Pain and diabetic neuropathy

Diabetic neuropathy: achieving best practice

Peripheral neuropathy is one of the commonest complications of diabetes and the commonest form of neuropathy in the developed world.1 Diabetic polyneuropathy encompasses several neuropathic syndromes, and the commonest presentation is chronic distal symmetrical neuropathy (DSP). DSP, often associated with autonomic neuropathy, has two clinical consequences: namely neuropathic pain and foot ulceration. Both often occur in the same individual, and cause severe curtailment of quality of life. The other, less common presentations of diabetic polyneuropathy include acute painful neuropathies, and focal neuropathies (amyotrophy, pressure palsies, truncal radiculopathies, mononeuropathies and mononeuritis multiplex).2 Table 1 shows a recent classification of diabetic polyneuropathy based upon the natural history of the various syndromes.3

Repeated Dosing with NCX1404, a Nitric Oxide-Donating Pregabalin, Re-establishes Normal Nociceptive Responses in Mice with Streptozotocin-Induced Painful Diabetic Neuropathy

NCX1404 [(3S)-5-methyl-3-(((1-(4-(nitrooxy)butanoyloxy)ethoxy)carbonylamino) methyl)hexanoic acid] is a novel nitric oxide (NO)-donating pregabalin that is readily absorbed and processed in vivo to pregabalin and NO. We determined the antiallodynic response of NCX1404 after acute or after 7, 14, and 21 days of repeated daily oral dosing in mice with streptozotocin (STZ)-induced painful diabetic neuropathy (PDN). Pregabalin and its combination with the NO donor isosorbide mononitrate (ISMN) were used for comparison. The blood levels of pregabalin and nitrites, used as surrogate marker of NO release, after NCX1404 or pregabalin dosing were monitored in parallel experiments using liquid chromatography with tandem mass spectrometry (LC-MS/MS). NCX1404 and pregabalin resulted in similar pregabalin levels as it was their antiallodynic activity after acute dosing in STZ mice. However, NCX1404 resulted in disease-modifying properties when administered daily for 21 days, as indicated by the time- and dose-dependent reversal of STZ-induced mechanical allodynia (paw withdrawal threshold [PWT]Veh_21d= 1.3 ± 0.15 g for vehicle; PWTNCX1404_21d= 1.4 ± 0.5 g, 2.9 ± 0.2 g* and 4.1 ± 0.2 g*, respectively for 19, 63, and 190μmol/kg, oral gavage [PO] of NCX1404; *P< 0.05 versus vehicle). This effect was not shared by pregabalin at equimolar doses (190μmol/kg, PO, PWTPregab_21d= 1.4 ± 0.1 g*, *P< 0.05 versus equimolar NCX1404). In addition, the NO donor ISMN (52.3μmol/kg, PO) alone or combined with pregabalin (63μmol/kg) was active at 7 days (PWTVeh_7d= 1.7 ± 0.16 g; PWTISMN_7d= 3.9 ± 0.34 g*; PWTPregab_7d= 1.3 ± 0.07 g; PWTISMN+pregab_7d= 3.8 ± 0.29 g*; *P< 0.05) but not at later time points. The long-term effect of NCX1404 was independent of residual drug exposure and lasted for several days after the treatment was stopped. In summary, like pregabalin, NCX1404 is an effective antiallodynic agent. Differently from pregabalin, repeated dosing of NCX1404 re-established normal nociceptive responses in STZ-induced PDN in mice.

