On the function of the endocrine glands in myotonic muscular dystrophy

Primary Hyperparathyroidism and Hyperthyroidism in a Patient with Myotonic Dystrophy: A Case Report and Review of the Literature

Various endocrine manifestations are commonly described in myotonic dystrophy (MD), including primary hypogonadism, diabetes mellitus, and thyroid and parathyroid dysfunction. We describe a 46-year-old woman with a family history of MD with her son. She was diagnosed with cardiac arrhythmia and required the implantation of a pacemaker. She was noted to have a bilateral cataract. She complained of muscle weakness, diffuse myalgia, and palpitation. The electromyography (EMG) showed myotonic discharges. Laboratory tests showed high serum calcium 2.83 mmol/L, serum phosphate 1.2 mmol/L, parathormone 362.5 pg/mL, thyroid stimulating hormone TSH 0.02 mIU/L (normal range: 0.34–5.6 mIU/L), FT4 21.17 ng/mL, and negative anti-thyroperoxidase antibodies. Cervical ultrasound revealed a multinodular goiter. The 99mTc-MIBI scintigraphy localized a lower right parathyroid adenoma. The clinical data, the family history of MD, EMG data, and endocrine disturbances were strongly suggestive of MD associated with hyperthyroidism and primary hyperparathyroidism.

Hypogonadism is common in men with myopathies

BACKGROUND

Hypogonadism has been described in patients with myotonic muscular dystrophy type 1 but has not been evaluated in other myopathies.

METHODS

We measured total and free serum testosterone levels in 59 men with myotonic muscular dystrophy type 1 (N = 12), facioscapulohumeral muscular dystrophy (N = 11), dystrophinopathy (N = 12), metabolic myopathy (N = 7), and inclusion body myositis (N = 17) and compared these with the normal reference interval.

RESULTS

Thirty-two of the 59 (54%) participants had low total testosterone, 23 (39%) had low total and free values, and 5 (8%) had low free with normal total levels. There were no significant differences in the prevalence of hypogonadism between those with myotonic muscular dystrophy type 1 and the other groups even after considering age as a confounder.

CONCLUSIONS

Hypogonadism is common in men with myopathies, and with the importance of testosterone in the maintenance of muscle mass, treatment of hypogonadism should be considered.

Hyperkalaemia and selective hypoaldosteronism in myotonic dystrophy

Myotonic dystrophy (MyD) is a common genetic neuromuscular disorder in which chromosome 19 gives rise to an abnormal expansion of CTG-trinucleotide repeats. MyD is a highly variable multisystem disorder with muscular and nonmuscular abnormalities. Increasingly, endocrine abnormalities, such as gonadal, pancreatic, and adrenal dysfunction are being uncovered. Herein we present three unrelated cases with MyD with abnormally elevated serum potassium; 2 of the 3 cases presented clinically with cardiac dysrhythmias. Hyperkalaemic conditions such as renal failure, cortisol deficiency, pseudohyperkalaemia, and hyperkalaemic periodic paralysis were excluded. Further endocrine evaluation revealed baseline hypoaldosteronism associated with elevated renin activity. Perturbation of the renin-angiotensin-aldosterone system resulted in appropriately enhanced renin activity but with a subnormal aldosterone response, which appeared to be due to adrenal hyporesponsiveness. The treatment of all cases with fludrocortisone was without effect. Whether the apparent mineralocorticoid abnormality in MyD is due to associated hormonal perturbations (i.e. excessive ACTH responsiveness. elevated cytokines, elevated atrial natriuretic hormone, etc.), adrenal atrophy, and/or a manifestation of the underlying kinase dysfunction is uncertain, but merits further evaluation in view of the clinical consequence of hyperkalaemia.

