Suspected acute myocardial infarction in a dystrophin-deficient dog

Golden retriever muscular dystrophy (GRMD) is a model for the genetically homologous human disease, Duchenne muscular dystrophy (DMD). Unlike the mildly affected mdx mouse, GRMD recapitulates the severe DMD phenotype. In addition to skeletal muscle involvement, DMD boys develop cardiomyopathy. While the cardiomyopathy of DMD is typically slowly progressive, rare early episodes of acute cardiac decompensation, compatible with myocardial infarction, have been described. We report here a 7-month-old GRMD dog with an apparent analogous episode of myocardial infarction. The dog presented with acute signs of cardiac disease, including tachyarrhythmia, supraventricular premature complexes, and femoral pulse deficits. Serum cardiac biomarkers, cardiac-specific troponin I (cTnI) and N-terminal prohormone of B-type natriuretic peptide (NT-proBNP), were markedly increased. Echocardiography showed areas of hyperechoic myocardial enhancement, typical of GRMD cardiomyopathy. Left ventricular dyskinesis and elevated cTnI were suggestive of acute myocardial damage/infarction. Over a 3-year period, progression to a severe dilated phenotype was observed.

Postoperative hyperthermia, rhabdomyolysis, critical temperature, and death in a former premature infant after his ninth general anesthetic

An 8-mo-old infant born at 24-wk of gestation died unexpectedly 12 h after his ninth uneventful general anesthetic. Preoperatively, he required low-flow nasal oxygen due to bronchopulmonary dysplasia, chronic diuretic therapy, and IV alimentation. As planned preoperatively, the infant remained tracheally intubated after his elective surgery and went to the Neonatal Intensive Care Unit in stable condition. However, over the next 6 h, he developed fever. The diagnosis of postoperative sepsis was considered. One hour before his death his temperature reached 43 degrees C. Autopsy documented Duchenne's muscular dystrophy and renal tubules containing myoglobin.

Cardiac involvement in Beagle-based canine X-linked muscular dystrophy in Japan (CXMDJ): electrocardiographic, echocardiographic, and morphologic studies

Background

Cardiac mortality in Duchenne muscular dystrophy (DMD) has recently become important, because risk of respiratory failure has been reduced due to widespread use of the respirator. The cardiac involvement is characterized by distinctive electrocardiographic abnormalities or dilated cardiomyopathy, but the pathogenesis has remained obscure. In research on DMD, Golden retriever-based muscular dystrophy (GRMD) has attracted much attention as an animal model because it resembles DMD, but GRMD is very difficult to maintain because of their severe phenotypes. We therefore established a line of dogs with Beagle-based canine X-linked muscular dystrophy in Japan (CXMD_J) and examined the cardiac involvement.

Methods

The cardiac phenotypes of eight CXMD_J and four normal male dogs 2 to 21 months of age were evaluated using electrocardiography, echocardiography, and histopathological examinations.

Results

Increases in the heart rate and decreases in PQ interval compared to a normal littermate were detected in two littermate CXMD_J dogs at 15 months of age or older. Distinct deep Q-waves and increase in Q/R ratios in leads II, III, and aVF were detected by 6–7 months of age in all CXMD_J dogs. In the echocardiogram, one of eight of CXMD_J dogs showed a hyperechoic lesion in the left ventricular posterior wall at 5 months of age, but the rest had not by 6–7 months of age. The left ventricular function in the echocardiogram indicated no abnormality in all CXMD_J dogs by 6–7 months of age. Histopathology revealed myocardial fibrosis, especially in the left ventricular posterobasal wall, in three of eight CXMD_J dogs by 21 months of age.

Conclusion

Cardiac involvement in CXMD_J dogs is milder and has slower progression than that described in GRMD dogs. The distinct deep Q-waves have been ascribed to myocardial fibrosis in the posterobasal region of the left ventricle, but our data showed that they precede the lesion on echocardiogram and histopathology. These findings imply that studies of CXMD_J may reveal not only another causative mechanism of the deep Q-waves but also more information on the pathogenesis in the dystrophin-deficient heart.

