Meralgia paresthetica (MP) or lateral femoral cutaneous nerve (LFCN) entrapment syndrome is a common lower limb entrapment neuropathy with an incidence of 3–4/10000 person-years and a male preponderance. It usually manifests in the fifth decade. It is mostly idiopathic in nature with some predisposing factors such as obesity, wearing tight belts, beepers, cell phones, and pregnancy, while on occasion it may be secondary to prone position surgery, trauma, tumors, etc.
LCFN is a pure sensory nerve arising from the lumbar plexus formed by L2–L3 root that appears along the lateral border of the psoas major, runs on the anterior surface of the iliacus to reach the inguinal ligament, and exits under the inguinal ligament medial to anterior superior iliac spine at a variable distance. It supplies the anterolateral aspect of the thigh. Diagnosis is based on symptomatology and clinical examination including provocation tests such as pelvic compression tests. Electrophysiology may aid in confirmation with a ratio of the sensory nerve action potential (SNAP) on both sides >2.3 or a SNAP of <3 μV on the affected side suggestive of nerve entrapment.
Treatment strategies can be divided into conservative and interventional. Conservative measures include weight reduction, avoidance of tight-fitting trousers/belts, and use of neuropathic pain medications like gabapentin, pregabalin, carbamazepine, or non-steroidal anti-inflammatory drugs (NSAIDs).
It is a general consensus that MP often responds to conservative management only. There are very few studies reporting the proportion of patients responding to conservative therapy alone. Ecker et al. (1938) found complete resolution of symptoms over 2 years in 62% of cases with conservative therapy akin to Chhuttani et al. (1966) who reported complete resolution in 60% of cases.
Therefore, approximately one-third of patients may not respond to conservative measures. Here comes the role of interventional therapies, which include either local nerve blocks with or without steroids or surgical interventions such as neurolysis or neurectomy in refractory cases.
The local nerve block is a safe and practical alternative to surgery for patients non-responsive to conservative measures. Steroids inhibit transmission in C-type fibers and ectopic release. Local anesthetics block A-delta and C fiber transmission and block sodium channels in sympathetic neurons as well. This results in the release of nitric oxide, which enhances vascular microcirculation and reduces inflammation.
Various studies have reported the effectiveness of local infiltration of the nerve with anesthetics with or without steroids in MP, though the response rate has been variable across the studies. Ivins (2000) reported complete recovery in 33% of cases to up to 85% response rate observed by Dureja et al. (1995). Elavarasi et al. studied the effect of local steroids (triamcinolone) in eight MP patients, five of them required multiple injections and six patients (75%) showed complete improvement. There is a dearth of high-quality randomized controlled trials (RCTs) for the assessment of nerve blocks vs conservative measures in MP. A study conducted by Okur et al. (2017, n = 38) reported that local nerve blocks were more effective than pregabalin in MP.
The LCFN has a highly variable exit course, which is one of the major reasons for the failure of the local nerve blocks. The use of ultrasound guidance for nerve blocks has shown a cent percent success rate as reported by Tagliafico et al. and Khodair et al. though the proportion of the population requiring multiple injections varied among the two studies.
A recent RCT by Kilic et al. (2020) compared ultrasound-guided local injection (n = 17) vs transcutaneous electrical nerve stimulation (TENS) (n = 16) vs control (n = 21). Outcome measures included pain assessment using a visual analog scale and painDETECT questionnaire, cutaneous pressure threshold using Semmes–Weinstein monofilament test (SMWT), and quality of life evaluation using the Pittsburgh sleep quality index (PSQI) and Short Form Health Survey (SF-36), which were performed before and after treatment at day 15 and at 1 month. Local injections showed a statistically significant reduction in pain scores and SMWT scores but did not show a significant reduction in quality-of-life scores. Pain reduction and SMWT scores showed a statistically significant reduction in local injection groups vs the sham and TENS groups.
It is surprising that so few RCTs have been undertaken for the management of this disturbing and disabling painful disorder, identified way back in 1878.
The current study compared local anesthetics with or without steroids for the management of MP. They found that both modalities resulted in a statistically significant reduction in pain scores (i.e., numerical rating scale) but no statistically significant difference in pain scores was noted on comparing the two groups. It is a well-conducted, well-balanced, and adequately blinded study.
The use of an ultrasound-guided nerve block by the authors was a wise decision as it increases the precision of the given treatment. USG-guided nerve blocks should be introduced into the standard protocol for the treatment of MP.
The limitations of the study were a small patient population, lack of control or sham group, and short duration of study follow-up. A control group would have helped us to determine the effectiveness of nerve blocks against conservative treatment. This study would have been more relevant in relation to refractory cases. In a given clinical scenario, it could have helped in selecting patients where steroids can have an additional benefit.
In addition, the authors used only a subjective scale as an outcome measure; to make results more robust and unbiased, objective pain threshold scores and quality of life assessment scores could have been included.
There are several unsolved problems regarding the therapy of MP, which need to be addressed, notably:
- How effective are local nerve blocks when compared to conservative therapies?
- How many sessions of local nerve blocks are needed for complete remission and long-term follow-up of these patients is largely not known.
- High-quality studies comparing local steroids against surgical interventions in treatment-refractory cases are needed.
TC – Conception, design, drafted the manuscript, accountable for all aspects of the work
VG – Conception, design, revision, and final approval of the manuscript, accountable for all aspects of the work.
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