Scintigraphically assessed cardiac sympathetic dysinnervation in poorly controlled type 1 diabetes mellitus: one-year follow-up with improved metabolic control

A Comparative Assessment of Cardiovascular Autonomic Reflex Testing and Cardiac 123I-Metaiodobenzylguanidine Imaging in Patients with Type 1 Diabetes Mellitus without Complications or Cardiovascular Risk Factors

AIM

To compare the cardiovascular autonomic reflex tests (CARTs) with cardiac sympathetic innervation imaging with ¹²³I-metaiodobenzylguanidine (MIBG) in patients with type 1 diabetes mellitus (T1DM).

PATIENTS AND METHODS

Forty-nine patients (29 males, mean age 36 ± 10 years, mean T1DM duration 19 ± 6 years) without cardiovascular risk factors were prospectively enrolled. Participants were evaluated for autonomic dysfunction by assessing the mean circular resultant (MCR), Valsalva maneuver (Vals), postural index (PI), and orthostatic hypotension (OH). Within one month from the performance of these tests, patients underwent cardiac MIBG imaging and the ratio of the heart to upper mediastinum count density (H/M) at 4 hours postinjection was calculated (abnormal values, H/M < 1.80).

RESULTS

Twenty-nine patients (59%) had abnormal CARTs, and 37 (76%) patients had an H/M_4 < 1.80 (p = 0.456). MCR, PI, Vals, and OH were abnormal in 29 (59%), 8 (16%), 5 (10%), and 11 (22%) patients, respectively. When using H/M_4 < 1.80 as the reference standard, a cutoff point of ≥2 abnormal CARTs had a sensitivity of 100% but a specificity of only 33% for determining CAN.

CONCLUSIONS

CARTs are not closely associated with ¹²³I-MIBG measurements, which can detect autonomic dysfunction more efficiently than the former. In comparison to semiquantitative cardiac MIBG assessment, the recommended threshold of ≥2 abnormal CARTs to define cardiovascular autonomic dysfunction is highly sensitive but of limited specificity and is independently determined by the duration of T1DM.

Insulin restores myocardial presynaptic sympathetic neuronal integrity in insulin-resistant diabetic rats

BACKGROUND

Diabetes is associated with increased sympathetic activity, elevated norepinephrine, impaired heart rate variability, and the added risk of cardiovascular mortality. The temporal development of sympathetic neuronal dysfunction, response to therapy, and relation to ventricular function is not well characterized.

METHODS AND RESULTS

Sympathetic neuronal integrity was serially investigated in high fat diet-fed streptozotocin diabetic rats using [(11)C]meta-hydroxyephedrine (HED) positron emission tomography at baseline, 8 weeks of diabetes, and after a further 8 weeks of insulin or insulin-sensitizing metformin therapy. Myocardial HED retention was reduced in diabetic rats (n = 16) compared to non-diabetics (n = 6) at 8 weeks by 52-57% (P = .01) with elevated plasma and myocardial norepinephrine levels. Echocardiography pulse-wave Doppler measurements demonstrated prolonged mitral valve deceleration and increased early-to-atrial filling velocity, consistent with diastolic dysfunction. Insulin but not metformin evoked recovery of HED retention and plasma norepinephrine (P < .05), whereas echocardiography measurements of diastolic function were not improved by either treatment. Relative expressions of norepinephrine reuptake transporter and β-adrenoceptors were lower in metformin-treated as compared to insulin-treated diabetic and non-diabetic rats. Diabetic rats exhibited depressed heart rate variability and impaired diastolic function which persisted despite insulin treatment.

CONCLUSIONS

HED imaging provides sound estimation of sympathetic function. Effective glycemic control can recover sympathetic function in diabetic rats without the corresponding recovery of echocardiography indicators of diastolic dysfunction. HED positron emission tomography imaging may be useful in stratifying cardiovascular risk among diabetic patients and in evaluating the effect of glycemic therapy on the heart.

Diagnostic and prognostic imaging of the cardiac sympathetic nervous system

Individuals with systolic dysfunction congestive heart failure may have decreased neuronal density, decreased neuronal function (reuptake or retention of norepinephrine), or a combination of these, plus reduction in postsynaptic beta-receptor density. Cardiac neuronal distribution and function can be imaged with standard gamma cameras and PET using radiolabeled analogs of norepinephrine. Postsynaptic beta-adrenergic receptor distribution and density can be determined using PET. Multiple imaging studies of the presynaptic component have reported that those individuals with the lowest retention or fastest washout of the radiolabeled analogs have a much greater annual mortality than do those with greater retention or slower washout rate. The results of some studies have suggested that the image abnormalities are better predictors of death than are more common predictors of outcome such as ejection fraction, heart rate variability, and microvolt T-wave alternans. The variability between these studies makes it unclear which measure of presynaptic dysfunction is the most predictive. beta-Receptor imaging has not been evaluated as extensively as a prognostic tool as has presynaptic imaging. Preliminary data suggest that regional mismatch between beta-receptors and presynaptic norepinephrine transporter function may serve as a marker for adverse outcome.