Impairment of cardiac autonomic function in patients with Duchenne muscular dystrophy: relationship to myocardial and respiratory function

BACKGROUND

Previous studies reported an impairment of nervous autonomic activity in patients with Duchenne muscular dystrophy (DMD). However, the relationship of the autonomic dysfunction to the impairment of cardiac mechanical function and of respiratory failure is not completely understood.

METHODS

We evaluated cardiac autonomic function by time- and frequency-domain heart rate variability (HRV) analysis on 24-hour Holter recordings in 60 patients with DMD (16.8 +/- 4.8 years) and 28 healthy control patients (15.2 +/- 4.6 years, P = not significant). The circadian rhythm of R-R interval, low frequency, high frequency, and low-frequency/high-frequency ratio was also assessed. In all patients, left ventricular ejection fraction was measured by 2D echocardiography; respiratory function was assessed by spirometry.

RESULTS

All HRV parameters were lower in patients with DMD than in control subjects, with the percentage of differences between adjacent R-R intervals >50 ms (11.6% +/- 8.5% vs 27.3% +/- 14.1%, P =.00001) and high frequency (23.9 +/- 10.3 ms vs 36.1 +/- 12.2 ms, P =.0001) showing the strongest differences. A significant circadian rhythm of HRV variables was present in both groups, but it was considerably flattened in patients with DMD. There was no correlation between left ventricular ejection fraction and HRV indexes except for a weak correlation with high frequency (r = 0.30, P =.02) and with low-frequency to high-frequency ratio (r = -0.29, P <.03). Similarly modest correlations were found between forced vital capacity and high frequency (r = 0.4, P =.007) and low-frequency/high-frequency ratio (r = -0.32, P =.026). Multiple regression analysis did not show any independent predictive variable for the autonomic impairment.

CONCLUSIONS

Our data show a marked impairment of cardiac autonomic function in patients with DMD, which appears to mainly involve the parasympathetic branch and appears to have a multifactorial origin.

Nine-year follow-up study of heart rate variability in patients with Duchenne-type progressive muscular dystrophy

OBJECTIVES

The purpose of this study was to investigate the progression of autonomic dysfunction in patients with Duchenne-type progressive muscular dystrophy (DMD) over time by using heart rate variability.

BACKGROUND

Although previous studies suggest the presence of autonomic dysfunction in patients with DMD, the precise cause is not known. On the other hand, it is well known that analysis of heart rate variability provides a useful, noninvasive means of quantifying autonomic activity. High frequency power is determined predominantly by the parasympathetic nervous system, whereas low frequency power is determined by both the parasympathetic and sympathetic nervous systems.

METHODS AND RESULTS

Frequency and time domain analyses of heart rate variability during ambulatory electrocardiographic monitoring were performed in 17 patients with DMD over a 9-year period. At the time of entry, the mean patient age was 11 years and the mean Swinyard-Deaver stage was 4. In the first year, high frequency power was significantly lower and the ratio of low frequency to high frequency was significantly higher in patients with DMD than in the normal control subjects. These differences become significantly greater as the disease progressed. At the time of entry, low and high frequency powers increased at night in both groups. However, over time, high and low frequency powers at night tended to decrease. All of the time domain parameters were significantly lower in the patients with DMD at all time points compared with the normal control subjects.

CONCLUSIONS

We concluded that DMD patients have either a decrease in parasympathetic activity, an increase in sympathetic activity, or both as their disease progresses.

Circadian rhythm and variability of heart rate in Duchenne-type progressive muscular dystrophy

Using 24-hour Holter monitoring and time domain and power spectral measurements, we evaluated the variability of the heart rate and its circadian rhythm in 55 male patients with Duchenne-type progressive muscular dystrophy (DMD) to characterize their autonomic function versus findings in 20 normal controls. Comparisons were also made in patients with mild, moderate, and severe stages of DMD. The percent difference between successive RR intervals that exceeded 50 ms, a measure of parasympathetic tone, was significantly lower even in patients with early stage of DMD than in controls (p < 0.01). This trend became marked with disease progression. Power in the high-frequency (HF) range (0.15 to 0.40 Hz), a measure of parasympathetic tone, was lower (p < 0.01), and the ratio of the power in the low-frequency (LF) range (0.04 to 0.15 Hz) and that of HF range (LF/HF ratio), a measure of sympathetic tone, was higher in DMD patients versus controls (p < 0.01). This trend was also marked with disease progression. Patients with mild or moderate disease had a slight circadian alteration in HF and LF/HF ratio. Patients with severe disease had virtually no circadian rhythm in HF. Their LF/HF ratio was higher at night (p < 0.01), lower in the morning (p < 0.01), and still lower during the day (p < 0.01), the opposite of control findings. The autonomic abnormalities in DMD were thus characterized by a significant increase in sympathetic activity and a significant decrease in parasympathetic activity. Thus, heart rate variability and circadian rhythm were useful in assessing autonomic dysfunction in DMD.

