N. D. Harris

Altered ventricular repolarization during hypoglycaemia in patients with diabetes

There is circumstantial evidence implicating hypoglycaemia in the sudden overnight death of young patients with insulin‐dependent (Type 1) diabetes mellitus (IDDM), the mechanism of which is unknown. We have investigated the effects of hypoglycaemia on the electrocardiogram in 15 patients with diabetes (8 with IDDM and 7 with NIDDM) using a high resolution computer‐based system. Patients were randomized to either 2 h of euglycaemia or hypoglycaemia (at around 3 mmol l−1 ) during the afternoon, using hyperinsulinaemic glucose clamps, the two visits separated by a period of at least 4 weeks. Corrected QT interval (QTc), plasma potassium, and adrenaline were measured at baseline and at 0, 60, and 120 min. The degree of QTc lengthening (from baseline) during clamped hypoglycaemia was greater compared to the euglycaemic control period in patients with IDDM (median{range} at 60 min, 156{8 to 258 } vs 6{−3 to 28} ms, p <0.02) and NIDDM (120 min, 128{16 to 166} vs 4{ −3 to 169} ms, p <0.05). The fall in plasma potassium was greater during clamped hypoglycaemia compared to euglycaemia in those with NIDDM ( p <0.03) but not with those with IDDM ( p >0.06). The rise in plasma adrenaline was greater during clamped hypoglycaemia in both groups (IDDM p <0.02, NIDDM p <0.02) and there was a strong relationship between the rise in adrenaline and increase in QTc ( r = 0.73, p <0.0001).These data demonstrate alteration of ventricular repolarization with lengthening of the QT interval during hypoglycaemia and suggest a possible mechanism by which hypoglycaemia could cause ventricular arrhythmias.

Impaired blood flow and arterio-venous shunting in human diabetic neuropathy: a novel technique of nerve photography and fluorescein angiography.

SummaryNew techniques of sural nerve photography and fluorescein angiography which are able to provide an index of nerve blood flow have been developed. Under local anaesthetic, 3 cm of sural nerve was exposed at the ankle using an operating microscope. Without disturbing the epineurium, vessels were identified and photographed at a standard magnification (× 30). These were independently graded by an ophthalmologist not otherwise involved with the study. Fluorescein angiography was then carried out on the exposed nerve. The fluorescein appearance time and intensity of fluorescence were quantified, using computer analysis of digitised images. Thirteen subjects with chronic sensory motor neuropathy, five non-neuropathic diabetic and nine normal control subjects were studied. The mean epineurial vessel pathology score of the neuropathic group was significantly higher than the combined normal control and non-neuropathic diabetic groups (p <0.01). Direct epineurial arteriovenous shunting was observed in six neuropathic and one non-neuropathic diabetic patients and not in any of the normal control subjects. The nerve fluorescein appearance time was significantly delayed in subjects with chronic sensory motor neuropathy (51.5 ± 12 s) compared to both normal (34.7 ± 9 s, p <0.01) and non-neuropathic diabetic subjects (33.4 ± 11 s, p <0.025). The mean intensity of fluorescence at 96, 252 and 576 s, was significantly lower in subjects with chronic sensory motor neuropathy compared with both of the other groups (p <0.05). The epineurial vessel pathology score was significantly related to reduced sural (p <0.01) and peroneal (p <0.001) nerve conduction velocities, elevated vibration (p <0.01) and thermal (p <0.001) perception and the severity of retinopathy (p <0.002). The fluorescein appearance time was significantly related to reduced sural sensory (p <0.02) conduction velocity, elevated vibration (p <0.01) perception and epineurial vessel (p <0.002) pathology score, but it failed to relate to peroneal motor (p = 0.06) conduction velocity, thermal (p = 0.1) perception and the severity of retinopathy (p = 0.3). Intensity of fluorescence was significantly related to fluorescein appearance time (at 96 s, p <0.001; at 576 s, p <0.05) but did not relate to measures of neuropathic severity. These techniques have enabled us to observe that epineurial vessel anatomy is abnormal and that nerve blood flow is impaired in subjects with chronic sensory motor neuropathy. In addition epineurial arterio-venous shunting may be a feature of diabetic neuropathy. These techniques may further be applied to study nerve blood flow in early diabetic neuropathy.

