Hello, my name is Devin Rubin, and this Teachable Moment will be focusing on neuralgia parasitica. The objectives of this Teachable Moment are to review the clinical features of neuralgia parasitica, discuss the causes and etiologies, and discuss the electrodiagnostic and neuromuscular ultrasound approach or evaluation to patients with suspected lateral femoral cutaneous neuropathy or neuralgia parasitica. So this morning, a patient came to our EMG lab. She was a woman who was sent for further evaluation of complaints of burning pain in the anterolateral aspect of her right thigh, so somewhere around this area of her thigh. And her story was that she had experienced this burning pain that came on insidiously without any trauma or precipitating factor about three months prior to her presentation this morning. The symptoms were in the area in the anterior thigh and the lateral thigh. She could draw the area with her finger where she was experiencing the symptoms. She did have some slight symptoms in the left thigh in the same area, but it was predominantly in the right. She described a burning pain. It was sometimes a sharp, stabbing pain, and it seemed to be worse at night when she was laying down and trying to sleep, but she did have some during the day. She also felt some numbness in that area, and when she would touch the area, it was just not as sensitive as it was on the rest of her thigh. Her neurologic examination demonstrated normal strength in her leg, normal reflexes, including in her quadriceps reflex, and she did have decreased pinprick sensation in the area that she was experiencing the pain. So it was really in the anterolateral aspect of the thigh. So this is not an uncommon presentation, and when we're faced with a patient that comes in complaining of symptoms in the anterolateral thigh, there are several localizations that one might consider to account for the patient's symptoms. So thinking about this from a proximal to distal localization, one consideration is a radiculopathy, particularly the L3 or perhaps the L4 root. Lumbar plexopathy would be a consideration, although most patients with lumbar plexopathy will have significant weakness and not just pain or sensory loss. I added femoral nerve problem here, but remember that the anterolateral aspect of the thigh is supplied by the lateral femoral cutaneous nerve, and that does not come off of the femoral nerve. It comes off directly from the lumbar plexus. And then there's the lateral femoral cutaneous nerve of the thigh or neuralgia parasthetica, and also consider hip pathology, and there are patients who will present with pain more so than numbness in the thigh or in the groin due to hip pathology. So these are all potential considerations. So in this patient, the concern and the clinical features were very consistent with impingement or injury to the lateral femoral cutaneous nerve. This is a nice diagram demonstrating the nerve. This nerve comes off of the lumbar plexus, so as I just said, it does not emanate from the femoral nerve itself, which would be in this area, comes off of the plexus into the pelvic region, and it runs just underneath the inguinal ligament by the anterior superior iliac spine of the iliac bone, and then it runs underneath that inguinal ligament, and it has multiple branches supplying the sensation to the anterolateral thigh. And in the clinical condition of neuralgia parasthetica, this nerve is impinged or compressed in this area between the bone and the inguinal ligament. So you can see in this hand-drawn picture, there's the erythema, the irritation of the nerve and the compression of the nerve. And as you know, when a nerve is irritated or compressed, it causes sensory symptoms if it's a sensory nerve. So what are the symptoms of neuralgia parasthetica? Well, they may be a combination of symptoms that include tingling sensation, numbness, or a decrease in sensation. Some patients will describe a burning sensation or electrical sharp stabbing type of sensation or a combination of all of the above. And the distribution is, of course, the distribution of the dermatome of the lateral femoral cutaneous nerve of the thigh. So this is the general distribution, and every patient's distribution is going to be slightly different. Sometimes the nerve will extend more anteriorly, but it's mostly the anterolateral aspect of the thigh. And you can see it in this picture as well in the redden or erythematous area. So what are the causes of neuralgia parasthetica? Well, there are a variety of causes. Classically, it's described when the nerve is compressed, and it's reported to occur when people wear tight clothing, such as tight belts or tool belts, if they've gained weight with obesity or pregnancy or some medical condition where there's fluid accumulation in the abdomen or any type of procedure around that nerve. But many patients don't complain or don't note any of these things. They haven't gained weight. They aren't wearing any different belts or clothing, and it occurs idiopathically without any known etiology. I will say that although this isn't often listed as a possible cause, any change or increase in exercise may irritate that nerve. And on a personal note, I had symptoms in that distribution about a year