FRCA Notes

Meralgia Paraesthetica

This topic is included owing to a BJA Education article, and is both a cause ofperipheral neuropathic pain and positioning-related nerve complications from anaesthesia/surgery.

Resources


  • A common but under-recognised mononeuropathy of the lateral femoral cutaneous nerve (LFCN)
  • Leads to pain, paraesthesia and dysaesthesia over the anterolateral thigh ± sensory loss

Epidemiology

  • Incidence 4.3-32.6 per 100,000 patient years
  • Most common between 41 - 60yrs
  • Slight male preponderance

Risk factors

  • Diabetes mellitus (7.5x increased incidence vs. baseline population)
  • Obesity
  • Pregnancy
  • Carpal tunnel syndrome

Origin

  • A purely sensory nerve arising from the dorsal divisions of the lumbar plexus
  • Most commonly L2 and L3 (~60%), but may arise from L1-L2 or L2 alone
  • This variability explains the variable clinical presentation, and why the diagnosis should not be dismissed if symptoms are atypical

Course

  • Emerges from the lateral border of psoas major
  • Runs obliquely across iliacus, deep to the fascia iliaca, towards the anterior superior iliac spine (ASIS)
  • Passes under, through or above the inguinal ligament to enter the thigh
  • Divides into anterior and posterior branches distal to the inguinal ligament, supplying the anterolateral thigh

Anatomical variants

  • The course of the LFCN as it leaves the pelvis has been classified into five types (Aszmann)
  • Types A-C are superficial to the inguinal ligament and were thought to be most vulnerable to trauma and compression
  • Ultrasound studies suggest type B is the most common variant in patients with meralgia paraesthetica

Type Frequency Course
A 4% Posterior to the ASIS, across the iliac crest
B 27% Anterior to the ASIS, within the substance of the inguinal ligament
C 23% Medial to the ASIS, within the tendinous origin of sartorius
D 26% Medial to sartorius, deep to the inguinal ligament
E 20% Most medial, deep to the inguinal ligament overlying iliopsoas


Idiopathic

  • Mechanical
    • Direct entrapment of the LFCN as it passes the inguinal ligament is the most common cause
    • Increased intra-abdominal pressure: pregnancy, obesity, ascites, abdominopelvic tumours
    • External compression: tight belts, corsets and trousers
    • Leg length discrepancy

  • Metabolic
    • Diabetes mellitus
    • Alcoholism
    • Lead poisoning

Iatrogenic

  • Direct trauma
    • Inguinal hernia repair
    • Pelvic osteotomy and iliac crest bone graft harvest
    • Laparoscopic procedures (port sites)
    • Intramuscular injections into the thigh

  • Indirect trauma
    • Self-retaining retractors in gynaecological-oncology surgery
    • Psoas haematoma
    • Positioning (see below)

Positioning-related injury

  • Lithotomy e.g. prolonged attempts at vaginal delivery prior to Caesarean section
  • Prone positioning for spinal surgery
    • Jackson table and Relton-Hall frame both compress the LFCN at the ASIS
    • Reduced blood flow in the vasa nervorum leads to nerve dysfunction
  • Prone ventilation on ICU

  • Prevention
    • Additional padding (gel or foam pads) over the iliac crests and ASIS
    • Care with hip angle when positioning
    • Limit duration in lithotomy where possible

Symptoms

  • Numbness, tingling, burning pain and dysaesthesia over the anterolateral thigh
  • Exacerbated by standing, walking or hip extension
  • Usually unilateral; bilateral in ~20%

Examination

  • Sensory disturbance mostly confined to the anterolateral thigh
  • Variable root supply means some crossover to the medial thigh is possible
  • Any motor weakness or altered reflexes should suggest another diagnosis as the LCFN is purely sensory

Differential diagnosis

  • Femoral neuropathy
  • Lumbar plexopathy
  • Lumbar radiculopathy (L2-L3)

Investigations

  • Diagnosis is primarily clinical
  • Imaging (X-ray, CT, MRI) may be used to exclude differentials e.g. spinal or pelvic pathology
  • Ultrasound
    • Can identify the nerve and its anatomical variants
    • Abnormal nerve appears hypoechoic, with abnormal intraneural vascularity and an abrupt change in calibre
    • Increased cross-sectional area: normal ~1.7mm2, pathological >2.65mm2
  • Diagnostic block
    • Resolution of symptoms after ultrasound-guided LA infiltration around the LFCN supports the diagnosis
  • Electrophysiological studies
    • SNAP, sensory nerve conduction velocity and SSEPs
    • Useful where diagnostic doubt exists, but rarely used in clinical practice

  • Often self-limiting: ~90% resolve with conservative management
  • Reassurance for mild symptoms

Non-pharmacological

  • Avoid tight-fitting clothing and belts
  • Weight loss in patients living with obesity
  • Physiotherapy

Pharmacological

  • Simple analgesics
  • Antineuropathic agents e.g. gabapentinoids
  • Tricyclic antidepressants e.g. amitriptyline
  • See neuropathic pain for more detail on the pharmacological management of neuropathic pain

Ultrasound-guided infiltration

  • LA and corticosteroid infiltration around the LFCN
    • Both diagnostic and therapeutic
    • Symptoms resolve or improve in ~80-90% of patients
    • Similar success to surgery, but quicker, safer and cheaper

Landmark technique Ultrasound-guided technique
Failure rate up to 60% Nerve located in ~80-85%
Unreliable due to anatomical variability in nerve course Visualises anatomical variants directly
No information on nerve morphology Can assess nerve morphology at the same time

Neuromodulation

  • Reserved for refractory cases; evidence limited to case series and case reports
  • Pulsed radiofrequency (PRF)
    • Non-destructive; promising results in small case series
    • Electrode position and duration of treatment may influence success
  • Radiofrequency ablation (RFA)
    • Thermal denervation of the LFCN
  • Peripheral nerve stimulation (PNS)
    • Temporary externalised leads
    • Benefit may persist after lead removal, possibly by modulating central sensitisation
  • Spinal cord stimulation

  • Considered when conservative and interventional measures fail
  • Both procedures are effective, with neurectomy having a higher success rate
Neurolysis Neurectomy
Procedure Decompression of the LFCN at the inguinal ligament Transection of the LFCN
Role Usually first-line surgical option Higher success rate; often used after failed neurolysis
Drawbacks Lower rate of complete pain relief Permanent numbness of the anterolateral thigh; risk of deafferentation pain