Arterio-venous shunting and proliferating new vessels in acute painful neuropathy of rapid glycaemic control (insulin neuritis)

SummaryInsulin neuritis, or painful neuropathy following rapid improvement in glycaemic control, is well recognised but its aetiology is unclear. An understanding of the processes involved in the genesis of acute painful neuropathy of rapid glycaemic control may give an insight into the early pathogenetic factors leading to diabetic nerve damage in general. We have identified five subjects with insulin neuritis including one who developed severe autonomic neuropathy following treatment with insulin. Subjects underwent: 1) assessment of neuropathic symptom and deficit scores; 2) quantitative sensory and electro-physiological studies and 3) sural nerve epineurial vessel photography and fluorescein angiography in vivo. The sural nerve photographs were independently graded by an ophthalmologist. All subjects with insulin neuritis presented with severe sensory symptoms but clinical examination and electrophysiological tests were normal except in the subject with the severe autonomic neuropathy in whom all the tests were abnormal. On nerve photography, there was an abundance of epineurial nutrient vessels although these showed severe abnormalities including arteriolar attenuation, tortuosity and arterio-venous shunting in all subjects. Proliferating neural ‘new vessels’ which bear striking similarities to those found in the retina and that were more leaky to fluorescein than normal vessels, were observed in three subjects. Venous distension and/or tortuosity was also observed in three subjects and this was most marked in the subject with severe autonomic neuropathy. This study shows that epineurial nutrient vessel anatomy is abnormal in subjects with acute painful neuropathy of rapid glycaemic control, a condition previously thought to be purely metabolic in origin. The presence of epineurial arterio-venous shunting and a fine network of vessels resembling the new vessels of the retina, may lead to a ‘steal’ effect rendering the endoneurium ischaemic. This process may be important in the genesis of neuropathic pain, and further supports the importance of vascular factors in the pathogenesis of diabetic neuropathy.

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

SummarySevere 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 (x 3) 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<0.01, mean increase 5.07 m/s) and non-neuropathic diabetic subjects (p<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<0.01, mean increase 2.07°C) and non-neuropathic diabetic subjects (p<0.001, mean increase 2.52 °C) but not in neuropathic subjects (mean increase 0.15 °C). However, sural sensory conduction velocity increased by 1.2 m · s−1. °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. °C−1). The impairment of exercise conduction increment in diabetic neuropathy suggests impaired nerve blood flow in diabetic neuropathy.

Digital imaging: an accurate and easy method of measuring foot ulcers.

Aims A progressive reduction in the area of foot ulcer on serial measurement is traditionally done by tracing the margin of the ulcer on a transparent film and counting the number of squares on a graph paper underneath. We set out to use and validate the measurement of foot ulcers using a digital imaging technique and compare this with the traditional method.

Prolonged but partial impairment of the hypoglycaemic physiological response following short-term hypoglycaemia in normal subjects

SummaryRecent studies have reported reduced endocrine and symptomatic responses to hypoglycaemia 18–24 h after antecedent hypoglycaemia in both non-diabetic subjects and those with insulin-dependent diabetes mellitus. We examined these and peripheral physiological responses in eight nondiabetic subjects aged 23–35 years in the week following antecedent hypoglycaemia. Blood glucose levels were held at plateaus of 5 mmol/l and 2.5 mmol/l for 30 min during hyperinsulinaemic (60 mU · m−2 · min−1) morning clamps on days 1, 3 and 8 of two study periods separated by at least 4 weeks. Measurements were made at time 0, 15 and 30 min of each plateau on each day. On the afternoon of Day 1 we also induced either euglycaemia with a blood glucose level of 5 mmol/l (control week) or hypoglycaemia of 2.9 mmol/l (hypo week) for 2 h in random order. The adrenaline response to morning hypoglycaemia (p<0.01 on all days) was attenuated on Day 3 (p<0.05) and Day 8 (p<0.05) compared to Day 1 of hypo week only. Sweating was also attenuated on Day 3 (p<0.05) and Day 8 (p<0.02) of hypo week only. Noradrenaline levels and tremor increased during hypoglycaemia on each study day (p<0.05) but did not differ between days in either week. During hypo week only, the total symptom score response to hypoglycaemia was attenuated on Day 3 (p<0.03) but not Day 8 (p=0.10). Autonomic symptoms were similarly affected. In summary, the physiological responses to hypoglycaemia are affected differentially by antecedent hypoglycaemia with sweating and adrenaline responses remaining impaired for at least 5 days.