Hypothalamic-pituitary-testicular function in 70 patients with myotonic dystrophy

Hypothalamic-pituitary-testicular function was studied in 70 patients with myotonic dystrophy (MD). The diagnosis was confirmed by electromyography. The mean age of the patients was 36.2 +/- 13.2 yr and the duration of the disease was 11.17 +/- 8.01 yr. Testicular atrophy (testes less than or equal to 12 ml on a Prader orchidometer) was present in 65.5% of patients. Fertility among married patients was 66.6%. Mean testosterone plasma levels were 438 +/- 298 ng/dl vs 520 +/- 185 ng/dl in the control group (P = NS). Basal plasma FSH and LH levels, and their response after the administration of 100 mcg of LH-RH were significantly increased although a wide dispersion was observed. Sperm count was carried out in 27 cases, showing a normal count in 7, oligospermia in 12, and azoospermia in 8 patients. Testicular biopsy was performed in 45 patients being normal in 2, showing mild testicular damage in 8, moderate in 14, and severe in 18; it was nule in 3 of them. A significant relationship between testicular atrophy and the sperm count (p less than 0.01), testicular damage and testicular atrophy (p less than 0.025), and sperm count and testicular damage (p = 0.017) was found. Basal plasma FSH and LH level were significantly related to the degree of damage in the testicular biopsy. All these findings indicate a primary testicular pathology, prevailing tubular over interstitial damage. We have not found any association between the duration of the disease and gonadal dysfunction.

Thyroid function in patients with myotonic dystrophy

In order to investigate endocrine disturbances in patients with myotonic dystrophy (MD), 12 patients and 20 normal controls were studied. All patients were clinically euthyroid and there were no significant differences between circulating levels (mean +/- SD) of T4 (114.7 +/- 26.8 vs 129.9 +/- 28.3 nmol/l), FT4 (16.6 +/- 4.5 vs 18.4 +/- 3.8 pmol/l), T3 (1.61 +/- 0.29 vs 1.86 +/- 0.33 pmol/l), TSH (2.7 +/- 1.3 vs 2.4 +/- 1.4 mU/l), TBG (26.7 +/- 5.5 vs 27.6 +/- 4.9 mg/l), T4/T3 (84.3 +/- 18.4 vs 82.1 +/- 15.3), and FT4/FT3 (0.28 +/- 0.05 vs 0.33 +/- 0.08). Serum FT3 (4.3 +/- 1.4 pmol/l) in patients were significantly lower than those (5.3 +/- 0.9 pmol/l) in normal controls (P less than 0.02). Thyroidal 131I-uptakes (8.7 +/- 4.3%) in patients were significantly lower than those (25.8 +/- 7.4%) in controls (P less than 0.01). The mean maximal TSH responses following TRH stimulation were significantly less in patients with MD (11.4 +/- 4.5 vs 17.0 +/- 6.2 mU/l; P less than 0.02). Neither circulating thyroid microsomal nor thyroglobulin antibodies were detectable in the 11 patients tested. Serum thyroglobulin concentrations were within the normal range in all patients but one. In conclusion, it is suggested that normal levels of serum T4, T3, FT4, TSH, TBG, T4/T3 and FT4/FT3, slight but significant decrease of serum FT3, reduced TSH response to TRH and a decrease of thyroidal radioiodine uptake might be due to a slight functional failure of TSH secretion in patients with myotonic dystrophy.

Progressive myotonia in foals resembling human dystrophia myotonica

A severe and progressive neuromuscular disorder accompanied by clinical, electrophysiological, and pathological features resembling human dystrophia myotonica was observed in three foals. This disorder was apparent as early as 1 month of age and involved progressive skeletal muscle dysfunction, initially characterized by proximal muscle hypertrophy and hypertonicity with subsequent muscle stiffness, weakness, and atrophy. Multisystem involvement was manifested in one case by testicular hypoplasia, early cataract formation, and borderline glucose intolerance. Prolonged dimpling of these large rear-limb muscles was elicited by percussion. Myotonic discharges were identified by electromyography. Percussion dimpling and the typical myotonic discharges persisted after neuromuscular blockade. Distinctive histologic muscle changes included ring fibers, sarcoplasmic mass formation, variation in fiber diameter size, and internally positioned nuclei.