Recent advances in the management of diabetic distal symmetrical polyneuropathy

There is now little doubt that poor blood glucose control is an important risk factor for the development of diabetic peripheral neuropathy (DPN). Furthermore, traditional cardiovascular risk factors for macrovascular disease appear to be associated with an increased risk of DPN. The recently established International Expert Group on Diabetic Neuropathy has recommended new criteria for the diagnosis of DPN in the context of clinical and research settings. Studies in experimental diabetes examining the pathogenesis of DPN have identified a number of metabolic abnormalities including polyol pathway hyperactivity, increased advanced glycation end‐point formation, alterations in the protein kinase C beta pathway through diacylglycerol and oxidative stress. There is now strong evidence implicating nerve ischemia as the cause of DPN. Studies in human and animal models have shown reduced nerve perfusion and endoneurial hypoxia. These endoneurial microvascular changes strongly correlate with clinical severity and the degree of nerve‐fiber pathology. Unfortunately, many compounds that have been effective in animal models of neuropathy have not been successful in human diabetic neuropathy. The only compounds found to be efficacious in human diabetic neuropathy, and are in clinical use, are the anti‐oxidant, α‐lipoic acid and the aldose reductase inhibitor, epalrestat. Overall, the evidence emphasizes the importance of vascular dysfunction, driven by metabolic change, in the etiology of DPN, and highlights potential therapeutic approaches. Epidemiological data on diabetic painful neuropathic pain (DPNP) are limited. In one population‐based study, the prevalence of DPNP, as assessed by a structured questionnaire and examination, was estimated at 16%. It was notable that, of these patients, 12.5% had never reported symptoms to their doctor and 39% had never received treatment for their pain. Thus, despite being common, DPNP continues to be underdiagnosed and undertreated. Pharmacological treatment of DPNP include tricyclic compounds, serotonin noradrenalin reuptake inhibitors, the anti‐oxidant α‐lipoic acid, anticonvulsants, opiates, membrane stabilizers, topical capsaicin and so on. Management of the patient with DPNP must be tailored to individual requirements and will depend on the presence of other comorbidities. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00083.x)

Central nervous system involvement in diabetic neuropathy

Diabetic neuropathy is a chronic and often disabling condition that affects a significant number of individuals with diabetes. Long considered a disease of the peripheral nervous system, there is now increasing evidence of central nervous system involvement. Recent advances in neuroimaging methods detailed in this review have led to a better understanding and refinement of how diabetic neuropathy affects the central nervous system. Recognition that diabetic neuropathy is, in part, a disease that affects the whole nervous system is resulting in a critical rethinking of this disorder, opening a new direction for further research.

Effective treatment of symptomatic diabetic polyneuropathy by high-frequency external muscle stimulation

AIMS/HYPOTHESIS

Diabetic distal symmetrical sensory polyneuropathy (DSP) affects 20-30% of diabetic patients. Transcutaneous electrical nerve stimulation (TENS) and electrical spinal cord stimulation have been proposed as physical therapies. We performed a controlled, randomised pilot trial to compare the effects of high-frequency external muscle stimulation (HF) with those of TENS in patients with symptomatic DSP.

METHODS

Patients with type 2 diabetes and DSP (n=41) were randomised to receive treatment with TENS or HF using strata for non-painful (n=20) and painful sensory symptoms (n=21). Both lower extremities were treated for 30 min daily for three consecutive days. The patients' degree of symptoms and pain were graded daily on a scale of one to ten, before, during and 2 days after treatment termination. Responders were defined by the alleviation of one or more symptoms by at least three points.

RESULTS

The two treatment groups were similar in terms of baseline characteristics, such as age, duration of diabetes, neurological symptoms scores and neurological disability scores. The responder rate was significantly higher (p<0.05) in the HF group (80%, 16 out of 20) than in the TENS group (33%, seven out of 21). Subgroup analysis revealed that HF was more effective than TENS in relieving the symptoms of non-painful neuropathy (HF: 100%, seven out of seven; TENS: 44%, four out of nine; p<0.05) and painful neuropathy (HF: 69%, nine out of 13; TENS: 25%, three out of 12; p<0.05). The responders did not differ in terms of the reduction in mean symptom intensity during the trial.

CONCLUSIONS/INTERPRETATION

This pilot study shows, for the first time, that HF can ameliorate the discomfort and pain associated with DSP, and suggests that HF is more effective than TENS. External muscle stimulation offers a new therapeutic option for DSP.

Diabetic neuropathy: the painful foot

Diabetic neuropathy typically present as a mixture of sensory, motor and autonomic involvement. The development and severity of the neuropathy varies. This article briefly reviews the types of diabetic neuropathy and their relationship to pain and discusses the proposed etiologies.

