This is a common clinical condition that presents with pain & hyperaesthesia on antero-lateral aspect of thigh along distribution of lateral cutaneous nerve of thigh. The nagging discomfort over the lateral part of thigh happens because of compression of the lateral cutaneous nerve of thigh. This is a common type of entrapment neuropathy where the nerve is compressed.
The lateral cutaneous nerve of thigh arises from lumbar plexus (L2,3). It comes into the thigh by piercing the inguinal ligament approximately 2 cm medial and 2 cm inferior to the anterior superior iliac spine. It supplies the skin on the antero-lateral side of the thigh as far as the knee and also anterior part of gluteal region.
The nerve may be compressed commonly under the inguinal ligament but also at the level psoas muscle and in the fascia lata. The exact cause of compression is unknown in majority of cases but it may be due to trauma, tumours, surgical injury, retroperitoneal haematoma, aortic aneurysm, prolonged pressure from belts etc.
Patients commonly complain of burning pain, allodynia (pain on touch) and hyperesthesia. The pain may be reproduced by pressure over the inguinal ligament or by a reverse straight leg-raising test.
The diagnosis is made clinically which can be confirmed by nerve conduction study or by diagnostic nerve block.
Infiltration local anaesthetic mixed with depo-steroids around the nerve at the suspected point of compression can solve this particular pain problem. As mentioned earlier, often it gets compressed between anterior superior iliac spine and inguinal ligament. The needle is inserted perpendicularly at a point 2 cm medial & 2-3 cm inferior to anterior superior iliac spine. As the needle passes through the external and internal oblique muscles, resistance to needle is felt, which is lost and a pop is felt. As the tip of the needle enters the area of the nerve one may get paraesthesia. This depth should not be more than 1.5 to 2.5 cm. After reaching this area about 10 ml of bupivacaine 0.5% mixed with about 40-80 mg of depot -steroid is injected in a fan-wise fashion. Patient will feel numbness of lateral part of thigh. Patients usually require about 3-5 such injections at weekly intervals. If situation does not respond to these injections, then surgical decompression might be needed.