The iliohypogastric nerve arises primarily from the ventral primary rami of L1 and occasionally with a twig from T12. This nerve has a pathway similar to that of the intercostal nerves in the thoracic region. The iliohypogastric nerve traverses the psoas major muscle, piercing the lateral border of the muscle anterior to the quadratus lumborum muscle and posterior to the kidney to traverse the lateral abdominal wall. The nerve penetrates the transverse abdominal muscle near the iliac crest, coming between it and the internal oblique musculature. The nerve supplies the lower fibers of the transverse abdominal muscle and the internal oblique, and divides into the lateral and anterior cutaneous branches.
The anterior cutaneous branch continues anteriorly between the internal oblique and transverse abdominal muscle, then pierces the internal oblique and becomes cutaneous through an opening in the fascial aponeurosis of the external oblique muscle, approximately 2-3 cm above the superficial inguinal ring. The distribution of the cutaneous sensation is a small region just superior to the pubis.
The iliohypogastric nerve is rarely injured in isolation. The most common causes of injury are surgical procedures. These include transverse lower abdominal incisions, as in hysterectomies, or injuries from procedures such as inguinal herniorrhaphy and appendectomies. The injuries mainly occur if the incision extends beyond the lateral margin of the inferior rectus abdominis fibers. The damage can result from direct surgical trauma, such as passing a suture around the nerve and incorporating it into the fascial repair, or postoperative entrapment in scar tissue or neuroma formation. Sports injuries, such as trauma or muscle tears of the lower abdominal muscles, may also result in injury to the nerve. Injury may also occur during pregnancy, owing to the rapidly expanding abdomen in the third trimester. This is called the idiopathic iliohypogastric syndrome and is rare.
Symptoms include burning or lancinating pain immediately following the abdominal operation. The pain extends from the surgical incision laterally into the inguinal region and suprapubic region. Discomfort may occur immediately or up to several years after the procedure, and may last for months to years. This discomfort is possibly because of the formation of scar tissue in the region. Occasionally, the pain may extend into the genitalia because of significant overlap with other cutaneous nerves. Loss of sensation is usually minimal and not problematic. Iliohypogastric nerve entrapment causing symptoms similar to trochanteric bursitis refractory to conventional therapy has been reported.
On examination, pain and tenderness are usually present in the area of scarring or entrapment. Hyperesthesia or hypoesthesia may occur in the area supplied by this nerve. Diagnosis is difficult, owing to the small area of cutaneous supply that this nerve provides. There may be overlap in sensory supply with the genitofemoral and ilioinguinal nerves.
Three major criteria are used to diagnose this nerve injury. The first is a history of a surgical procedure in the lower abdominal area, although spontaneous entrapment can occur. Pain can usually be elicited by palpating laterally about the scar margin, and the pain usually radiates inferomedially toward the inguinal region and into the suprapubic and proximal genital area. Second, a definite area of hypoesthesia or hyperesthesia should be identified in the region of supply of the iliohypogastric nerve. Third, infiltration of a local anesthetic into the region where the iliohypogastric and ilioinguinal nerves depart the internal oblique muscle and where symptoms can be reproduced on physical examination by palpation should provide symptomatic relief.
If no relief is obtained with injection, a different etiology should be sought for the discomfort. Alternate diagnoses include upper lumbar or lower thoracic nerve root pathology, or discogenic etiology of the pain. If the iliohypogastric nerve is identified clearly as the source of pain, and a favorable response is not obtained with local anesthetic injection, then surgical exploration and resection of the nerve should be considered. No reliable electrodiagnostic techniques are available to define the integrity of this nerve, although needle electromyography of the lower abdominal musculature may serve as an adjunct in the diagnosis.
Treatment includes local injection of an anesthetic (as noted above), oral medications, or physical therapy. The oral medications may include antiseizure medications, such as gabapentin , carbamazepine (Tegretol), as well as nonsteroidal anti-inflammatory drugs (NSAIDs), tricyclic antidepressant medications (amitriptyline), capsaicin cream, topical lidocaine patches, or tramadol. With physical therapy, cryotherapy or a transcutaneous electrical nerve stimulation (TENS) unit may be tried. When conservative measures are not successful, surgical excision may result in relief of pain with few potential complications. Krahenbuhl and colleagues reported an endoscopic approach.
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