Laboratory Studies
Indicated laboratory and radiologic studies for lateral femoral cutaneous nerve (LFCN) pathology depend on the suspected etiology and clinical impression. Meralgia paresthetica (MP) caused by obvious benign compressive forces requires no further investigation; however, the following scenarios may necessitate more testing:
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Magnetic resonance imaging (MRI) to investigate the lumbar plexus
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Serum tests for diabetes
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Radiography for possible pelvic fracture or cancer
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Computed tomography (CT) scanning for retroperitoneal hemorrhage in patients who have undergone anticoagulation therapy
A study by Suh et al indicated that ultrasonography can aid in the diagnosis of MP by determining the cross-sectional area of the LFCN. The investigators suggested that, based on the values derived from the patients in the study with unilateral MP (n=23), as well as from the controls (n=12), the cross-sectional area’s optimal cutoff value for a diagnosis of MP was 5 mm2. [15]
Electromyography is very helpful for ruling out radiculopathy, plexopathy, generalized polyneuropathy, or other neuropathic causes for the symptoms. Nerve conduction studies and somatosensory evoked potentials of LFCN have shown abnormalities in patients with MP, but these tests are unreliable and are not necessary for the diagnosis.
Caution: Discomfort over the anterolateral thigh may not be representative of LFCN injury. Referred pain can be caused by neoplastic invasion of nearby femoral or pelvic bone. In the author's experience, the pain associated with a neoplasm or bone fracture is deep, boring, and severe. MP discomfort is superficial, with fluctuating dysesthesias, paresthesias, and cutaneous hypersensitivity; it is neuropathic in quality and usually is not disabling.
Procedures
See the list below:
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For classical meralgia paresthetica (MP), conservative therapy may be initiated without the necessity for invasive procedures. The diagnosis (ie, an MP secondary to trauma, irritation, or compression of the lateral femoral cutaneous nerve [LFCN] near the IL) can be verified by injecting a small quantity of lidocaine at the point of their intersection or at the point of tenderness. The discomfort should resolve transiently.
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Diagnosis of LFCN often is verified by nerve conduction studies. The LFCN is stimulated antidromically 1-2 cm medial to the ASIS above the IL; the response is recorded 12-20 cm distally over the lateral thigh. Excessively thick soft tissue makes surface stimulation technically difficult or inadequate; the examiner may choose needle stimulation instead. Right and left LFCNs are compared. Study findings are considered abnormal if there are significant side-to-side differences; there may be no response or a 50% drop in amplitude compared with the contralateral nerve. If there is no recordable response on the asymptomatic side, the results obtained when testing the symptomatic thigh are not interpretable. Asymptomatic individuals have widely variable LFCN action potentials in conduction studies.
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Basic anatomy of the lateral femoral cutaneous sensory nerve. The blue region over the anterolateral thigh outlines the area of cutaneous innervation.