Myocardial myotonia in myotonic muscular dystrophy

We sought to determine whether and to what degree myocardial myotonia might occur in myotonic muscular dystrophy. Cardiac involvement manifests itself chiefly as abnormalities of specialized tissues. Current echocardiographic techniques permit assessment of left ventricular diastolic filling properties and might detect subtle myocardial myotonia. Twenty patients (mean age 37 +/- 13 years) with myotonic muscular dystrophy were studied. Twenty normal subjects (mean age 34 +/- 12 years), served as controls. Each subject had two-dimensional targeted M-mode echocardiograms of the posterior left ventricular wall to measure the rate of early diastolic relaxation, which was defined as diastolic endocardial velocity maximum (DEVM). Global left ventricular function was quantified. Doppler recordings of mitral inflow measured peak E and A velocities, ratio of E to A (E/A), mitral deceleration time (DT) and isovolumic relaxation (IVR) time. Normal controls had DEVM = 19 +/- 3 cm/sec, IVR = 72 +/- 7 msec, E/A = 1.6 +/- 0.5, and DT = 193 +/- 18 msec. Two SDs below the mean normal DEVM was 13.3 cm/sec. Two patient groups emerged: group A (10 patients) had abnormally slow DEVM (< or = 13.2 cm/sec) and group B (10 patients) had normal DEVM (> 13.2 cm/sec) with DEVM = 11 +/- 2 cm/sec and 20 +/- 4 cm/sec, respectively. Mitral inflow parameters showed a longer DT and IVR, with lower E/A ratios for group A versus group B, with DT = 203 +/- 48 msec and 175 +/- 21 msec, IVR = 87 +/- 15 msec and 74 +/- 7 msec, E/A = 1.7 +/- 0.7 and 2.3 +/- 0.9, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of the transient outward potassium current in canine X-linked muscular dystrophy

BACKGROUND

The xmd dog develops a cardiomyopathy similar to that seen in Duchenne muscular dystrophy patients. In both the canine and human diseases, ECG abnormalities may precede the development of overt cardiac pathological lesions. The purpose of this study was to determine whether specific cellular electrical abnormalities occur in dystrophic ventricular tissue.

METHODS AND RESULTS

Action potentials were recorded in epicardial tissue strips obtained from normal and xmd dogs. Phase 1 amplitude was increased from 86.8 +/- 2.7 mV in normal dogs to 94.3 +/- 1.8 mV in xmd dogs (mean +/- SEM; P < .05). The 4-aminopyridine-sensitive transient outward potassium current (Ito), as recorded in isolated epicardial myocytes using the whole-cell patch-clamp technique, was reduced in xmd dogs compared with age-matched normal dogs. Cell capacitance also was reduced significantly in xmd compared with normal cells, as was the current density (3.6 +/- 0.3 versus 5.4 +/- 0.8 pA/pF, respectively). No differences were observed in the time constants of current decay or in the kinetics of recovery from inactivation between groups. The slope factor (k) of steady-state inactivation was significantly greater in xmd compared with normal cells (7.2 +/- 0.9 versus 5.4 +/- 0.5, respectively), whereas the V1/2 of inactivation did not differ (-38.2 +/- 2.4 versus -36.8 +/- 1.6 mV, respectively).

CONCLUSIONS

These data indicate that the magnitude of Ito is reduced in dystrophic epicardial myocytes, resulting in an increase in phase 1 amplitude. The reduction of Ito may alter the balance of inward and outward currents in dystrophic myocardium and thereby contribute to the development of cardiac pathology.