Evaluation of renal parathyroid hormone receptor function in myotonic dystrophy

Endocrine abnormalities in myotonic dystrophy (MyD) reflect some of the multi-systemic involvement resulting from this disorder. One of these, abnormal insulin secretion, is considered to be caused by receptor dysfunction. Bone abnormalities, cataract and calcium transport defect suggest the abnormal calcium metabolism in MyD. The calcium metabolism is chiefly regulated by parathyroid hormone (PTH). An interest in the similarity between MyD and pseudohypoparathyroidism, which is a disorder of PTH receptor dysfunction, encouraged the authors to evaluate renal PTH receptor function from the responses of urinary adenosine 3',5'-monophosphate (cAMP) and phosphate excretion after administration of human PTH(1-34). The responses of cAMP were high in 3 cases, low in one case, but normal in the 4 other cases. The phosphaturic responses were elevated in 3 cases, reduced in 3 cases, and normal in 2 other cases. Since these abnormal responses closely mimic those in hypoparathyroidism, there may also be renal PTH receptor dysfunction in some cases of MyD. The results of the present study suggest another peptide hormone receptor defect, similar to insulin, which supports the hypothesis of generalised receptor dysfunction in MyD.

Serum testosterone levels in young normal horses

Serum testosterone levels were measured in normal young male horses (29 to 34 weeks old). No differences were found between gelded and intact males. The values for all the horses were low. On the basis of their testosterone levels, all the horses were prepubertal.

Neuropsychological findings in myotonic dystrophy

Although the literature contains several references to clinically apparent cognitive deficits in patients with myotonic dystrophy (MYD), efforts to support these observations with formal testing have been lacking. The current study compared 17 MYD patients with 25 normal controls on an expanded Halstead-Reitan Battery. The MYD group scored worse than the controls on nearly every neuropsychological measure. Significant neuropsychological impairment was present even when tests of motor skills were excluded. There was no relationship between general neuropsychological impairment and degree of weakness, myotonia, or muscle atrophy in the MYD patients. These findings suggest that cognitive impairment can be an important and relatively independent component of the disability in MYD, which should be considered in the clinical evaluation and counselling of persons with this disease.

Thyroid abnormalities in patients with myotonic dystrophy

Myotonic dystrophy (MD) is a systemic genetic disorder predominantly affecting musculature. Although endocrine disturbances have been described, data relating to thyroid gland involvement are sparse and conflicting. To further investigate this aspect, twenty MD patients and twenty age-and sex-matched controls were studied. All were clinically euthyroid and there were no significant differences between circulating levels (mean +/- SD) of T4 (101 . 5 +/- 28.2 v. 103 . 8 +/- 27 . 7 nmol/l), T3 (1 . 86 +/- 0 . 57 v. 2 . 15 +/- 0 . 47 nmol/l), rT3 (0 . 39 +/- 0 . 14 v. 0 . 42 +/- 0 . 09) nmol/l) or TSH (2 . 6 +/- 0 . 5 v. 2 . 4 +/- 0 . 8 mu/l) or between T3 resin uptakes (97 +/- 8 v. 98 +/- 11%). TSH responses following TRH stimulation were significantly less in the MD patients (323 +/- 141 v. 529 +/- 240%; P less than 0.0025). The mean maximum TSH levels achieved were 6 . 7 +/- 3 . 6 mu/l (MD) and 17 . 1 +/- 7 . 2 mu/l (controls); P less than 0.005. Two patients had multinodular goitres (MNG) and one a single thyroid nodule. Thyroid scans were normal except for patchy uptake in the two MNG patients. There was no abnormal incidence of circulating thyroid microsomal or thyroglobulin antibodies, and thyrotrophin binding inhibitory immunoglobulin levels were normal in four patients. It is concluded that in MD: 1 palpable thyroid gland abnormalities occur with increased frequency, 15% (three/twenty) in the present study and 20% (twenty/102 case reports) from a review of the literature; 2 TSH responses to TRH are significantly reduced; 3 circulating thyroid hormone levels are usually normal.

Insulin receptors in myotonic dystrophy: a study with mononuclear leucocytes and erythrocytes

We evaluated insulin receptor activity on mononuclear leucocytes and erythrocytes in 9 patients with myotonic dystrophy and in 9 controls. The results demonstrated that in myotonic dystrophy: 1. insulin binding to specific receptors was significantly impaired (P less than 0.01) because of a reduction in the number of high and low affinity receptors. 2. the affinity constants were not significantly affected 3. there was no correlation between receptor activity, insulin behaviour and glucose tolerance. These data obtained in two different cellular systems suggest that the constant numerical reduction of insulin receptors was probably due to a systemic membrane defect, typical of myotonic dystrophy.