ago when I was training for a long hike, and I was wearing a backpack that had a hip belt and doing lots of walking and climbing stairs and went on a 10-day hike. And at the end of the hike, I noted symptoms of some burning and some numbness in the lateral femoral cutaneous nerve distribution. Fortunately for me, after the hike was over and I stopped doing all the exercising and walking and wearing the backpack, my symptoms improved over time. But you can have these symptoms from change of activity if that nerve is being irritated or compressed or repetitively stretched. So how do we evaluate this in the electrodiagnostic laboratory? Well, this is a tough nerve to evaluate, the lateral femoral cutaneous nerve. It's a challenging nerve. It's a small sensory nerve, and it's a technically difficult nerve to test. So the first step is to perform routine nerve conduction studies, which are usually normal. And these consist of the fibular motor or the tibial motor and maybe a sensory such as the serral, looking for evidence of a more diffuse process. Needle EMG is also important to exclude an upper lumbar radiculopathy or plexopathy. So this may include some of the vastus muscles, the adductor longus, or perhaps the iliopsoas. And if these are abnormal, then the diagnosis is not a lateral femoral cutaneous neuropathy, but would raise the possibility of another etiology. Then the lateral femoral cutaneous nerve conduction studies can be performed to test that nerve. These are technically challenging conduction studies, particularly in obese patients. And you can use neuromuscular ultrasound to identify the nerve, which may help with nerve conduction studies, and to look at the size of the nerve or look for any structure abnormality that may be compressing the nerve. To perform the lateral femoral cutaneous nerve conduction study, this is a picture depicting the technique. So the G1 and G2 electrodes are placed in the anterolateral aspect of the thigh. And if you look at the hand-drawn picture of the nerve, you'll note that there are a number of branches. So this is where it becomes technically challenging, because you often need to move the electrodes to find the largest branch or the branch that you obtain the best response. We can then stimulate the nerve, usually just underneath the inguinal ligament at the site of the nerve exiting underneath the inguinal ligament. And the stimulation and the electrode distance varies from about 10 centimeters to 12 centimeters. I find that 10 centimeters might give a better response. Some textbooks state to place these 12 centimeters. And again, you have to move these electrodes around. Some people will stimulate both underneath the inguinal ligament and then more proximal, although this is technically difficult, because you're now getting into the pelvic region and abdomen, and there's more adipose tissue in most patients. So in our laboratory, we only stimulate at the distal site. We're looking for the amplitude or an amplitude change. And in this patient from this morning, these were the lateral femorocutaneous sensory conduction studies. So on the affected side, we did not obtain a response. And on the unaffected side, we obtained a response. This was a reliable response. And the amplitude is only 4 microvolts. So it was very low, even on the normal unaffected side. And these are typically very low amplitudes. And lastly, we can use neuromuscular ultrasound to identify the nerve. And you can scan the nerve underneath the inguinal ligament. You can scan it more proximally. And when you scan it more distal to the ligament, the nerve kind of looks like a little eyeball here. You can see the nerve circled here. And you can follow the course of the nerve here on the affected side. You still see the nerve. And you actually see the two branches of the nerve. These are both the lateral femorocutaneous branches. So here, the nerve was a little bit larger on the normal side compared to the affected side, although this might reflect that this is prior to the nerve branching, where this is post-branching. So it's a little bit hard to interpret the size difference. But if you don't see a response on the affected side and you have a nice nerve, you can identify the nerve nicely. That might support that there's atrophy and axonal degeneration of the nerve. So these are some clinical pearls, clinical features, and electrodiagnostic and ultrasound features of neuralgia parasitica. This is primarily a clinical diagnosis. And in patients who we can't get lateral femorocutaneous nerve conduction studies on either the good side and the affected side, we have to basically say that this could be due to technical factors and we can't reliably confirm or exclude this on electrodiagnostic testing. So this is primarily a clinical diagnosis. Electrodiagnostic testing is helpful to exclude alternative diagnosis and sometimes support this condition if you can obtain lateral femorocutaneous nerve conduction studies and neuromuscular ultrasound may be helpful to identify the nerve and help with the techniques of the nerve conduction studies and also help confirm the diagnosis. Thank you.

neuralgia parasitica

Dr. Devin Rubin

clinical features

nerve conduction studies

neuromuscular ultrasound

ncs