Meralgia Paraesthetica
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Direct answer
Meralgia paraesthetica is entrapment of the lateral femoral cutaneous nerve as it passes just medial to the anterior superior iliac spine, under or through the inguinal ligament. It causes burning pain, tingling or numbness over the anterolateral thigh, and because the nerve is purely sensory, power and reflexes stay normal. Most patients improve with weight loss, looser clothing and neuropathic pain medication, and surgical decompression is reserved for refractory cases.
What you must remember
- Nerve involved: the lateral femoral cutaneous nerve, usually arising from the L2 and L3 roots, which carries sensation only.
- Site of entrapment: the inguinal ligament, just medial to the anterior superior iliac spine.
- Symptoms: burning pain, tingling or numbness over the anterolateral thigh, with normal motor power and reflexes.
- Risk factors: obesity, tight belts, pregnancy, diabetes, and trauma or surgery at the iliac crest, including bone graft harvest.
- Key differential: L2 or L3 radiculopathy, which produces motor and reflex change, plus femoral nerve lesions and hip disease.
- Diagnostic clue: relief after local anaesthetic infiltration near the anterior superior iliac spine supports the diagnosis.
- First-line care: weight loss, loose clothing, activity change and neuropathic pain medication, with surgery reserved for refractory cases.
How the nerve is tested and treated
A patient with burning thigh pain and numb skin needs a nerve-based diagnosis, because the commonest mimic is a lumbar root lesion that needs a different plan. The steps below separate the two.
- Map the sensory loss: a deficit confined to the anterolateral thigh suggests the lateral femoral cutaneous nerve, whereas a dermatomal band reaching below the knee points to a root lesion.
- Check power and reflexes: normal hip flexion, knee extension and reflexes favour meralgia paraesthetica, while weakness suggests L2 to L4 or femoral nerve involvement.
- Take a history of triggers: ask about recent weight gain, pregnancy, tight belts, diabetes and any iliac crest bone graft or hip surgery.
- Perform a diagnostic block: infiltration with local anaesthetic just medial to the anterior superior iliac spine that relieves the pain strongly supports the diagnosis.
- Image when the picture is unclear: MRI of the lumbar spine excludes root compression, and pelvic or hip imaging looks for a mass or another cause.
- Escalate only after conservative care fails: persistent symptoms despite weight loss, clothing change and neuropathic pain medication are the indication for surgical decompression or neurolysis.
Where the exam tries to catch you
Exam stems about thigh numbness often hinge on whether the motor examination is normal.
- Trap: choosing L3 radiculopathy for burning anterolateral thigh pain with normal power Correct: an isolated sensory deficit over the anterolateral thigh, with normal motor and reflex findings, points to the lateral femoral cutaneous nerve.
- Trap: ignoring the triggers and labelling every case idiopathic Correct: obesity, pregnancy, diabetes and iliac crest harvest are the expected causes, and a careful history often finds one.
- Trap: operating early in an obese patient with burning thigh pain Correct: weight loss and clothing advice come first, because surgery is reserved for disease that fails conservative care.
Frequently asked questions
Which nerve is affected in meralgia paraesthetica?
The lateral femoral cutaneous nerve, usually arising from the L2 and L3 roots, is entrapped in meralgia paraesthetica. Because this nerve is purely sensory, the motor examination of the thigh stays normal in this condition.
Why is weakness not a feature of meralgia paraesthetica?
The lateral femoral cutaneous nerve supplies only the skin of the anterolateral thigh and has no motor branches. Weakness of hip flexion or knee extension therefore suggests a lumbar root or femoral nerve lesion rather than meralgia paraesthetica.
What are the commonest causes of meralgia paraesthetica?
Obesity, tight belts or clothing, pregnancy and diabetes are the commonest causes of meralgia paraesthetica. Trauma or surgery near the anterior superior iliac spine, including iliac crest bone graft harvest, is a classic iatrogenic cause that exam questions often mention.
Which investigation supports the diagnosis of meralgia paraesthetica?
The diagnosis of meralgia paraesthetica is largely clinical and is supported by relief after a local anaesthetic block near the anterior superior iliac spine. Imaging of the lumbar spine is reserved for cases in which root pathology is suspected.