Despite its relative common occurrence, definitive diagnosis of small fiber neuropathy (SFN) remains problematic. In practice, patients with pain, numbness, and/or paresthesias in their lower limbs are diagnosed with SFN if found to have dissociated sensory loss in their feet, that is, impaired pinprick perception (PP) but relatively preserved vibration. We sought to assess the sensitivity and specificity of clinical examination and various diagnostic tools available for screening SFN.
Medical records of 56 patients diagnosed with SFN were reviewed. Diagnosis was based on symptoms, detailed neurological examination that included PP, and abnormal results on at least one testing modality—quantitative sudomotor axon reflex (sweat) test (QSART), quantitative sensory testing (QST), and heart rate variability (HRV) testing.
Sensitivity of PP was relatively consistent between modalities of about 63% in presence of appropriate sensory symptoms. Laboratory testing diagnosed 88% of patients when both QSART and QST are employed. QST was most sensitive for detection of SFN with the heat-pain testing having higher sensitivity than cooling. Heart rate variability testing revealed low correlation across all groups.
The diagnostic yield for SFN increases by combining clinical features with various testing modalities. In symptomatic patients, we propose the following diagnostic criteria for diagnosis of SFN: Definite SFN—abnormal neurological examination and both QSART and QST; Probable SFN—abnormal neurological examination, and either QSART or QST; Possible SFN—abnormal neurological exam, QSART, or QST.