Increased sural nerve epineurial blood flow in human subjects with painful diabetic neuropathy

AIMS/HYPOTHESIS

The pathogenesis of painful diabetic neuropathy remains unknown. As a consequence we still do not have any effective, rational treatments and a greater understanding of the mechanisms is urgently required. Previous studies have shown no consistent morphological differences in the nerves of patients with and without painful neuropathy. The aim of this study was to compare epineurial haemodynamics in patients with chronic painful and painless neuropathy.

METHODS

The techniques of microlightguide spectrophotometry and fluorescein angiography were used to measure epineurial intravascular oxygen saturation and blood flow respectively. Eleven patients with painful and eight with painless neuropathy were studied, with the groups matched carefully in terms of severity of neuropathy and diabetes control.

RESULTS

Intravascular oxygen saturation was higher in the painful neuropathy group compared to those without pain (median 73.8% vs 67.7%, respectively; p=0.021). Fluorescein rise time was also faster in those with painful symptoms (median 18.3 s vs 53.6 s; p=0.046) indicating higher epineurial blood flow in these subjects.

CONCLUSION/INTERPRETATION

These results indicate that there are distinct differences in haemodynamics within the epineurium of the sural nerve in subjects with painful and painless neuropathy. Haemodynamic factors could therefore have an important role in the pathogenesis of neuropathic pain and might offer further insight into potential treatments for this distressing condition.

Understanding the impact of painful diabetic neuropathy

Painful neuropathy is a common and often distressing complication of diabetes. It has considerable impact on the social and psychological well-being of affected individuals. There are two distinct forms of painful neuropathy: an acute and self-limiting form that resolves within a year or a chronic form that can go on for years. There are now a number of drugs available for the treatment of neuropathic pain. However, some may fail to respond to these drugs or may have unacceptable adverse side effects. When this is the case, the patient's quality of life can be severely affected. Health care professionals need to assess the full impact of painful neuropathy. In this article we review a number of instruments that are used to assess the severity of painful neuropathy and its impact on the quality of life.

Treatment of chronic painful diabetic neuropathy with isosorbide dinitrate spray: a double-blind placebo-controlled cross-over study

OBJECTIVE

Considerable evidence implicates impaired nitric oxide (NO) generation in the pathogenesis of diabetic neuropathic pain. We therefore conducted a pilot study to examine the effects of isosorbide dinitrate (ISDN), a NO donor with local vasodilating properties, in spray form in the management of chronic neuropathic pain.

RESEARCH DESIGN AND METHODS

The study was of double-blind, randomized, placebo-controlled, and two-period cross-over design. After a 2-week run-in period, 22 diabetic patients (13 men, 20 with type 2 diabetes, age [mean +/- SE] 63.7 +/- 1.8 years, duration of diabetes 9.1 +/- 1.5 years, duration of painful neuropathy 2.6 +/- 0.4 years) were randomized to receive ISDN or placebo sprays for 4 weeks, exchanging their treatment for a further 4 weeks after a 2-week wash-out period. The patients administered the spray to both feet before bedtime. Biweekly pain and other sensory symptoms were assessed using a visual analog scale (VAS) and the Lickert scale, respectively.

RESULTS

ISDN spray reduced overall neuropathic pain (P = 0.02) and burning sensation (P = 0.006). No treatment difference was observed with other sensory modalities (hot/cold sensation, tingling, numbness, hyperesthesia, and jabbing-like sensation). At study completion, 11 patients (50%) reported benefit and wished to continue using the ISDN spray, 4 (18%) preferred the placebo spray, and the remaining 7 (32%) were undecided.

CONCLUSIONS

ISDN spray offers an alternative and effective pharmacological option in relieving overall pain and burning sensation in the management of painful diabetic neuropathy. The potential of ISDN spray in alleviating other specific sensory symptoms associated with diabetic peripheral neuropathy merits further study.