Myotonic dystrophy as a disease of abnormal membrane receptors: an hypothesis of pathophysiology and a new approach to treatment

Muscle weakness, clinical myotonia, and multiple systemic organ disturbances characterize myotonic dystrophy. Many of these systemic disturbances involve target organ insensitivity to aminergic or peptidergic (hormonal) stimulation. Reduced density or impaired function of aminergic and peptidergic (hormonal) receptors throughout the body may underlie this disorder. Tricyclic antidepressant augmentation of aminergic function ameliorates depressive symptoms, increases strength, and improves clinical myotonia in myotonic dystrophy patients.

Evaluation of insulin receptor in myotonic dystrophy

Only 2 of 6 patients with myotonic dystrophy had glucose intolerance and hyperinsulinemia. All, however, had markedly reduced insulin binding to specific receptors on circulating monocytes. A significant difference in receptor affinity for insulin was observed, but the difference in affinity was not as marked as that in the amount of 125I-insulin bound per cell, which was sixfold greater in control cells. No evidence for a circulating factor competing with insulin for binding sites could be demonstrated. These alterations in binding did not directly correlate with glucose intolerance or hyperinsulinemia and are consistent with a postulated generalized membrane defect. Study of the insulin receptor in this disorder may uncover regulatory mechanisms in normal and pathological conditions, including myotonic dystrophy.

Dystrophia myotonica and hypothyroidism

Two cases of dystrophia myotonia associated with hypothyroidism are reported. This association is not frequent. In both cases the hypothyroidism precipitated an otherwise mild muscular weakness due to dystrophia myotonia. Treatment with dried thyroid improved the muscular power, while the myotonia remained unchanged. Since dystrophia myotonica and hypothyroidism have many presenting symptoms in common, a complete assessment of the thyroid function in every case of dystrophia myotonica seems worthwhile.

Insulin secretion in patients with myotonic dystrophy and their relatives

Oral glucose, i.v. tolbutamide and i.v. arginine tolerance tests were performed in 11 patients with myotonic dystrophy and 9 of their clinically unaffected relatives. Five of the myotonic patients had glucose intolerance; 7 had exaggerated immunoreactive insulin (IRI) response to glucose. One of the 9 relatives demonstrated glucose intolerance and none had exaggerated insulin response to glucose. Three relatives, all obese, hyperresponded to arginine and one of them also responded excessively to tolbutamide. The results indicate that an exaggerated IRI response to glucose is common in myotonic dystrophy. Enhanced responses to other stimuli are less frequent. Although glucose intolerance occurred in half of the patients, the fact that the highest IRI levels were seen in non-diabetic patients suggests that this excessive response may protect against glucose intolerance. Our studies in relatives do not support the potential usefulness of testing for hyperinsulinemia in the early detection of myotonic dystrophy.

Hormonal regulation of intraocular pressure

A detailed review of the hormonal effects on intraocular pressure is presented. There is evidence that corticotropin, vasopressin, thyroxin, insulin, glucocorticoids and mineralocorticoids may play a role in the physiologic regulation of intraocular pressure. Growth hormone, melanocyte stimulating hormone, progesterone, estrogen, chorionic gonadotropin and relaxin may influence intraocular pressure when administered in pharmacologic doses. Whether the key to understanding primary open-angle glaucoma lies in recognizing abnormal endocrine mechanisms, especially involving glucocorticoids, remains unclear at the present time.

Protein kinase activity in erythrocyte ghosts of patients with myotonic muscular dystrophy

Myotonic muscular dystrophy is a disorder of humans that involves many organ systems. Physiological studies have suggested that the fundamental defect is of membrane origin. Heretofore, no reproducible metabolic abnormalities have been demonstrated. In the present studies we used erythrocyte ghosts as a convenient source of purified membranes that do not possess changes of denervation, dystrophy, and fibrosis that might complicate the interpretation of muscle membrane changes. Our experiments demonstrated a significant difference in the phosphorylation of erythrocyte ghost protein by [gamma-(32)P]ATP, with endogenous protein kinase of erythrocyte membrane as the enzyme source. After ghosts were kept for 1 week at -20 degrees , phosphorylation of membrane protein in eight controls was twice as high as endogenous protein kinase activity measured in fresh preparations. No stimulation was seen in preparations from seven myotonic dystrophy patients from three different families. This reproducible difference in normal and myotonic membranes may represent an important new approach to studies of this debilitating inborn error of metabolism.