Dottorato in TECNOLOGIE INNOVATIVE NELLE MALATTIE DELLO SCHELETRO,
DELLA CUTE E DEL DISTRETTO ORO-CRANIO-FACCIALE

Titolo della tesi: Small fibre damage and neuropathic pain in peripheral neuropathies. New insights from amyloid, lupus, and diabetic polyneuropathy.

BACKGROUND. Neuropathic pain is one of the most frequent and disabling symptoms in patients with peripheral neuropathies and its pharmacological treatment still represents an unmet clinical need, due to limited understanding of the underlying pathophysiological mechanisms. Neuropathic pain has been traditionally attributed to small nerve fibers damage in peripheral neuropathies. However, several studies failed to demonstrate differences in intraepidermal nerve fibre density between patients with painful and painless polyneuropathy at skin biopsy, the most widely agreed diagnostic tool to evaluate small nerve fibers. AIM. This study series aims at defining the relation between neuropathic pain symptoms and small fibre damage in patients with amyloid, lupus, and diabetic polyneuropathy, as assessed by detailed clinical, neurophysiological, and skin biopsy small fibre testing. Secondarily, our findings provide an overview of the prevalence and clinical impact of small fibre damage in polyneuropathies, and define peculiar characteristics distinguishing pure small fibre-neuropathy (SFN) in different etiological conditions. RESULTS. Study 1 - Hereditary transthyretin amyloidosis with polyneuropathy (ATTRv-PN). In 30 patients with ATTRv-PN we showed that functional tests assessing small-fibre damage, i.e. Quantitative Sensory Testing (QST) and Sudoscan, better reflected neuropathic pain and autonomic symptoms than skin biopsy morphometric variables, thus representing potential valuable tools for patients’ follow up. Small fibre damage was ubiquitous in ATTRv-PN. 53% of patients showed a mixed fibre neuropathy (MFN) with small and large fibers involvement, whereas 47% of patients, with shorter disease duration, had SFN, possibly representing an early manifestation of ATTRv-PN. Study 2- Systemic Lupus Erythematosus (SLE) related polyneuropathy. In 50 patients with SLE and painful symptoms we couldn’t find significant correlations between painful symptoms and small fibre tests outcome variables, possibly due to high comorbidity with fibromyalgia syndrome, which may have conditioned pain symptoms and skin biopsy variables in patients with SLE. However, small fibre damage was present in all the SLE patients diagnosed with neuropathy (52%). 14% of patients, of older age, suffered from MFN, whereas SFN with anon-length dependent distribution was found in 38%. SFN was associated with hypocomplementemia, thus showing a relation with disease immunologic activity, and was also related to disease modifying therapy. Study 3- Diabetic distal symmetric polyneuropathy (DSPN). In 65 patients with painful and painless diabetic DSPN we found that patients with specific neuropathic pain symptoms, like burning and cold pain, and tingling paresthesia, had higher densities of TRPV1 immunoreactive intraepidermal nerve fibers at skin biopsy respect to patients without these pain types, thus highlighting the possible role of TRPV1 as contributors to neuropathic pain pathogenesis and as therapeutic targets. Also in diabetic DSPN small fibre damage was highly prevalent in the form of MFN (74%) and SFN (23%), and patients with SFN scored lower at neuropathic pain and small fibre related symptoms questionnaires in comparison to MFN. CONCLUSIONS. Our findings allowed us to draw several conclusions: - Small fibre damage is an ubiquitous phenomenon in peripheral neuropathies of different etiologies, like amyloid, lupus, and diabetic neuropathy, manifesting either in the context of MFN or SFN. - We gathered evidence that SFN is an earlier manifestation of neuropathy respect to MFN, as shown in amyloid and lupus polyneuropathy, and that it is characterised by milder symptoms. These findings resize the role of SNF as a separate entity and suggest that polyneuropathies should be regarded as a spectrum, within which SFN may represent an early form. Small fibre related testing should be implemented in clinical practice, to facilitate SFN early recognition and patients’ follow up. - Neuropathic pain was highly prevalent in all the examined conditions. In the 3 studies, the evaluation of intraepidermal nerve fibre density as assessed by the pan-axonal marker PGP9.5 with skin biopsy, the gold standard diagnostic tool to assess small thermal-nociceptive fibers, was unable to distinguish patients with painful and painless polyneuropathies. This finding raises the hypothesis that pain may depend on mechanisms other than the mere nerve fibers loss, and that other biochemical and functional changes are required. - Our findings in ATTRv-PN showed that functional tests like QST reliably reflect neuropathic pain symptoms, thus strengthening the hypothesis that functional changes in small nerve fibers may underlie neuropathic pain. - Skin biopsy analysis with sensory biomarkers other than the conventional PGP9.5 may help better characterise small fibre damage in neuropathic pain conditions. The finding of increased TRPV1 immunoreactive fibers in diabetic patients with specific neuropathic pain subtypes supports the hypothesis that changes affecting membrane proteins and signalling pathways in sensory neurons may contribute to neuropathic pain.