PREVALENCE AND MANAGEMENT OF REFRACTORY MERALGIA PARESTHETICA IN LUMBAR SPINE SURGERY: 5 YEARS OF EXPERIENCE

ABSTRACT Objective The objective of our study was to report 5 years of experience in the recognition and management of refractory meralgia paresthetica (MP) in patients who had undergone posterior approach lumbar surgery. Methods Patients who were submitted to procedures in the lumbar spine from January 2010 to January 2015 in three different hospital centers in Belo Horizonte/MG were selected for an evaluation of the postoperative development of MP. A prospective observational comparative case series study. Level of evidence III. Evaluation of the following parameters: type of support for the patient, surgical time, body mass index. Results 367 posterior approach lumbar spine surgeries for degenerative pathologies of the lumbar spine were performed. MP was observed in 81 patients (22%). In 65 of those patients (80%), there was complete resolution of the symptoms with conservative management (local measures and medications for neuropathic pain) in less than two months. Twelve patients improved with a corticosteroid depot injection in the inguinal ligament and four patients required a surgical procedure in the third month. Pneumatic support was the least involved in the development of MP, as well as surgical time <1h and body mass index <25. Conclusion Refractory MP may occur in patients submitted to posterior approach lumbar spine surgeries. Management includes local measures, medications for neuropathic pain, and corticosteroid injection in the inguinal ligament. Decompression surgery is reserved for rare refractory cases. Level of evidence III; Prospective observational study with comparative case series.


INTRODUCTION
Meralgia paresthetica is the compressive mononeuropathy of the lateral cutaneous nerve of the thigh (or lateral cutaneous femoral nerve) and can cause discomfort and neuropathic pain in the anterolateral region of the thigh. 1 It is estimated that the incidence of meralgia paresthetica following lumbar spine surgery in the prone position is approximately 20%. 2,3 Despite being a relatively common complication, there are few reports in the international literature and none in the Latin American, [3][4][5] as well as an absence of focus on the management of refractory cases.
Objective: To report the experience with and management of refractory meralgia paresthetica in patients submitted to posterior approach lumbar spine surgery.
Patients with degenerative lumbar spine disease who were going to be submitted to some posterior approach procedure were selected after filling out the Informed Consent Form and approval of the study by the Institutional Review Board. Oncological surgeries, spinal cord surgeries, spinal cord trauma surgeries, and surgeries of the upper lumbar levels L1, L2, and L3 were excluded.
A diagnosis of meralgia paresthetica was based on clinical evaluation and should meet the two criteria: positive Tinel's sign in the inguinal ligament and neuropathic pain on the anterolateral surface of the thigh.
A prospective, observational study was conducted in these three centers with the following evaluation criteria: type of patient support, surgical time, and body mass index.
Success of the treatment was confirmed by a score of less than 4, mild pain, on the Verbal Numeric Scale (VNS), and by the disappearance of the Tinel's sign on the inguinal ligament and of the burning on the lateral surface of the thigh.
The times of appearance and disappearance of the symptoms were noted. Refractory cases were defined as the lack of improvement -Verbal Numeric Scale for Pain (VNS) > 5 and persistence of symptoms beyond two months, despite medication and local measures.
Treatments with medication, depot injection of corticosteroids, and surgical decompression of the cutaneous femoral nerve were evaluated.

