Pathophysiology/Complications ORIGINAL ARTICLE The Relationship Among Pain, Sensory Loss, and Small Nerve Fibers in Diabetes 1,2 LEA SORENSEN, RN, BHSC ing our own, have shown this not to be 1 LYNDA MOLYNEAUX, RN the case (9–11). However, in view of the 1,2 DENNIS K. YUE, MD, PHD, FRACP pivotal role played by small nerve fibers in the transmission of pain sensation, fur- ther studies are obviously of importance. OBJECTIVE — Many individuals with diabetes experience neuropathic pain, often without Direct examination of intraepidermal objective signs of large-fiber neuropathy. We examined intraepidermal nerve fibers (IENFs) to nerve fibers (IENF) using skin biopsy evaluate the role of small nerve fibers in the genesis of neuropathic pain. technique is a proven procedure to iden- tify small-fiber abnormalities. Several RESEARCH DESIGN AND METHODS — Twenty-five diabetic subjects with neuro- studies using this technique have shown pathic pain and 13 without were studied. The pain was present for at least 6 months for which the density of IENF to be reduced in id- no other cause could be found. Punch skin biopsies were obtained from the distal leg. IENFs were stained using antibody to protein gene product 9.5 and counted with confocal microscopy. iopathic and nondiabetic neuropathies Neuropathy was graded by vibration perception and cold detection thresholds and the Michigan (12–14). This technique has also shown Neuropathy Screening Instrument. that people with diabetes have reduced IENF and altered nerve morphology RESULTS — In the total cohort, IENF density was significantly lower in those with pain (14,15). However, to our knowledge, no compared with those without (3 [1–6] vs. 10 [3–19], respectively, P ϭ 0.02). There were studies have been specifically conducted significant inverse correlations between IENF and severity of neuropathy, with the pain group to examine IENF in the context of its role having a flatter gradient than their pain-free counterparts (P Ͻ 0.02). The difference in IENF in the genesis of pain in diabetes. There- Յ density was greatest in subjects with less objective evidence of neuropathy (P 0.01). fore, the current study was conducted to compare subjects with or without pain to CONCLUSIONS — More severe loss of IENF is associated with the presence of neuropathic pain only in those with little or no objective sign of neuropathy. Thus, loss of IENF cannot determine the relationship among pain, explain pain in all cases, suggesting that different mechanisms underpin the genesis of pain at sensory loss, and IENF density. various stages of neuropathy. RESEARCH DESIGN AND Diabetes Care 29:883–887, 2006 METHODS — Subjects were recruited from patients attending our Diabetes Cen- eripheral neuropathy is a common usually limited to nerve conduction stud- tre. As the focus is on chronic neuropathic complication of both type 1 and ies that measure large–nerve fiber func- pain (rather than transient neurogenic P type 2 diabetes (1). Most com- tion. The results of these tests are often pain, which may occur during acute fluc- monly, it manifests as sensory loss, which normal and unable to provide an expla- tuation in glycemic control), for the pur- predisposes subjects to foot abnormalities nation for the presence of pain (7,8). This pose of this study patients were only and high risk of ulceration. However, it conundrum reflects a common clinical recruited if they have had pain for Ͼ6 has been reported that between 4 and observation that pain is often present in months. All patients had symmetrical foot 33% of subjects with diabetes suffer from the absence of objective signs of neurop- pain. Patients were deemed to have neu- the painful type of neuropathy, which can athy. Even in the presence of abnormal ropathic pain by obtaining a detailed his- become chronic and produce unremitting nerve conduction studies, some individ- tory of the nature of the pain and pain for which there is little satisfactory uals may experience pain while others performing a physical examination to ex- treatment (2–4). with the same degree of electrophysiolog- clude nociceptive pain such as arthritis or It is known that pain transmission in ical abnormalities are completely asymp- peripheral vascular disease. Blood was peripheral nerves occurs along the small tomatic. It is commonly assumed that collected to eliminate other possible A␦ and C-type fibers (5,6). However, con- more specific testing of small-fiber func- causes of pain such as vitamin B12 defi- ventional clinical investigation of individ- tion may better discriminate those with or ciency, hypothyroidism, and where clini- uals with painful diabetic neuropathy is without pain, but several studies, includ- cally relevant, monoclonal gammopathy. ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● Chest X-ray was performed if indicated to From 1The Diabetes Centre, Royal Prince Alfred Hospital, Camperdown, New South Wales, Australia; and exclude paraneoplastic phenomena, and the 2Discipline of Medicine, The University of Sydney, New South Wales, Australia. physical examination and history ex- Address correspondence and reprint requests to Lea Sorensen, Diabetes Centre, Royal Prince Alfred cluded spinal or hereditary causes. Where Hospital, Level 6 West Wing, Missenden Road, Camperdown NSW 2050, Australia. E-mail: [email protected]. no other abnormality was found, the nsw.gov.au. Received for publication 8 November 2005 and accepted in revised form 22 December 2005. cause of pain was deemed to be diabetes. Abbreviations: CDT, cold detection threshold; IENF, intraepidermal nerve fiber; MNSI, Michigan Neu- A total of 38 patients with diabetes were ropathy Screening Instrument; VPT, vibration perception threshold. studied, composed of 13 without and 25 A table elsewhere in this issue shows conventional and Syste`me International (SI) units and conversion with painful neuropathy. factors for many substances. Apart from noting its presence, the © 2006 by the American Diabetes Association. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby severity of pain was recorded using a marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 10-cm visual analog scale. Subjects were DIABETES CARE, VOLUME 29, NUMBER 4, APRIL 2006 883 Small nerve fibers and painful diabetic neuropathy Table 1—Demographic and clinical profiles of subjects with or without pain Five indicators were measured for each foot (appearance of foot, presence of ul- No pain Pain Test statistic and P value ceration, ankle reflexes, vibration percep- tion, and ability to feel the 10-gm n 13 25 monofilament). Each indicator may be Age (years) 57 (54–65) 59 (53–62) Z ϭ 0.08; P ϭ 0.9 assigned a score of 0, which indicates nor- Duration of diabetes (years) 7 (2–13 ) 7 (5–12) ZϭϪ0.5; P ϭ 0.6 mal, 0.5, or 1, respectively, for intermedi- Male (n) 13 15 Fisher’s exact P ϭ 0.008 ate grade or gross abnormalities. Each foot Type 2 diabetes (n)1121␹2 ϭ 0.002; P ϭ 0.96 was assessed independently, giving a possi- Height (m) 1.8 (1.7–1.8) 1.8 (1.7–1.8) Z ϭ 1.5; P ϭ 0.1 ble total maximum score of 10. ϭ ϭ HbA1c (%) 7.2 (6.6–8.9) 7.1 (6.4–8.9) Z 0.5; P 0.7 Skin biopsies were obtained and ana- VPT (volts) 33 (21–50) 35 (22–50) Z ϭϪ0.2; P ϭ 0.9 lyzed according to the protocol developed Absent ankle reflexes (%) 28.4 48 ␹2 ϭ 0.5; P ϭ 0.5 by McArthur et al. (17). Skin specimens MNSI 1.0 (0.5–5.5) 3.0 (1.0–6.0) Z ϭϪ0.8; P ϭ 0.4 were obtained using a 3-mm punch bi- CDT (°C) Ϫ2.67 Ϫ3.42 Z ϭϪ0.5; P ϭ 0.6 opsy from the leg 10 cm above the lateral Pain score 0 8.0 (6.5–8.0) Z ϭϪ5.2; P Ͻ 0.001 malleolus. The skin sections were placed IENF (density/3 mm) 10 (3–19) 3 (1–6) Z ϭ 2.4; P ϭ 0.02 in 2% paraformaldehyde/lysine/ Data are median (interquartile range), unless otherwise indicated. periodate. Fifty-micron freezing mic- rotome sections were immunostained asked to grade pain at the level they feel and second toes. The voltage is slowly in- with the panaxonal marker PGP 9.5. they experience most of the time. Control creased from 0, and patients are asked to IENFs of the whole biopsy were counted ϫ subjects were patients who were pain free, indicate when they can first feel the vibra- using confocal microscopy at 40 mag- with similar age, duration of diabetes, and tion. Three measurements are taken on nification in a standardized manner. Indi- glycemic control. As we have shown pre- each foot and the average of these re- vidual fibers were counted when they viously, overall, subjects with pain have corded as the VPT. CDT was measured crossed the dermal-epidermal junction, more sensory loss associated with neu- using the Computer-Aided Sensory Eval- and secondary fibers that branched from ropathy and the cohort was further char- uator machine (CASE IV; Medical Elec- within the epidermis were excluded. The acterized by measurement of large-fiber tronics, Stillwater, MN). A series of cold length of the epidermis was measured us- function with vibration perception stimuli of different temperatures is deliv- ing a computerized tracing system, and threshold (VPT), small-fiber function ered with a sensor placed on the dorsum the results were expressed as IENF num- with cold detection threshold (CDT), and of the foot, and the patient indicates ber per 3 mm of the section.