Haemodynamics in microvascular complications in type 1 diabetes

Type 1 diabetes is commonly associated with microvascular complications. Most of the microvascular blood vessels are involved but those in the kidney, retina and large nerves exhibit the more significant pathology. Haemodynamic and metabolic factors both alone and through the activation of a common pathway contribute to the characteristic dysfunction observed in diabetic vasculopathy. The haemodynamic abnormalities in type 1 diabetes are characterized by increased systemic blood pressure and altered blood flow with subsequent activation of various vasoactive factors, which can contribute to the maintenance of the haemodynamic alterations and to the development and progression of the microvascular complications. These vasoactive factors include vasoconstrictors such as angiotensin II, and endothelin, as well as vasodilators such as nitric oxide (NO). Systemic hypertension and vasoactive factors independently and in interaction with the metabolic pathway activate intracellular second messengers, nuclear transcription factors and various growth factors which lead to the typical functional and structural alterations of diabetic microvascular complications. Therapeutic strategies involved in the management and prevention of diabetic complications currently include antihypertensive agents, particularly those that interrupt the renin-angiotensin system. Further understanding of the interactions among the vasoactive factors, the intracellular second messengers and the growth factors may help to identify novel strategies to influence the action of the vasoactive factors. These novel therapies, together with specific inhibitors of the metabolic pathway or the common pathway, may provide the possibility of preventing or even reversing the progression of diabetic microvascular complications.

Spinal cord stimulation in diabetic lower limb critical ischaemia: transcutaneous oxygen measurement as predictor for treatment success

OBJECTIVES

to evaluate whether transcutaneous oxygen tension (TcpO(2)) measurements could be used as a specific prognostic parameter in selecting diabetic patients for permanent device implantation.

METHODS

sixty consecutive diabetic patients (28 with autonomic neuropathy), classified as Fontaine stage III or IV, underwent spinal cord stimulation (SCS) for ischaemic pain, after failed conservative or surgical treatment. Pedal TcpO(2)on the dorsum of the foot and ankle-pressure Doppler measurements were performed before, and 2 and 4 weeks after implantation.

RESULTS

limb salvage and good pain relief were achieved in 35 patients, while in 12 partial pain relief and limb salvage for at least 6 months were obtained. In 13 patients the method failed and the ischaemic limbs were amputated. Only 3 of the 28 patients with neuropathy had any long-term benefit. Limb salvage was achieved in those patients with a significant increase in TcpO(2)within 2 weeks of stimulation. The stage of the neuropathy was inversely related to the success of SCS therapy. The ankle-brachial pressure index (ABPI) did not change after stimulation.

CONCLUSIONS

diabetic patients with significant increase of TcpO(2)and pain relief during a 2-week test period may be successfully treated by long-term SCS unless they have advanced autonomic neuropathy.

Does autonomic neuropathy influence spinal cord stimulation therapy success in diabetic patients with critical lower limb ischemia?

BACKGROUND

Spinal cord stimulation (SCS) improves microcirculatory blood flow and relieves diabetic neuropathic and ischemic pain, reducing the amputation rate in patients with peripheral arterial occlusive disease (PAOD). The purpose of this study was to evaluate whether the presence of autonomic neuropathy in diabetic patients with PAOD influences the success of SCS therapy.

METHODS

Sixty consecutive diabetic patients (15 with early and 13 with definite and/or combined autonomic neuropathy) with an ankle/brachial systolic pressure index (ABI) less than 0.20 +/- 0.08, underwent spinal cord stimulation after failed conservative or surgical treatment. The neuropathic status of the patients was evaluated before implantation and pedal TcpO2 measurements on the dorsum of the foot were performed.

RESULTS

Limb salvage and pain relief >75%, evaluated with the visual analogue scale, were achieved in 35 patients, whereas in 12 a partial success with pain relief >50% and limb salvage for at least 6 months were obtained. In 13 patients the method failed and the ischemic limbs were amputated. Among the 28 diabetic patients with autonomic neuropathy the treatment failed or resulted in only partial success in 25, whereas in all 32 patients without neuropathy limb salvage and pain relief >75% were achieved (p < 0.0001). Partial success in 10 patients with early neuropathy and in two with definite was achieved (p = 0.008), whereas in 11 patients with definite neuropathy and in two with early the method failed (p < 0.001). The stage of the neuropathy was inversely related to the success of SCS therapy, independent of the stage of the disease. The method's success was related to the presence of adequate paraesthesias and warm feeling in the painful area with size reduction of the trophic lesions.

