14.3 Femoral, Sciatic, and Lateral Femoral Cutaneous Neuropathies

Not every lower-limb sensory complaint is a distal entrapment. Femoral, sciatic, and lateral femoral cutaneous nerve (LFCN) lesions sit more proximally, produce distinctive SNAP maps, and appear after hip surgery, intramuscular injection, and prolonged lithotomy. Outline topics VI.A.2.b (femoral) and VI.A.2.d (sciatic), together with LFCN/meralgia cases, belong in the same localization toolkit as the fibular-head and tarsal-tunnel studies in the previous sections. Independent OpenExamPrep teaching here keeps motor and sensory NCS in the technologist’s domain and treats needle EMG as a physician study that you only need to understand anatomically.

Femoral nerve: iliacus, inguinal ligament, and the saphenous SNAP

The femoral nerve arises from L2–L4, forms within psoas major, emerges from its lateral border, and descends between psoas and iliacus before giving a motor branch to iliacus and traveling under the inguinal ligament into the thigh. Distal to the ligament it innervates sartorius, pectineus, and the quadriceps — rectus femoris, vastus medialis (VM), vastus lateralis, and vastus intermedius. Terminal sensory fibers continue as the saphenous nerve to the medial leg and medial malleolus. Anterior femoral cutaneous branches supply the anterior thigh.

Two lesion sites change the expected NCS picture:

• Intrapelvic / iliacus compartment (retroperitoneal hematoma, iliopsoas bleed on anticoagulation, stretch): iliacus and quadriceps are both affected; the saphenous SNAP falls if the lesion is axonal.
• Inguinal ligament (lithotomy, extreme hip flexion, tight compression, femoral catheterization): quadriceps and saphenous fibers are at risk; iliacus may be relatively spared because it was innervated more proximally.

Saphenous SNAP is the routine sensory study for the femoral nerve. It is technically demanding: the nerve is small, the recording site at the ankle or distal medial leg is prone to artifact, and obesity or edema can make the response unobtainable even in healthy people. Always attempt a side-to-side comparison on the same day with identical distances. A unilaterally absent saphenous SNAP with weak knee extension supports femoral (or lumbar plexus) involvement. Bilateral absent saphenous responses in an older patient with polyneuropathy are much less localizing.

Femoral motor NCS, when performed, stimulates just below the inguinal ligament and records over VM or rectus femoris. A prolonged latency or a side-to-side CMAP amplitude drop supports a femoral motor lesion. The study does not replace imaging for hematoma, but it documents axonal loss for the physician. Keep stimulus artifact low; inguinal stimulation is uncomfortable and can activate neighboring nerves if cathode placement is sloppy.

Clinically, femoral neuropathy produces knee-extension weakness, difficulty climbing stairs, a reduced patellar reflex, and sensory change on the anterior thigh and medial leg. Hip flexion may be weak if iliacus is involved. Foot dorsiflexion is spared — that finding alone steers you away from a sciatic or fibular story.

Sciatic nerve: thigh lesions and the fibular-division bias

The sciatic nerve (L4–S3) exits the pelvis through the greater sciatic foramen, typically inferior to piriformis, and runs deep in the posterior thigh. It is already organized into a tibial division (medial) and a common fibular division (lateral) long before the popliteal fossa. The fibular division is more often, and more severely, injured than the tibial division: it is more lateral, more taut, and more exposed during posterior hip approaches and misplaced buttock injections.

NCS consequences of that bias:

• Fibular motor studies (EDB, TA) may look like a severe “fibular palsy,” including a small CMAP, but there is no focal conduction block at the fibular head
• Superficial fibular SNAP is low if the fibular division is axonal
• Sural SNAP is often low because the medial sural cutaneous contribution travels with the tibial division and the communicating branch with the fibular complex — sciatic lesions sit proximal to both
• Tibial motor responses (AH, abductor digiti minimi pedis) may be relatively preserved when the fibular division takes the brunt, or reduced when both divisions are involved
• Short-head biceps femoris (physician needle EMG) is abnormal in sciatic fibular-division lesions and normal in isolated fibular-head compression — the anatomic pearl from Section 14.1

A true fibular-head lesion should leave sural and tibial studies intact. When foot drop arrives with a low sural SNAP, think sciatic, plexus, or polyneuropathy before you stop at the fibular neck.

