On neurologic examination, the patient is asked to outline the distribution of discomfort. He or she can invariably do so by taking a finger to delineate areas of abnormal sensation; typically this is a small football-like elliptic area of dysesthesias. The remaining neurologic examination is normal and, in particular, there is no weakness in the femoral-innervated muscles.
Almost all LFCNs develop insidiously without the identification of a specific pathophysiologic mechanism. On rare occasions, an individual unaccustomed to utilizing a tight harness or a backpack belt tightly positioned across the iliac crest, may report more painful dysesthesias appropriate to the LCFN innervation. Other mechanisms include compression with orthopedic appliances or athletic injuries—from direct blunt trauma to the thigh in high-energy sports or, in female gymnasts, due to the repetitive impact on the thigh by parallel bars. Almost all idiopathic LFCN lesions gradually resolve. Reporting this to the patient is very reassuring.
When the patient’s history is atypical, such as because of constant boring pain leading to postural changes or difficulty sleeping, it is important to obtain CT scans or MRIs to evaluate the possibility of other lesions, such as a potentially fatal soft tissue sarcoma.
ILIOHYPOGASTRIC, ILIOINGUINAL, AND GENITOFEMORAL NERVES
The iliohypogastric, ilioinguinal, and genitofemoral nerves are primary sensory nerves innervating the lower abdomen, inguinal region, the upper and medial anterior thigh, and part of the genitalia. Each has an L1 origin; sometimes there are T12 and L2 contributions. The iliohypogastric and ilioinguinal nerves track laterally, similar to other cutaneous thoracoabdominal nerves. The genitofemoral nerve descends distally through the psoas, emerging to pass under the inguinal ligament, dividing into genital and femoral branches. Its genital branch accompanies the ilioinguinal nerve and has a similar cutaneous distribution; it also innervates the cremaster muscle. The femoral branch, and the ilioinguinal nerve innervate small areas of the most proximal anterior thigh. The ilioinguinal nerve innervates the skin above the inguinal ligament; the base of the penis and upper scrotum in men, or, in women, the mons pubis and labium majus; and the upper medial thigh. The iliohypogastric nerve innervates the distal abdominal wall musculature, its adjacent skin, a small area above the pubis, and a minor portion of the upper buttocks.
These nerves are most likely to be affected subsequent to inguinal hernia repair. Entrapment may occur as the ilioinguinal nerve emerges from the abdominal wall near the iliac crest. This is typified by iliac fossa, inguinal allodynia, or hyperesthesia radiating to the genitalia, often exacerbated by walking and hip extension. Relief may occur with hip flexion. This neuropathy is often clinically difficult to prove; sometimes a local nerve block may confirm the diagnosis and provide therapy. Either a retroperitoneal lymphoma or intrapelvic/inguinal endometriosis merits differential diagnostic consideration.
OBTURATOR NERVE
Originating within the lumbar plexus and derived from the anterior divisions of the L2, 3, 4 nerve roots, these nerves unite within the posterior psoas muscle, forming the obturator nerve. This descends through the iliopsoas to emerge medially near the upper sacroiliac joint. The obturator nerve then courses along the pelvis, lying lateral to the ureter and internal iliac vessels, and bending anteroinferiorly to follow the lateral pelvic wall. It next passes anterior to the obturator vessels while lying on the obturator internus muscle, to reach the obturator groove, and then it enters the obturator canal. Here it descends to the medial thigh, supplying the obturator externus, pectineus, adductor longus, adductor brevis, and gracilis muscles, as well as hip and knee joints and medial thigh.
Obturator mononeuropathies are exceedingly rare in both adults and children. Various pathophysiologic mechanisms include pelvic and hip fractures, rarely obturator hernias, malignancies, surgery involving the hip and pelvis, particularly when patients are placed in the lithotomy position, and various laparoscopic interventions.
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