A new minimally invasive technique to show nerve ischaemia in diabetic neuropathy.

Aims/hypothesis. Experimental studies have shown that abnormalities of nerve microcirculation are important factors in the pathogenesis of diabetic neuropathy but there have been few clinical studies. We have applied microlightguide spectrophotometry to measure intravascular oxygen saturation (HbO2%) and blood flow in human sural nerve. Methods. We studied ten patients with mild-moderate sensory motor diabetic neuropathy, nine patients without neuropathy and nine control subjects. We took 300 measurements of oxygen saturation under direct visual control through a 1.9 mm rigid endoscope over three regions of the nerve. Spectrophotometric measurements of nerve fluorescence were taken after an intravenous injection of sodium fluorescein and the rate of increase in nerve fluorescence (rise time) was used as an indicator of nerve blood flow. Results. Nerve oxygen saturation was reduced in patients with neuropathy compared with control subjects (67.1 ± 2.2 % vs 76.7 ± 2.1 %, p = 0.006). Fluorescein rise time was prolonged in patients with neuropathy compared with the control group (48.5 ± 7.0 s vs 14.0 ± 3.1 s, p = 0.001) suggesting impaired nerve blood flow. There was a correlation between rise time, nerve oxygen saturation, glycaemic control and sural nerve sensory conduction velocity (p < 0.01). Conclusion/interpretation. The combination of microlight-guide spectrophotometry and micro-endoscopy provides a valuable minimally invasive technique for clinical investigation of nerve microcirculation. We have shown reduced nerve oxygenation and impaired blood flow in diabetic neuropathy and these findings strongly support a central role of microvascular disease in the pathogenesis of diabetic neuropathy. [Diabetologia (1999) 42: 737–742]

Can changes in QT interval be used to predict the onset of hypoglycemia in type 1 diabetes

Episodes of hypoglycaemia in adults and children with Type I diabetes induce abnormalities in cardiac repolarization, including lengthening of the QT interval and QT dispersion. The authors have used a computerised ECG system to measure overnight changes in the heart rate corrected QT interval (QTc) in diabetic patients. The aim of the study was to determine whether QT prolongation is associated with nocturnal hypoglycaemia (blood glucose <3.5 mmol/l) and to investigate whether such changes might be used as the basis for a hypoglycaemia alarm. The authors used a Hewlett Packard 200LX (pocket PC attached to a single channel high gain amplifier with a serial data interface. Recordings were made for 30 seconds at 15 minute intervals throughout the night and the data downloaded to a PC for analysis. Patients were asked to measure their blood glucose at 23:00, 03:00 and 07:00 hours. The authors have studied 20 non-diabetic subjects (8F/12M), with a mean age 32 years and 14 patients with diabetes (7 M/7F), with a mean age 49 years. In nondiabetic subjects, the QTc remained stable throughout the night with a mean change in QC of 2.8 (SD 22.1) ms. In the diabetic patients, the mean change in QTc during 13 normoglycaemic nights was 6.9 (SD 10.0) ms and during hypoglycaemic nights 6.5 (SD 8.9) ms. The QTc cumulative mean change and SD was calculated throughout the night. Episodes where there were two consecutive increases in QTc above mean +2 SD within an hour were considered significant. Using these criteria, there was I significant event during the normoglycaemic nights but only 4 episodes were correctly identified out of 6 hypoglycaemic nights. Automated measurements of nocturnal QT interval can be used to monitop changes in cardiac repopulation associated with hypoglycaemia but more sophisticated analysis of T wave morphology or other factors will be required if such a system is to be used to reliably detect the onset of hypoglycaemia.