CONCLUSIONS

Diabetic patients with peripheral arterial occlusive disease presenting with intractable pain may be successfully treated with spinal cord stimulation unless they have associated severe autonomic neuropathy.

Epidural spinal cord electrical stimulation in diabetic critical lower limb ischemia

Spinal cord stimulation (SCS) has been suggested to improve microcirculatory blood flow to relieve ischemic pain and to reduce amputation rate in patients with peripheral arterial occlusive disease (PAOD). The aim of this study was to evaluate the specific prognostic parameters in the prediction of successful SCS, in diabetic patients, performing a retrospective data analysis. To perform this evaluation, 64 diabetic patients (39 men, 25 women; mean age, 69 years) classified as Fontaine's stage III and IV, with PAOD, were treated with SCS for rest pain and trophic lesions with dry gangrene, after failed conservative or surgical treatment. In clinical controls, pedal transcutaneous oxygen tension (TcPO(2)), ankle/brachial blood pressure index (ABI), and toe pressure Doppler measurements were utilized to select and follow-up the patients. After 58 months of follow-up (range, 20-128 months), pain relief greater than 75% and limb salvage were achieved in 38 diabetic patients. A partial success was obtained in nine patients with pain relief greater than 50% and limb salvage for at least 6 months. The method failed in 17 patients or the device was removed due to technical problems, and the limb was amputated in these patients. TcPO(2) was assessed on the dorsum of the foot. Clinical improvement and SCS success were associated with increase of TcPO(2), before and after implantation. Limb salvage was achieved in the patients who had significant TcPO(2) increase within the 2 weeks of the testing period, independently of the stage of the disease. A TcPO(2) increase of more than 50% in the first 2 months after implantation was predictive of success, and was related to the presence of adequate paresthesias in the painful area during the trial period. TcPO(2) significantly increased after long-term follow-up in all patients with limb salvage (from 22.1 to 43.1 mm Hg in the rest pain patients, from 15.8 to 36.4 mm Hg in those with trophic lesions of less than 3 cm(2), and from 12.1 to 28.1 in those with trophic lesions of greater than 3 cm(2), (p < 0.01). ABI did not changed under stimulation. In diabetic patients with PAOD, the SCS increases the skin blood flow, is associated with significant pain relief, and could be proven an excellent alternative therapy, improving the life quality. Significant TcPO(2) increase within the 2-week test period, is a predictive index of therapy success and should be considered before the final decision in terms of cost effectiveness, before the permanent implantation.

Acupuncture for the treatment of chronic painful peripheral diabetic neuropathy: a long-term study

Forty-six diabetic patients with chronic painful peripheral neuropathy were treated with acupuncture analgesia to determine its efficacy and long-term effectiveness. Twenty-nine (63%) patients were already on standard medical treatment for painful neuropathy. Patients initially received up to six courses of classical acupuncture analgesia over a period of 10 weeks, using traditional Chinese Medicine acupuncture points. Forty-four patients completed the study with 34 (77%) showing significant improvement in their primary and/or secondary symptoms (P < 0.01). These patients were followed up for a period of 18-52 weeks with 67% were able to stop or reduce their medications significantly. During the follow-up period only eight (24%) patients required further acupuncture treatment. Although 34 (77%) patients noted significant improvement in their symptoms, only seven (21%) noted that their symptoms cleared completely. All the patients but one finished the full course of acupuncture treatment without reported or observed side effects. There were no significant changes either in the peripheral neurological examination scores, VPT or in HbA1c during the course of treatment. These data suggest that acupuncture is a safe and effective therapy for the long-term management of painful diabetic neuropathy, although its mechanism of action remains speculative.

