1 Introduction

Meralgia paresthetica (MP) is a painful mononeuropathy affecting the lateral femoral cutaneous nerve (LFCN). It occurs at a rate of 4.3 cases per 10,000 person-years in the general population and typically occurs between the ages of 30 and 40 years [1].

Regarding the anatomy of the LFCN in the thigh, it is a pure sensory nerve originating from L2 and L3 nerve roots. It traverses beneath the inguinal ligament and then bifurcates into anterior and posterior branches. The primary division typically involved in MP is the anterior branch, which provides sensory innervation to the anterior thigh extending up to the knee [2].

MP manifests by pain, tingling, and numbness in the anterolateral side of the thigh [3].

This condition is linked to factors such as obesity, diabetes mellitus (DM), extended surgical procedures, as well as other entrapment causes like pregnancy, wearing constrictive clothing. Furthermore, numerous case reports have been documented concerning atypical sources of traumatic injury and compression of LFCN by pelvic and retroperitoneal tumors [4].

In this case report, we present a case of right LFCN compression by a cyst in right adnexa adjacent to ovary.

To the best of our knowledge, this is the first report indicating the association between an adnexal cyst and MP in the literature.

We report this case after the patient's consent.

2 Case History/Examination

A 37-year-old housekeeper female (height, 160 cm; weight, 60 kg; body mass index, 23.4) was admitted to the Electrodiagnostic Department of Al-Zahra Hospital, Isfahan, Iran. She mentioned experiencing constant paresthesia in the anterolateral side of her thigh for the past 2 months, explaining that her discomfort initially began without any associated injury or trauma. The pain has progressively worsened and has been unresponsive to conservative therapies, including NSAIDs, Gabapentin, and physical therapy. She did not mention any specific aggravating or easing factors and denied changes in bowel or bladder function.

Furthermore, she denied any history of low back pain, radicular pain, or weakness in the lower extremities. However, she did note an unintentional weight loss of 6 kg over the last 2 months without any changes to her dietary habits or exercise routine. Additionally, she reported experiencing night sweats, insomnia, and one episode of menstrual irregularity in the preceding month.

Her past medical history was negative for chronic disease including diabetes.

Upon physical examination, gait was normal and nonantalgic. Muscle strength in the lower limbs was normal. The straight leg raise (SLR) test yielded negative results bilaterally. There was no tenderness noted in the lumbar vertebrae or sacroiliac joints. Reflexes of lower limbs were 2+ and symmetrical. However, there was reduced light touch sensation in the right lateral thigh region compared to the opposite and distal sides of the same limb. Rest of neurological exams, such as cranial nerve examination, primitive reflexes, and physical exams regarding upper motor neuron disease (e.g., extensor plantar reflex), were all normal.

Although the patient's history and physical examination were consistent with MP, the patient was referred to us for electrodiagnostic evaluation of the LFCN to confirm the diagnosis.

3 Investigations and Treatment

The electrodiagnostic evaluation, encompassing nerve conduction studies and electromyography, yielded normal results overall, except for the absence of response from the right LFCN. There was no evidence of myopathy, peripheral polyneuropathy, lumbosacral plexopathy, or lumbosacral radiculopathy.

Consequently, based on both clinical observation and electrodiagnostic assessment, a diagnosis of right meralgia paresthetica (MP) was made. Because the etiology of MP in this case was unclear and the patient had mentioned some red flags in her history such as rapid unexplained weight loss and night sweating, further investigations were recommended to rule out secondary causes such as compressive lesions.

Routine biochemistry tests were within the normal range, with specific markers such as CA-125 at 11.5 U/mL, CA19-9 below 3 U/mL, and a negative result for BHCG.

The patient underwent cystectomy. Her symptoms relieved promptly after the surgery.

4 Discussion

Erhardt–Roth syndrome, more commonly recognized as MP, was initially described in 1878 [5]. It is a widely recognized neuropathy affecting the LFCN, characterized by pain and abnormal sensations experienced over the anterolateral aspect of the thigh. Walking, standing, and hip extension exacerbate the symptoms [6].

The primary source of the LFCN is L2 and L3 nerve roots. It courses along the lateral edge of the psoas muscle and the iliacus muscle, extending toward the ventrolateral region of the thigh, near the anterior superior iliac spine (ASIS) and the lateral inguinal ligament [7].

MP can be classified into spontaneous or iatrogenic categories. Spontaneous causes encompass mechanical factors like obesity and heightened intra-abdominal pressure, limb length inequality, and pelvic crush fractures. Metabolic factors, such as diabetes, alcoholism, and infections like leprosy, also fall within the spontaneous category. Iatrogenic causes involve orthopedic interventions such as pelvic osteotomy, spinal surgery, total hip replacement, and various laparoscopic procedures [8].

One of the most important differential diagnoses of MP is L2-L3 radiculopathy, as both cause anterolateral thigh numbness. However, there are some points that distinguish them. First, the sensory symptoms in MP are limited to the anterolateral thigh, and there is no associated motor weakness or reflex changes, unlike radiculopathies [9]. Additionally, electrodiagnostic studies can effectively differentiate these conditions.

Several case reports have mentioned various mass lesions as causative factors for compression on the LFCN. These potential causes can occur anywhere along the course of the LFCN, from proximal to distal. For instance, as mentioned, compression at the spinal nerve level can induce symptoms resembling MP, prompting the recommendation for an MRI in such patients [9].

A systematic review revealed unusual causes of MP, including those involving mass lesions as the underlying etiology. Among the different masses leading to MP identified in the study were a metastatic tumor of the L2 vertebral body, a schwannoma originating from L2, a uterine fibroid tumor, a renal tumor, a pancreatic pseudocyst, an abdominal aortic aneurysm, a synovial cyst in the iliac fossa, a hematoma of the iliac muscle, and a metastasis to the iliac crest. This study emphasized the importance of being vigilant in the presence of accompanying symptoms such as back pain, fever, and oncological symptoms. In our case, the patient had reported one episode of menstrual irregularity in the preceding month [10]. Thus, obtaining a thorough medical history and conducting a neurological examination are crucial steps in reaching a diagnosis. Nonetheless, it is imperative to rule out red flags such as tumors and lumbar disk herniations [11].

Our patient did not exhibit any of the risk factors associated with MP. She did not wear tight belts, her weight was normal, and she did not have a history of diabetes. However, she had been experiencing unexplained weight loss. Therefore, we hypothesized a space-occupying lesion that confirmed after imaging.

What makes this case unique is that, while most instances of MP are idiopathic without any underlying etiology, there are rare conditions where an important cause is present.

5 Conclusion

We have shown when assessing patients with MP, it is crucial to contemplate the likelihood of prior trauma or compression due to a mass lesion. It is essential to examine for indicative signs that could suggest a traumatic origin or compression by a mass lesion.

Author Contributions

Maryam Behroozinia: conceptualization, data curation, methodology, writing – original draft, writing – review and editing. Elham Sharifian: conceptualization, data curation, writing – original draft, writing – review and editing. Saeid Khosrawi: conceptualization, project administration, supervision, writing – review and editing.

Ethics Statement

The case report meets ethical guidelines and adheres to the local legal requirements.

Consent

Written informed consent was obtained from the patient to publish this report in accordance with the journal's patient consent policy.

Conflicts of Interest

The authors declare no conflicts of interest.