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15. Case Mastery: Clinical Scenarios

Case 1: The "Functional" Faller

Patient: 30-year-old woman. Difficulty walking in dark. "Legs feel strong but don't obey." Exam: Normal power. Absent reflexes. Sensory ataxia (Positive Romberg). Diagnosis: Guillain-Barré Syndrome (Acute Ataxic variant) or Sjogren's Ganglionopathy. Need Lumbar Puncture.

Case 2: The "Painful Blue Foot"

Patient: 55-year-old smoker. Excruciating burning pain in feet, asymmetric. Blue toes. Diagnosis: Vasculitis (Mononeuritis Multiplex). This is a medical emergency. Action: Start High-Dose Steroids immediately after confirming diagnosis.

Case 3: The "Clumsy" Family

Patient: 18-year-old with "bad ankles". Dad has high arches. Exam: Pes Cavus, Champagne bottle legs (distal wasting), inverted reflex (supinator). Diagnosis: Charcot-Marie-Tooth (CMT) Type 1A. Genetic counseling needed.

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16. Appendix: Additional Resources

Useful Abbreviations

Online Tools

• Neuropathy Commons (Patient Support)
• Diabetes UK - Foot Care