RESULTS
There was a total of 367 posterior approach lumbar spinal surgeries for degenerative lumbar spinal pathologies. Oncological surgeries, intra-spinal surgeries, trauma surgeries, and surgeries of upper lumbar levels L1, L2, and L3, which can mimic meralgia paresthetica through the manipulation of the nerve roots involved at these levels, were excluded.
Meralgia paresthetica was observed in 81 patients (22%), 77 (95%) of whom were diagnosed during the first postoperative week; in four patients (5%) it occurred during the second week. Sixty-five patients (80%) had full recovery of symptoms in under two months with conservative management, that is, local measures and medications for neuropathic pain. Meralgia refractory to drug treatment was observed in 16 patients, 12 of whom improved with depot corticosteroid and anesthetic injections at the location where there was a positive Tinel's sign in the inguinal ligament, and the condition resolved in six months. In four patients, a surgical procedure was necessary in the third month with cutaneous femoral nerve decompression under local anesthesia and sedation. One patient required a neurectomy in the sixth postoperative month.
Regarding the operating table supports used, 48 surgeries were performed using the Wilson Bridge, 162 surgeries were performed using cloth cushions, and 157 were performed using automobile inner tubes as supports. (Table 1) The other variables analyzed in this study, such as surgical time and body mass index, are shown in Tables 2 and 3, respectively.

DISCUSSION
Meralgia paresthetica (MP) is a relatively common compressive mononeuropathy of the lower limbs, first described by Bernardt-Roth in 1878. 6 The first cutaneous femoral nerve decompression surgery of the thigh was performed by Harvey Cushing in 1900. 6 The main risk factors for this syndrome are obesity, tight clothing, surgeries in which grafts are taken from the iliac crest, pregnancy, and prolonged surgeries performed with the patient in the prone position. 5,6 Posterior approach lumbar spine surgeries, even without iliac crest graft removal, have been reported to be predisposed to this syndrome 3,5,6 . (Figure 1) In this study, the prevalence of MP was 22%, which is consistent with the case series described in the literature.
This study includes aspects that have not yet been addressed in the literature, such as the type of support used and how to proceed in refractory cases that have not improved after two months of treatment.
In most cases, meralgia paresthetica is a common benign condition with complete resolution after a few weeks. In our study, we adopted conservative treatment with medications and local measures, that is, massage, compresses, and loose clothing for up to two months following surgery. The two-month timeframe was based on the work of Yang et al., 7 in which remission within two months was reported in all cases. Table 4 shows the progression of the MP treatment. It begins with local measures and medications and continues with a corticosteroid injection, eventually leading to surgery in refractory cases.
In cases where there was no improvement after the first two months, we opted for an injection of 10 ml of depot corticosteroids (methylprednisolone acetate) associated with a local anesthetic (ropivacaine 1%) at the point of pain on percussion (positive Tinel's sign) in the inguinal ligament. (Figure 2) This procedure aimed to confirm both the diagnosis 6 and the therapy.
Following the injection of depot corticosteroid with anesthetic solution, we observed improvement in 75% of the cases (12 of the 16 refractory patients). This procedure has not yet been described in the specific management of MP in patients submitted to lumbar spine surgery and we believe that it can be a valid tool in the management of refractory cases or in those in which there is doubt about postoperative lumbar radiculopathy. One month after said procedure, the pain profile recurred in four patients and we decided to perform cutaneous femoral nerve decompression surgery. (Figure 3) In one of these four patients, we performed a neurectomy of the cutaneous femoral nerve two months following decompression surgery. All the patients were followed-up for more than one year.
Another aspect analyzed that has not been emphasized in the literature is the type of support used for the prone position. We observed that pneumatic support (automobile tire inner tube) was the type of support least associated with the appearance of MP, followed by cloth cushions and the Wilson Bridge.
The risk factors observed in the study associate the pathogenesis of MP with the point of greatest pressure. This finding indicates that the use of more rigid supports is a probable compression risk factor in the genesis of postoperative lumbar MP.
The main limitations of this study are its observational character and the absence of a control group for the treatment of refractory cases. Therefore, we cannot state whether treatment with inguinal ligament injection or surgical treatment is better than simple observation and adequate drug treatment.

CONCLUSION
Refractory MP may occur in patients who underwent posterior approach lumbar spine surgery. Its management includes local measures, medication for neuropathic pain, and corticosteroid injections into the inguinal ligament. In refractory cases, decompression surgery or retroperitoneal neurectomy may be performed.
All authors declare no potential conflict of interest related to this article.