Impaired cardiac autonomic function in women with prior gestational diabetes mellitus

BACKGROUND

Cardiac autonomic neuropathy is a common dysfunction in manifest diabetes mellitus and is associated with duration of diabetes and/or an inadequate glycaemic control. Heart rate variability (HRV) reflects autonomic heart function. The aim of the present study was to investigate whether in women with prior gestational diabetes (GD; pre-type 2 diabetes) alterations of cardiac autonomic function can be observed after delivery in relation to insulin sensitivity and glycaemic control.

MATERIALS AND METHODS

Forty-eight healthy women with prior GD were consecutively admitted to the study. HRV was analysed by both time, as well as frequency, domain methods using 24-h Holter monitoring. In addition, 20 women with normal glucose tolerance during and after pregnancy were investigated as control subjects. All women underwent a frequently sampled intravenous glucose tolerance test (FSIGT) for measurement of insulin sensitivity.

RESULTS

Time domain analysis (standard deviation of normal RR intervals; SDNN) showed a reduced HRV in 25 out of the 48 (52%) women with prior GD. Frequency domain analysis revealed that in these 25 subjects both low and high frequency components of power spectral density (reflecting mainly sympathetic respectively parasympathetic activity) were reduced, indicating that sympathetic as well as parasympathetic functional impairment may be assumed. However, a relative predominance of the sympathetic over parasympathetic cardiac function was observed. The impairment of cardiac autonomic function (reduced SDNN) was correlated with HbA1c values and the 2-h blood glucose concentration (oral glucose tolerance test) but not with insulin sensitivity.

CONCLUSION

The present results demonstrate that in 52% of the women examined who had prior GD, an impairment of cardiac sympathetic as well as parasympathetic function was present, which related to glycaemic control, but not to insulin sensitivity. This infers that functional autonomic changes could be an early prognostic indicator in pre-type 2 diabetes.

Relationship between cardiovascular dysfunction and hyperglycemia in streptozotocin-induced diabetes in rats

Streptozotocin (STZ)-induced diabetes in rats is characterized by cardiovascular dysfunction beginning 5 days after STZ injection, which may reflect functional or structural autonomic nervous system damage. We investigated cardiovascular and autonomic function, in rats weighing 166 +/- 4 g, 5-7, 14, 30, 45, and 90 days after STZ injection (N = 24, 33, 27, 14, and 13, respectively). Arterial pressure (AP), mean AP (MAP) variability (standard deviation of the mean of MAP, SDMMAP), heart rate (HR), HR variability (standard deviation of the normal pulse intervals, SDNN), and root mean square of successive difference of pulse intervals (RMSSD) were measured. STZ induced increased glycemia in diabetic rats vs control rats. Diabetes reduced resting HR from 363 +/- 12 to 332 +/- 5 bpm (P < 0.05) 5 to 7 days after STZ and reduced MAP from 121 +/- 2 to 104 +/- 5 mmHg (P = 0.007) 14 days after STZ. HR and MAP variability were lower in diabetic vs control rats 30-45 days after STZ injection (RMSSD decreased from 5.6 +/- 0.9 to 3.4 +/- 0.4 ms, P = 0.04 and SDMMAP from 6.6 +/- 0.6 to 4.2 +/- 0.6 mmHg, P = 0.005). Glycemia was negatively correlated with resting AP and HR (r = -0.41 and -0.40, P < 0.001) and with SDNN and SDMMAP indices (r = -0.34 and -0.49, P < 0.01). Even though STZ-diabetic rats presented bradycardia and hypotension early in the course of diabetes, their autonomic function was reduced only 30-45 days after STZ injection and these changes were negatively correlated with plasma glucose, suggesting a metabolic origin.