Electrical spinal-cord stimulation for painful diabetic peripheral neuropathy

BACKGROUND

Conventional treatment for painful peripheral diabetic neuropathy is largely symptomatic and often ineffective, with unacceptable side-effects. We tested electrical spinal-cord stimulation for the management of chronic neuropathic pain.

METHODS

Ten diabetic patients who did not respond to conventional treatment (mean age 51 [SD 9.3] years, six with type II diabetes, mean duration of diabetes 12 [6.3] years, mean duration of neuropathy 5 [2.1] years) were studied. The electrode was implanted in the thoracic/lumbar epidural space. Immediate neuropathic pain relief was assessed by visual analogue scale (VAS) after connecting the electrode, in a random order, to a percutaneous electrical stimulator or to a placebo stimulator. Exercise tolerance was assessed on a treadmill.

FINDINGS

Eight subjects had statistically significant pain relief with the electrical stimulator (p < 0.02) and were therefore converted to a permanent system. Statistically significant relief of both background and peak neuropathic pain was achieved at 3 months (n = 7, p = 0.016), at 6 months (n = 7, p = 0.03), and at the end of the study (14 months, n = 7, background pain p = 0.06, peak pain p = 0.03). One patient died 2 months after the start of the study of unrelated cause while continuing to benefit from treatment and another patient ceased to benefit at 4 months. McGill pain questionnaire scores with the stimulator turned off did not change significantly from baseline scores, indicating that the severity of the underlying pain was unaltered. However, with the stimulator turned on, there was a statistically significant (p < 0.05) improvement in all four components of the score, by the end of the study. At the end of the study, six patients continued to gain significant pain relief and used the stimulator as the sole treatment for their neuropathic pain. For example, median background and peak pain scores at the end of study, were, respectively, 77 and 81 with the stimulator off and 23 and 20 with the stimulator on. Exercise tolerance significantly improved at 3 months (n = 7, median % increase 85 [IQR, 62-360], p = 0.015) and at 6 months (n = 6, 163 [61-425], p = 0.0007). Electrophysiological tests, vibration perception-threshold, and glycaemic control were unchanged.

INTERPRETATION

Electrical spinal-cord stimulation offers a new and effective way of relieving chronic diabetic neuropathic pain and improves exercise tolerance. The technique should be considered in patients with neuropathic pain who do not respond to conventional treatment.

Neurotropin prevents neurophysiological abnormalities and ADP-induced hyperaggregability in rats with streptozotocin-induced diabetes

Neurotropin, a non-proteinaceous extract from the inflamed dermis of rabbits inoculated with vaccinia virus, was administered for 8 weeks to rats with streptozotocin-induced diabetes. The physiological and biochemical changes of the nerves were studied as well as ADP-induced platelet aggregation. Neurotropin improved the caudal motor nerve conduction velocity, R-R variability, sciatic nerve blood flow, and platelet hyperaggregability in diabetic rats, despite having no effect on sorbitol and fructose accumulation or myoinositol depletion in the sciatic nerve. The correlation between nerve conduction velocity, R-R variability, nerve blood flow, and platelet aggregation were significant between each two parameters (p < 0.0001). Thus, the mechanism of action of neurotropin differed from that of aldose reductase inhibitors. These findings suggest that vascular factors may play an important role in the development of diabetic neuropathy, and that neurotropin may be useful for the treatment of this condition.

Vascular factors in diabetic neuropathy

Despite considerable research we still do not have a comprehensive explanation for the pathogenesis of diabetic neuropathy. Although chronic hyperglycaemia is almost certainly involved, it is not known whether the primary pathology is metabolic, microvascular, or an interaction between the two. Hyperglycaemia-induced polyol pathway hyperactivity associated with nerve sorbitol accumulation and myo-inositol depletion may play a part in the genesis of diabetic neuropathy. The case for microvascular disease in diabetic neuropathy is now strong. Fibre loss in human sural nerve is multifocal, suggesting ischaemia. The degree of vessel disease has been related to the severity of neuropathy. People with chronic obstructive pulmonary disease develop the so called "hypoxic neuropathy" in which similar microvascular changes occur as in diabetic neuropathy. In rats with experimental diabetic neuropathy nerve blood flow is reduced and oxygen supplementation or vasodilator treatment improved the deterioration in conduction velocity and nerve blood flow. Similarly, in human diabetic neuropathy, there is impaired nerve blood flow, epineurial arterio-venous shunting and a reduction in sural nerve oxygen tension. At what stage during the development of nerve damage these changes occur is yet to be determined.