Similar Physiological and Symptomatic Responses to Sulphonylurea and Insulin Induced Hypoglycaemia in Normal Subjects

There is little information concerning the physiological response to hypoglycaemia induced by sulphonylureas. We compared the physiological and symptomatic responses to insulin and tolbutamide induced hypoglycaemia in 8 normal subjects. While infusing either insulin or tolbutamide, we used a glucose clamp to maintain blood glucose at 4.5 mmol l−1 for 30 min and lowered it to 2.9 mmol l−1 for a further 30 min. Mean peripheral insulin levels during the insulin infusion arm in comparison with the tolbutamide infusion were not significantly different during the euglycaemic plateau: 106 ± 4 vs 77 ± 15 mU l−1 (mean ± SEM) (mean difference 29 mU l−1, 95 % CI −22 to 80; p = NS) but were greater during the hypoglycaemic plateau: 106 ± 3.5 vs 21.0 ± 4.0 mU l−1 (mean difference 85 mU l−1, 95 % CI 72 to 98; p < 0.0001). Portal insulin concentrations, calculated from C‐peptide data were not significantly different during the euglycaemic plateau with insulin as compared to tolbutamide. However, during hypoglycaemia portal insulin concentrations were significantly higher 15 min from the start of the plateau, during insulin infusion. During hypoglycaemia induced by either insulin or tolbutamide there were similar peak responses of glucagon: 124 ± 14 vs 128 ± 7 ng l−1 (mean difference −4, 95 % CI −39 to 31; p = NS) and adrenaline: 2.9 ± 0.4 vs 2.8 ± 0.3 nmol l−1, (mean difference 0.1, 95 % CI −0.9 to 1.0; p = NS). Increases in tremor and sweating and deterioration in reaction time were similar during both periods of hypoglycaemia as were increases in total: 18.5 ± 1.4 vs 19.6 ± 2.2 (mean difference −1.0, 95 % CI −3.8 to 1.8; p = NS) and autonomic: 8.9 ± 0.9 vs. 9.9 ± 1.3 (mean difference −1.1, 95 % CI −5.9 to 3.6; p = NS) symptom scores. We conclude that there is no difference in the glucagon, sympathoadrenal, cognitive or symptomatic response during hypoglycaemia induced by either insulin or tolbutamide. This suggests that the different insulin concentrations produced by these contrasting models of hypoglycaemia had no effect on the physiological response and patients taking sulphonylureas can be expected to develop similar warning symptoms to those on insulin.

Combined microlightguide spectrophotometry and microendoscopy for measurement of oxygen saturation in peripheral nerves

Microlightguide measurements of the spectral composition of backscattered light may be used to determine local tissue oxygen saturation and monitor tissue perfusion using intravenous injection of fluorescein dye as a contrast agent. We have used a combination of microlightguide spectrophotometry and microendoscopy to measure intravascular oxygen saturation (HbSaO2%) and monitor blood flow in the sciatic nerve of 12 healthy male Sprague-Dawley rats. The microlightguide and endoscope combination is a relatively new measurement technique. The aims of this study were to determine whether microlightguide spectrophotometry and microendoscopy could be used to measure HbO2 and blood flow in peripheral nerves and to compare the measurements made using the flexible lightguide with the endoscope-lightguide combination. We found no significant difference between the two types of measurement over similar regions of the nerve. mean SaO2% values 77.1% (95% CI = 75.4-78.8) and 78.8% (95% CI = 77.5-80.1) respectively. During a period of hypoxia there was a similar fall in both arterial and nerve oxygen saturation. Following injection of fluorescein, the rate of increase in nerve fluorescence was used as a measure of perfusion. The combination of microlightguide spectrophotometry and microendoscopy allows the exact site of measurement to be directly visualized. The minimally invasive nature of this technique may allow its application to the study of peripheral nerves in human subjects in conditions such as diabetic neuropathy where vascular factors are thought to have an important role in aetiology.