Objective measures of nociceptor fibre loss or dysfunction can serve as important tools to help diagnose sensory small fibre neuropathy, and to distinguish neuropathy from other chronic pain states. These may also potentially serve as indicators of analgesic efficacy. The Contact Heat Evoked Potential Stimulator (CHEPS) can be used to stimulate a range of heat sensitive receptors expressed by Aδ and C fibres, and the resulting evoked potentials can been recorded and measured [1
]. To the best of our knowledge, this is the first time that the use of CHEPS has been described in patients with small fibre neuropathy. Contact heat evoked potentials recorded from patients with small fibre sensory neuropathy were of substantially reduced Aδ amplitude from the face, arm and leg, in comparison with controls.
In this study we have focused on Aδ amplitudes as they provided the most robust signal; C fibre traces were also obtained but were less robust. Aδ amplitudes for our controls were slightly lower than those recorded by Granovsky et al (2005), however, we have used a more proximal site for thermode placement (i.e. volar forearm arm, compared to thenar eminence or dorsum of hand).
The apparent abnormalities of small sensory fibre function in our patients observed with the contact heat evoked potentials measurements were validated by demonstrating correlations with other objective measures – skin IEF fibre counts and histamine-induced flares. Reduction of PGP 9.5 and TRPV1 IEF showed a significant correlation with leg evoked Aδ potential amplitudes. A significant positive correlation was also demonstrated between skin flare area and leg evoked potential Aδ amplitude. CHEPS appears to be a sensitive measure, detecting abnormalities in some patients with clinical symptoms, but who are within normal limits for other tests, including thermal thresholds – while it may be speculated that the decrease or absence of contact heat evoked potentials in such patients may indicate early small fibre dysfunction (possibly due to a mechanism other than axonal degeneration – such as a sensory ion channelopathy), further studies are necessary to explore and confirm these possibilities. Skin biopsies and flares have the advantage over CHEPS and QST in that they localise the pathology within the peripheral nervous system. QST may reveal additional phenomena such as paradoxical sensations (burning sensation on cooling). It should also be noted that subsets of small fibres may be affected independently in small fibre neuropathies [19
], which reflects the heterogeneous aetiology of the condition. The use of a full array of tests to assess and localise small fibre dysfunction is thus desirable, including QST, skin biopsies, skin flares and CHEPS, for the diagnosis of the condition, and to advance clinico-pathological correlations.
Skin biopsy is a sensitive method for the quantitative assessment of small sensory fibres, and has been considered the "gold standard" for diagnosing small fibre neuropathy [11
]. The European Federation of Neurological Sciences (EFNS) guidelines commend skin biopsy for this diagnosis in preference to sural nerve biopsy [22
]. However, analysis of skin biopsy is technically challenging for some investigators; the biopsy leaves a small scar and has a risk, albeit low, of complications, for example infection or keloid formation. Skin biopsies are avoided over some affected regions e.g. the sole, or palm. Our skin biopsy results compare favourably to others utilising skin biopsy with the same tissue section thickness [13
Epidermal thickness in biopsies taken from our patients was significantly reduced compared to biopsies taken from controls, in keeping with a diagnosis of neuropathy. It is well known that denervation of the skin causes thinning of the epidermis and this has been illustrated experimentally in rat [23
] and mouse models [26
]. Swellings of nerve fibres and their varicosities were rarely observed in our patient cohort. We noted marked swellings on both large and fine calibre nerve fibres in only one patient in our study. This may reflect the stable symptomatology in most of our patients. The low detection frequency of swellings in our cohort of patients may alternatively reflect our routine use of 10 μm tissue sections, although some of our studies using thicker (50 μm) sections revealed no increase in the number of patients with this morphological phenomenon. We have seen higher frequencies of axonal swellings in diagnostic skin biopsies from patients with more acute neuropathies, in accord with other reports. Nerve fibre swellings have been reported previously in rodents and humans [21