Exercise-induced conduction velocity increment: a marker of impaired peripheral nerve blood flow in diabetic neuropathy

Severe microvascular disease exists at the stage of clinical diabetic neuropathy. A non-invasive test that will identify those diabetic subjects who will eventually develop neuropathy is essential for early intervention. Sural sensory conduction velocity was recorded (x3) in 12 non-neuropathic diabetic subjects, 15 diabetic subjects with established neuropathy and 16 age-matched normal control subjects, before and after exercise to 80% age/sex predicted maximum heart rate. Fixed sural electrodes were used. Subcutaneous temperature was recorded by a needle thermocouple placed near the sural nerve. Sural sensory conduction velocity increased significantly after exercise in normal subjects (p less than 0.01, mean increase 5.07 m/s) and non-neuropathic diabetic subjects (p less than 0.02, mean increase 3.99 m/s) but not in neuropathic subjects (mean increase 0.99 m/s). Subcutaneous temperature rose significantly in normal subjects (p less than 0.01, mean increase 2.07 degrees C) and non-neuropathic diabetic subjects (p less than 0.001, mean increase 2.52 degrees C) but not in neuropathic subjects (mean increase 0.15 degree C). However, sural sensory conduction velocity increased by 1.2 m.s-1.degree C-1 following direct warming of the limb in six neuropathic subjects which was comparable to that of normal and non-neuropathic subjects (1.49 and 1.48 m.s-1. degree C-1). The impairment of exercise conduction increment in diabetic neuropathy suggests impaired nerve blood flow in diabetic neuropathy.

The effect of gamma-linolenic acid on human diabetic peripheral neuropathy: a double-blind placebo-controlled trial

Twenty-two patients with distal diabetic polyneuropathy confirmed both clinically and by objective nerve function studies, completed a double-blind, placebo-controlled study to assess the effect of dietary supplementation with gamma-linolenic acid on their neuropathy. Patients received either 360 mg gamma-linolenic acid (12 patients) or indistinguishable placebo capsules (10 patients) for 6 months. All patients were assessed at the beginning and end of the study period by neuropathy symptom and sign scoring, motor and sensory nerve conduction studies, and thermal threshold measurements. When compared with the placebo group, patients on gamma-linolenic acid showed statistically significant improvement in neuropathy symptom scores (p less than 0.001), median nerve motor conduction velocity (p less than 0.01) and compound muscle action potential amplitude (p less than 0.01), peroneal nerve motor conduction velocity (p less than 0.05) and compound muscle action potential amplitude (p less than 0.05), median (p less than 0.01) and sural (p less than 0.001) sensory nerve action potential amplitude and ankle heat threshold (p less than 0.001) and cold threshold (p less than 0.01) values. gamma-Linolenic acid therapy might have a useful role in the prevention and treatment of distal diabetic polyneuropathy.

Diabetic neuropathies and pain

Many of the diabetic neuropathic syndromes are characterized by painful symptoms with a sensation of burning and associated with troublesome hyperaesthesia. It is important to distinguish between the acute and chronic forms of peripheral sensory neuropathy; while the former carries an excellent prognosis for symptomatic improvement within one year, the latter may cause persistent symptoms for many years. In contrast to the acute form, in which symptoms are particularly severe but abnormal neurological signs are minimal, patchy stocking and glove sensory loss together with peripheral small muscle wasting are often present in chronic sensorimotor neuropathy. Peripheral polyneuropathies are more common in patients with poor metabolic control, although recent evidence implicates blood glucose flux as a possible contributory factor to neuropathic pain. It is possible that blood glucose flux or altered peripheral blood flow leads to increased spontaneous activity in nociceptive afferent fibres which are present in the axonal sprouts that characterize small fibre neuropathy. In the diagnosis of the neuropathies, exclusion of other aetiological factors is of paramount importance as there is no specific diagnostic test for diabetic nerve damage. If there is no symptomatic improvement after a period of stable and optimal metabolic control together with simple analgesics, then the tricyclic drugs should be regarded as first line therapy. The rapid effect of these drugs suggests a peripheral rather than central mode of action.

