Focal and atypical neuropathies: plexopathy, entrapment and treatment-induced

Severe acute limb pain followed by patchy weakness points to a plexopathy such as neuralgic or diabetic amyotrophy, not a disc; purely sensory anterolateral thigh pain is often meralgia paresthetica; and a rapid HbA1c fall can trigger painful neuropathy.

1. Neuralgic amyotrophy: pain first, weakness later. Pain occurs in 96%, reaches its maximum within hours at a median of 9/10 and often wakes patients; weakness can lag the pain by two weeks or longer ¹.
2. Upper plexus and long thoracic nerve dominate. Upper plexus involvement, with or without a winged scapula, is most common; anterior interosseous involvement weakens pinch grip, and sensory loss is present in the majority ¹.
3. The diagnosis is clinical. Ancillary tests mainly exclude other causes, and a short course of high-dose oral corticosteroids is advised for patients seen early and still in pain ².
4. Diabetic amyotrophy follows weight loss. It occurs mostly in type 2 diabetes, often with fairly well controlled glucose and recent weight loss; severe thigh pain precedes spreading leg weakness ³.
5. Rapid glucose correction can cause neuropathy. Treatment-induced neuropathy is acute neuropathic pain or autonomic dysfunction within 8 weeks of an HbA1c fall of at least 2 points over 3 months ⁴.
6. Meralgia paresthetica is purely sensory. The lateral femoral cutaneous nerve (L2–L3) usually passes deep to the inguinal ligament medial to the anterior superior iliac spine, where belts compress it; a local anesthetic block diagnoses and treats it ⁵.

Asymmetric leg weakness, atrophy and absent knee and ankle reflexes after severe thigh pain in diabetes are not one radiculopathy: needle EMG denervation cannot be localized to one root ³.

• Treatment-induced neuropathy occurred in 10.9% of 954 neuropathy clinic referrals, and pain severity correlated with the HbA1c fall (R = 0.84) ⁴.
• An HbA1c fall of 2–3 points over 3 months carried a 20% risk of treatment-induced neuropathy; a fall of more than 4 points, over 80% ⁴.
• Most patients with diabetic amyotrophy improve, but recovery can take up to 2 years and few return to normal ³.
• In a randomized trial, ultrasound-guided lateral femoral cutaneous blocks with or without corticosteroid relieved meralgia similarly over 4 weeks ⁶.
• Ultrasound guidance is advised for lateral femoral cutaneous blocks because the nerve's course varies ⁶.

• Sending plexopathy-pattern weakness for lumbar epidural injection when EMG cannot localize it to one root ³.
• Lowering HbA1c by several points within weeks without warning of painful neuropathy ⁴.
• Treating meralgia as an L2–L3 radiculopathy despite normal strength and reflexes ⁵.