Microalbuminuria, cardiovascular autonomic dysfunction, and insulin resistance in patients with type 2 diabetes mellitus

Urinary albumin excretion/microalbuminuria and cardiovascular autonomic dysfunction are associated with high mortality in type 2 diabetic patients. We tested the hypothesis that the presence of microalbuminuria would correlate with cardiovascular autonomic dysfunction and insulin resistance in type 2 diabetic patients. The study group consisted of 15 Japanese patients with type 2 diabetes and microalbuminuria (age: 56 +/- 10 years, mean +/- SD). The control group consisted of 19 age-matched patients with normalbuminuria (56 +/- 7 years). Cardiovascular autonomic function was assessed by baroreflex sensitivity (BRS), heart rate variability, plasma norepinephrine concentration, and cardiac 123I-metaiodobenzylguanidine (MIBG) scintigraphy. BRS was lower in the microalbuminuria group than in the normalbuminuria group (P < .05). Early and delayed 123I-MIBG myocardial uptake values were lower (P < .05 and P < .005, respectively) and the percent washout rate of 123I-MIBG was higher (P < .0005) in the microalbuminuria group than in the normalbuminuria group. Fasting plasma glucose (P < .05) and insulin concentrations (P < .05), and the homeostasis model assessment (HOMA) index (P < .01) were higher in the microalbuminuria group than in the normalbuminuria group. Multiple regression analysis showed that urinary albumin excretion was independently predicted by the myocardial uptake of 123I-MIBG at delayed phase, fasting plasma insulin concentration, and the HOMA index. Our results indicate that the presence of microalbuminuria in our Japanese patients with type 2 diabetes is characterized by depressed cardiovascular autonomic function and insulin resistance, and that the myocardial uptake of 123I-MIBG at delayed phase, fasting plasma insulin, and HOMA index are independent predictors of urinary albumin excretion.

Cardiac sympathetic innervation and blood flow regulation of the diabetic heart

A key problem in ischemia-induced impairment of the vascular performance of the diabetic heart is the often-unrecognized cardiac sympathetic dysfunction. Advanced single-photon emission computed tomography (SPECT) and positron emission tomography (PET) using the radiopharmaceuticals, (123)I-metaiodobenzylguanidine ((123)I-MIBG) and (11)C-hydroxyephedrine ((11)C-HED), have shown that dysfunction of cardiac sympathetic nerves is present to a large extent in both type 1 and type 2 diabetes. The pattern of sympathetic disturbances is heterogeneous with a predominant effect in the posterior myocardial region. Furthermore, myocardial blood flow assessment with PET has shown that endothelial-dependent vasodilatation is reduced in proportion to the magnitude of cardiac sympathetic dysfunction. These mechanisms are currently proposed to lead from early changes to advanced impairment of cardiac function in diabetes.

New imaging techniques for cardiovascular autonomic neuropathy: a window on the heart

Cardiovascular autonomic neuropathy (CAN) is a common complication of diabetes, which results in disabling clinical manifestations and may predispose to sudden cardiac death. Recently, direct scintigraphic assessment of cardiac sympathetic integrity has become possible with the introduction of radiolabeled analogues of norepinephrine, which are actively taken up by the sympathetic nerve terminals of the heart. This article reviews how these techniques have been utilized to improve understanding of CAN complicating diabetes. Quantitative scintigraphic assessment of cardiac sympathetic innervation heart is possible with either [123I]-metaiodobenzylguanidine (MIBG) and single photon emission computed tomography (SPECT) or [11C]-hydroxyephedrine (HED) and positron emission tomography (PET). Studies in diabetic patients have explored the sensitivity of these techniques to detect CAN, characterize the effects of glycemic control on the progression of CAN and evaluate the effects of CAN on myocardial electrophysiology, blood flow regulation and function. Deficits of left ventricular (LV) [123I]-MIBG and [11C]-HED retention have been identified in diabetic subjects without abnormalities on cardiovascular reflex testing consistent with increased sensitivity to detect CAN. Poor glycemic control results in the progression of LV tracer deficits, which can be prevented or reversed by the institution of near-euglycemia. Deficits begin distally in the LV and may extend proximally. Paradoxically, however, absolute HED retention is increased in the proximal segments of the severe CAN subjects consistent with regional "hyperinnervation." These regions also exhibit abnormal blood flow regulation. Impaired myocardial MIBG uptake correlates with altered LV diastolic filling and myocardial electrophysiological deficits and is predictive of sudden death. Scintigraphic studies have provided unique insights into the effects of diabetes on cardiac sympathetic integrity and the pathophysiological consequences of LV sympathetic dysinnervation. Future studies using complementary neurotransmitter analogues will allow different aspects of regional dysfunction to be characterized with the aim of developing therapeutic strategies to prevent or reverse CAN.