Aldose reductase inhibitors and late complications of diabetes

Neuropathy and retinopathy are two potentially serious late complications of diabetes. There is accumulating evidence that the development of these conditions is closely related to increased activity of the polyol pathway, which occurs in certain tissues as a consequence of long term hyperglycaemia. Symptomatic diabetic neuropathy may appear as one of many forms and is frequently accompanied by pain. Diabetic retinopathy is a progressive degeneration of the retina that represents one of the major causes of blindness in the developed world. A good prognosis for either of these conditions is believed to rely on early diagnosis and optimisation of glycaemic control as they become less reversible with progression of cellular damage. A new approach to the treatment of these and other late complications of diabetes may be offered by recently developed drugs, such as sorbinil, that inhibit the enzyme aldose reductase. In various animal models of late complications of diabetes sorbinil and other aldose reductase inhibitors have been shown to reverse some of the biochemical and physiological changes believed to underlie these complications. These include prevention or reversal of the accumulation of sorbitol and depletion of myo-inositol in nerve, lens and renal glomeruli. Sorbinil also counteracts the slowing of nerve conduction velocities, reverses the structural changes of Sipple stages I and II cataracts and prevents proteinuria in diabetic rats. Orally administered sorbinil is absorbed rapidly and reaches steady state plasma concentrations after 6 to 10 days' administration. Its elimination half-life is long (38-52 hours) and much greater than that of another aldose reductase inhibitor, tolrestat (10-12 hours). Within the dose range 50-250 mg about one-third of administered sorbinil appears in the urine as unchanged drug. In the small number of clinical studies of diabetic patients with neuropathies sorbinil has demonstrated limited therapeutic effects. There is now a requirement for studies of its prophylactic use and its therapeutic use in patients with diabetic neuropathy in the early stages of development.

Pain relief in diabetic neuropathy: the effectiveness of imipramine and related drugs

Experience of treating 80 consecutive patients with painful diabetic neuropathy and a double-blind study of imipramine in acute painful neuropathy are reported. A standard stepwise protocol was used, based on imipramine as the drug of first choice, substitution of amitriptyline or mianserin as second choice, and, thirdly, addition of phenothiazine or clonazepam if necessary. Sixty per cent were satisfied with imipramine alone and all but 3 (5%) were improved by the stepwise scheme. Paradoxically, the most effective analgesia was obtained in those with the most severe and unpleasant pain. Often 150 mg of imipramine or amitriptyline was necessary. The onset of analgesia and relapse after withdrawal of treatment were rapid, facilitating clinical use, and suggesting a peripheral rather than central drug action.

Breaking the Cycle of Pain In Diabetic Peripheral Neuropathy

An attitude of helplessness pervades both patients and health professionals when dealing with the problems of dia betic peripheral neuropathy. Those afflicted are often over whelmed by the pain and find themselves unable to fulfill the daily requirements of their diabetes regimen. The in consistencies in amounts and timing of meals, levels of ac tivity, and scheduling of insulin injections can result in hyperglycemia, a state that has been found to correlate strongly with the occurrence of nerve damage. Thus the pain perpetrates itself. For the health professional, having to confront this devastating pain without objective factors to assess and few scientific treatment measures to offer con tributes to a sense of inadequacy and ignorance. It is fre quently assumed that "nothing can be done." In reality, health professionals do have something to offer the person suffering from this condition. First, they can provide infor mation about the relationship between control and compli cations, and explain the need for the patient to change those behaviors that may be perpetuating the pain. Sec ond, and most importantly, health professionals can con vey support and confidence, without which many patients will be unable to make the changes in their behavior neces sary to resolve their pain.