]. However, the significance of these morphological changes is uncertain [30
]. Some studies suggest that they predict nerve degeneration [23
], and mark the activity of the neuropathic process or the pre-degeneration of nociceptive fibres. In addition, they have been correlated with the early development of neuropathic symptoms, abnormal heat pain thresholds [32
], and a decline in the density of epidermal nerve fibres [33
]. Other studies have shown that swellings are found in regenerated fibres after skin blister or capsaicin denervation, and have been associated with improved sensation and re-innervation [30
Previous work on the axon reflex flare has shown that C-nociceptor fibre activation is the main factor determining its area [35
]; the correlation with CHEPS responses suggests parallel pathology in Aδ and C fibres in our patients. Flares are known to be diminished in patients with small fibre neuropathies [8
]. Recently, axon reflex flares have been shown to represent a potential biomarker for TRPV1 antagonists: a reduction of heat-evoked pain and capsaicin-evoked flare area by the TRPV1 antagonist SB705498 was demonstrated in a study of 19 healthy human volunteers [36
]. CHEPS can be used in areas unsuitable for histamine-induced flares such as the face or glabrous skin. We have previously reported correlations of nerve growth factor (NGF), which regulates TRPV1 expression, with skin flare area in patients with diabetic neuropathy [14
The EFNS guidelines on neuropathic pain assessment [22
] recommend laser evoked potentials as a reliable method of assessing nociceptive pathways, and of diagnostic use in peripheral neuropathy. They have been used successfully in studies of patients with peripheral neuropathies [38
], trigeminal neuralgia [39
], post herpetic neuralgia [40
], and syringomyelia [41
]. Laser evoked potential suppression helps diagnose neuropathic pain states [39
], while laser evoked potential facilitation is described in fibromyalgic and chronic inflammatory pain [44
]. The similarity between contact heat evoked and laser evoked potentials has already been described [5
]. Lower amplitudes of laser evoked potentials were reported following stimulation of the trigeminal nerve in diabetic patients [46
], with a number of absent responses in affected patients, as in our study with contact heat evoked potentials. Similar changes in laser evoked potentials were also reported following stimulation of the feet in diabetic patients [47
] with no clinical or electrophysiological evidence of large fibre dysfunction. CHEPS thus appears to produce similar cerebral evoked nociceptor potentials to laser stimulation in patients, at least in the regions we have tested. In comparison with lasers, the CHEPS system is easy to operate and calibrate, and it allows for repetitive stimulation or "wind-up", avoiding any risk of superficial burns. However, CHEPS does involve contact with the skin, which may be uncomfortable in some patients with allodynia, and, in principle, the skin contact may affect pathways not activated by heat or noxious stimuli. A discussion comparing thermal conduction and thermal radiation in the study of the nociceptive system has been conducted [49
]. Conductive heat offers the advantage of control over temperature at the thermode-skin interface (via a thermocouple in the stimulator), and while lasers avoid the simultaneous stimulation of low threshold mechanoreceptors, they can cause variations in baseline temperatures. Also discussed, was the problem of estimating the activation temperature at the nerve receptor level with the CHEPS system. Heat is dissipated at the skin-thermode due to a heat sink effect, which is not seen with modern lasers, where varying the wavelength can change the depth of penetration.
CHEPS could potentially be used as a valuable tool in future trials of novel therapeutic agents in experimental pain models, as contact heat evoked potentials may help monitor the effects of analgesic intervention. It may be particularly useful in the development of TRPV1 antagonists for chronic pain states. TRPV1, a member of the vanilloid receptor family localizes mainly to small sensory fibres, and is a ligand-gated ion channel activated by vanilloids, noxious heat and protons. The association between TPRV1 immunoreactivity and tissue hypersensitivity has been demonstrated for a number of hypersensitivity states in humans [18
]. TRPV1 antagonists have been shown to relieve pain in rodent models [52
], and clinical trials are underway.