Teaching Case

55-year-old caucasian male with a history of pain and numbness on the anterolateral side of the distal part of the left upper leg since more than 5 months. Spontaneous onset, but aggravated with walking and running. The pain was continuous but worse with longer walks. Running was no longer possible.

MRI of the lumbar spine did not show any nerve involvement, specifically of the L2, L3 or L4 nerve roots on the left side. No signs of lumbar spinal canal stenosis or foraminal stenosis on the left side.

Ultrasound with bilateral comparison of the region of the anterior superior iliac spine showed clear swelling and hypo-echoic aspect of the sartorius tendon on the left side. A small calcification was present in the tendon. There was no hyperaemia. The nearby lateral cutaneous femoral nerve on the left side was clearly swollen (cross-sectional area 6 mm² on the left side, 3 mm² on the right side) and hypo-echoic (Figures 1a and 1b).

CT scan of the pelvis confirmed the calcification at the insertion of the sartorius tendon on the left anterior superior iliac spine (Figures 2a and 2b).

Meralgia paresthetica is caused by the entrapment of the lateral femoral cutaneous nerve (LFCN) at the level of the inguinal ligament. The LFCN (pure sensory) is formed from the L2 and L3 spinal nerve roots, travels downward lateral to the psoas muscle, and then crosses the iliacus muscle. At the inguinal ligament, it travels underneath the inguinal ligament, anterior, trough or posterior to the sartorius muscle and most commonly 10 mm medial to anterior superior iliac spine (ASIS) [1]. Once distal to the inguinal ligament, the LFCN branches into anterior and posterior divisions. However, the anatomy of the LFCN is variable. It may also travel superior or lateral to the ASIS, or the LFCN can even pass through the ASIS, inguinal ligament or sartorius. But in 86% of the cases its course is medial to the ASIS [2]. Injury is usually the result of external compression or trauma, but may also be caused by internal structural changes. To our knowledge, this is the first described case in which the LFCN  is compressed by calcific tendinosis of the sartorius tendon. Symptoms include numbness, tingling/neurogenic sensations, and dysesthesias to touch or rub over the anterior and/or lateral thigh.

Diagnosis can be made by nerve conduction studies. However, this is technically challenging, especially in obese patients. Ultrasound of the nerve can be used to confirm diagnosis and to uncover the underlying cause of compression. Look for anatomical variants, swelling and structural changes of the nerve and changes of nearby structures (like the sartorius tendon) that might compress the LFCN. Ultrasound also provides immediate ultrasound-guided treatment if needed [3]. Also, high-resolution 3-Tesla MRI can detect signal alterations in the LFCN with good sensitivity and specificity and may exclude urogenital or gynaecological conditions [4].

In this case, an ultrasound-guided injection with cortisone (depo-medrol 40mg®) around the nerve and the calcification resulted in quick improvement of the pain. Four weeks after the treatment, an ultrasound showed almost symmetrical thickness of the lateral cutaneous nerve close to the anterior superior iliac spine. The patient could walk pain-free and could restart running. There was persistent numbness at the anterolateral side of the distal part of the left upper leg. The prognosis of meralgia paresthetica is usually good: 85% of patients report recovery with conservative management [5]. If conservative treatment fails, radiofrequency treatment or surgery (decompression) may be considered.

Lateral femoral cutaneous nerve (LFCN) compression caused by calcific tendinopathy is a rare diagnosis. Ultrasound can be used to detect features attributable to neuropraxia of this nerve.