Common sciatic settings include hip arthroplasty (especially a posterior approach), injection palsy from a misplaced dorsogluteal intramuscular injection, prolonged lithotomy or other stretch at the sciatic notch, coma with compression, femur fracture, and penetrating trauma.

Meralgia paresthetica: isolated LFCN

Meralgia paresthetica is an isolated sensory neuropathy of the lateral femoral cutaneous nerve (LFCN), which is derived from L2–L3 and has no motor fibers. The usual compression site is where the nerve passes under the inguinal ligament near the anterior superior iliac spine (ASIS) — tight belts, tool belts, obesity, pregnancy, seat-belt trauma, and iliac-crest bone-graft harvest are classic.

Patients report burning, tingling, or numbness on the anterolateral thigh. Strength, knee reflex, and medial-leg (saphenous) sensation remain normal. That clinical isolation is the diagnosis. LFCN NCS can support it but is technically limited: the SNAP is small, side-to-side comparison is mandatory, and the response is frequently unobtainable in overweight patients and even in some normal controls. A unilaterally absent LFCN SNAP with a preserved saphenous SNAP and normal femoral motor studies fits meralgia. Do not over-read a bilaterally absent LFCN SNAP as bilateral meralgia without the clinical picture.

LFCN studies are not a substitute for femoral NCS when knee extension is weak. If the “thigh numbness” is accompanied by quadriceps wasting or a lost patellar reflex, you are no longer in isolated meralgia territory — move to femoral and lumbar plexus protocols.

SNAP pattern table

Read the table as a map, not as four mandatory studies on every patient. Choose the SNAPs that can be abnormal for the suspected nerve and the SNAPs that should be spared if your localization is correct.

Case: hip surgery, injection palsy, and prolonged lithotomy

Case A — hip arthroplasty. A 68-year-old woman has a posterior-approach total hip replacement. On postoperative day one she has a foot drop. Below- and above-fibular-head fibular motor amplitudes are equally low (no across-head block). Superficial fibular and sural SNAPs are low compared with the other leg. AH CMAP is only mildly reduced. Saphenous SNAP and femoral CMAP to VM are normal. This is sciatic neuropathy with fibular-division predominance, not a fibular-head compression from an operating-table strap and not a femoral palsy from anterior retractors.

Case B — injection palsy. A 22-year-old receives a dorsogluteal intramuscular injection and immediately feels electric pain down the posterior thigh, then cannot dorsiflex the foot. NCS days later shows very small fibular CMAPs without focal fibular-head block, a low superficial fibular SNAP, and a borderline sural SNAP. The history plus a postganglionic SNAP pattern localizes to the sciatic nerve in the buttock, often the fibular division.

Case C — prolonged lithotomy. A 55-year-old patient spends several hours in lithotomy for a pelvic procedure. Afterward, knee extension is weak, the patellar reflex is reduced, and there is numbness on the anterior thigh and medial leg. Saphenous SNAP is unilaterally low; femoral CMAP to rectus femoris is reduced; LFCN SNAP is obtainable and symmetric; fibular and tibial distal studies are normal. Lithotomy can compress or stretch the femoral nerve at the inguinal ligament or iliacus, and it can also stretch the sciatic nerve or compress the fibular nerve at the fibular head against a stirrup — so a complete lower-limb NCS screen is warranted. In this patient the SNAP map is femoral, not sciatic and not isolated meralgia.

When the referral simply says “post-op numbness,” build the protocol from the deficit: saphenous and femoral motor for quadriceps and medial-leg problems; sural, superficial fibular, and fibular/tibial motor for foot drop; LFCN only when the story is isolated anterolateral thigh sensory change.

Key Takeaways

• Saphenous SNAP (and femoral motor to VM/rectus when performed) maps the femoral nerve at the iliacus or inguinal ligament.
• Sciatic lesions often hit the fibular division harder and lower sural and SF SNAPs without across-fibular-head block.
• Meralgia is isolated LFCN sensory disease; the NCS is limited and never replaces a motor exam.
• Hip surgery, injection palsy, and lithotomy are the history clues that should expand the protocol beyond a distal entrapment screen.