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The clinical picture of small-fiber neuropathy is dominated by spontaneous and stimulus-evoked positive sensory symptoms—namely thermal and pinprick hypoesthesia—that can mask the signs of small-fiber loss. Only a few studies have attempted to correlate IENF density with validated clinical scales. In patients with diabetic neuropathy, a negative correlation between IENF density and neuropathy symptom score was reported.[53,56] These studies also showed that the extent of epidermal denervation correlated with the duration of diabetes but not with hemoglobin A1C levels, suggesting that IENF density might be useful as a marker of neuropathy progression. A recent study found a high concordance between reduced IENF density and loss of pinprick sensation in the foot.[61]
Skin biopsy has allowed small-fiber neuropathy to be demonstrated in restless legs syndrome[75] and erythromelalgia.[76] In systemic diseases, such as systemic lupus erythematosus, sarcoidosis, Sjögren's syndrome, celiac disease and hypothyroidism, skin biopsy has enabled correlations to be found between neuropathic symptoms and small-fiber degeneration.[52,65,77–79] Although IENF density is a general marker of axonal integrity in peripheral neuropathies, it cannot be used to directly address the question of etiology. Skin biopsy findings can, however, indirectly contribute to the assessment of etiology. For example, in 40% of patients with small-fiber neuropathy diagnosed only after skin biopsy, oral glucose tolerance testing revealed a previously undetected impaired glucose tolerance.[49] Similarly, the distribution of IENF loss can help to differentiate between a non-length-dependent sensory neuronopathy and a length-dependent axonal neuropathy,[78,80] thereby leading to focused screening for associated diseases.
The relationship between IENF density and neuropathic pain remains uncertain. In HIV neuropathy, IENF density correlated inversely with pain severity when assessed by the patient, but not when the Gracely Pain Scale was used.[66] Another study found only a trend towards an inverse correlation between IENF density and pain intensity in this setting.[81] In diabetic neuropathy, patients with pain had lower IENF densities than did asymptomatic patients, but IENF density did not correlate with pain intensity within the group of symptomatic patients.[82] In patients with impaired glucose tolerance, diet and exercise induced a slight recovery of IENF density that was associated with a reduction in pain symptoms.[83] Similarly, epidermal reinnervation coincided with pain reduction after steroid treatment.[71] In length-dependent neuropathies, therefore, more-severe IENF loss seems to increase the risk of developing pain, the intensity of which might decrease in parallel with recovery of IENF density.
In postherpetic neuralgia, on the basis of evidence of relatively preserved skin innervation in the area of severe allodynia, normal thermal sensory function, pain relief in response to topical lidocaine, and worsening of pain with application of capsaicin, surgical removal of painful skin has been attempted.[84] After initial relief, pain increased, became intractable, and spread to previously unaffected dermatomes, suggesting the involvement of central mechanisms in the pathogenesis of neuropathic pain.
Sural sensory nerve action potential (SNAP) amplitude, which reflects the integrity of largediameter fibers, showed concordance with IENF density in the distal part of the leg in patients with large-fiber or mixed small-fiber and largefiber neuropathy. Not surprisingly, skin biopsy analysis seemed to be more sensitive than sural nerve conduction studies for diagnosing smallfiber neuropathy.[62] One study,[85] however, showed that in patients with symptoms of small-fiber neuropathy and normal sural nerve conduction, reduced IENF density correlated with a decrease in SNAP amplitude in the medial plantar nerve. This finding suggests subclinical involvement of the most-distal large fibers in small-fiber neuropathy.
The detection of thermal and pain thresholds using quantitative sensory testing has been widely used to assess the function of small nerve fibers. Although this approach is useful in population studies, it is an unreliable tool for diagnosing small-fiber neuropathy in clinical practice.[86] Moreover, the size of the probe used for the test can affect the results.[87]
In view of the fact that unmyelinated fibers and thinly myelinated fibers convey warm and cold sensation, respectively, thermal thresholds would be expected to correlate with IENF density. In diabetic neuropathy, IENF density was found to be inversely correlated with thermal and pain thresholds, showing the highest correlation with warm threshold.[53,56,82] Similarly, in Guillain–Barré syndrome lower IENF density was associated with increased warm threshold.[67] One study reported a significant correlation between cold pain threshold and signs of large-fiber impairment.[59] By contrast, others studies did not find any correlation between quantitative sensory testing results and IENF density.[45,51,88]
As IENFs are somatic unmyelinated fibers, their density would not be expected to correlate with autonomic fiber function. Intriguingly, however, in patients with Guillain–Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy, lower IENF density was associated with a higher risk of developing dysautonomia.[64,67] These findings suggest that the integrity of IENFs might reflect the integrity of the whole class of small nerve fibers, including autonomic fibers. A few studies have investigated the correlation between IENF density and the results of a quantitative sudomotor axonal reflex test in patients with painful neuropathy and autonomic symptoms in order to test the hypothesis that IENF density and sweat output might decrease concomitantly. IENF density correlated with test results in one study,[63] but not in another.[51] In leprosy neuropathy, reduced nicotine-induced axon-reflex sweating correlated with decreased innervation of sweat glands.[88]
Laser-evoked potentials (LEPs) have been used to investigate peripheral and central nociceptive pathways in trigeminal neuralgia and peripheral neuropathies. Late LEPs, reflecting Aδ-fiber activation, are delayed in patients with neuropathic pain, but can be enhanced when the pain has a psychogenic origin.[89] Recording of ultralate LEPs, reflecting activation of unmyelinated C-fibers, is less reliable than recording of late LEPs, thereby limiting the overall usefulness of LEPs in clinical practice. LEPs and skin biopsy findings have been examined in single case reports.[90] In two patients with Ross syndrome, abnormal LEPs correlated with decreased IENF density and increased thermal thresholds.[91] No study has yet looked for a correlation between results of skin biopsy analysis and recording of contact heat-evoked potentials, a technique that was recently proposed for investigating smallfiber function, but which cannot be used to assess C-fiber-related responses.[92]
Microneurography allows single-fiber recordings from nerves in awake patients. This technique demonstrated loss of nociceptive and skin sympathetic C-fiber activity that correlated with IENF and sweat gland denervation in a patient with hereditary sensory and autonomic neuropathy type IV.[20] In two patients with generalized anhidrosis, C-fiber recording and sweat gland innervation analysis distinguished postganglionic autonomic nerve fiber impairment from eccrine gland dysfunction.[34]
The diagnosis of small-fiber neuropathy is better assessed by skin biopsy than by sural nerve biopsy.[57] IENF density can be reduced despite normal morphometry of unmyelinated and thinly myelinated fibers in sural nerve biopsy.[58] In a large comparative study,[62] skin and sural nerve biopsy findings were concordant in 73% of patients, but in 23% of patients IENF density was the only indicator of small-fiber neuropathy. Skin biopsy offers the opportunity to differentiate small nerve fibers with somatic function from those with autonomic function, thereby giving it a further advantage over nerve biopsy. In Charcot–Marie–Tooth disease and related hereditary neuropathies, a biopsy sample of the glabrous skin demonstrated the typical neuropathological abnormalities known from sural nerve studies.[5,6]
Immunohistochemical studies demonstrated IgM deposited specifically in the myelinated fibers of hairy and glabrous skin in patients with anti-myelin-associated-glycoprotein neuropathy.[93] Although skin biopsy can be contemplated in genetic and immune-mediated neuropathies, sural nerve biopsy should always be considered to confirm the diagnosis in inflammatory polyradiculoneuropathy with atypical presentation, or when vasculitic or amyloid neuropathy is suspected.
