The symptoms and most frequent causes of femoral nerve lesions; diabetic amyotrophy

1. Big picture

The femoral nerve is the main nerve for hip flexion, knee extension, the patellar reflex, and sensation over the anterior thigh and medial leg. A femoral nerve lesion is uncommon compared with peroneal, ulnar, median, or radial nerve lesions, but it is clinically important because the patient may suddenly lose the ability to extend the knee, causing falls and knee buckling.

The key pattern:

Femoral nerve palsy = weak knee extension + weak hip flexion + absent/reduced patellar reflex + sensory loss over anterior thigh and medial leg.

The important exam association is diabetic amyotrophy, now often called diabetic lumbosacral radiculoplexus neuropathy. It presents with severe thigh/hip pain followed by proximal weakness, wasting, and weight loss, usually in older patients with type 2 diabetes.

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