Myopathology of myotonic dystrophy. A morphometric study

Lipomatous muscular 'dystrophy' of Piedmontese cattle

Lipomatous myopathy is a degenerative muscle pathology characterized by the substitution of muscle cells with adipose tissue, sporadically reported in cattle, pigs, and rarely in sheep, horses and dogs. This study investigated the pathology of this myopathy in 40 muscle samples collected from regularly slaughtered Piedmontese cattle living in Piedmont region (Italy). None of the animals showed clinical signs of muscular disease. Muscle specimens were submitted to histological and enzymatic investigations. Gross pathology revealed a different grade of infiltration of adipose tissue, involving multiple or single muscles. The most affected regions were the ventral abdomen and the shoulders, especially the cutaneous muscles and the muscles of the thoracic group. Morphological staining revealed an infiltration of adipose tissue varying in distribution and severity, changes in muscle fibre size and increased number of fibres with centrally located nuclei, suggesting muscle degeneration-regeneration. Necrosis and non-suppurative inflammatory cells were also seen. Furthermore, proliferation of connective tissue and non-specific myopathic changes were present. Chemical and physical characteristics of the affected tissue were also evaluated. The authors discuss about the aetiopathogenesis and classification of this muscle disorder whose histological lesions were similar to those reported in human dystrophies.

Muscle function and morphology in myotonic dystrophy

Patients with myotonic dystrophy classified clinically into function groups were studied. Muscle strength for knee extension and flexion was, with few exceptions, moderately or markedly reduced with successively more severe clinical disability. The reduction in maximal walking speed showed a similar tendency. There was a large percentage of both type I and in relation to normal findings of type II C fibers in biopsies from most patients. The fiber area varied greatly even in the patients in the best function groups, some of whom had large type II fibers. The most common-histopathological changes, found in all function groups, were fiber atrophy with small angular fibers, internal nuclei, splitting, fibrosis and moth-eaten fibers. A high percentage of type I fibers can already be seen in patients with practically no other morphological changes and without significant functional deterioration.

Structural and functional alterations of muscle fibres in the novel mouse model of facioscapulohumeral muscular dystrophy

We recently generated a mouse model of facioscapulohumeral muscular dystrophy (FSHD) by selectively overexpressing FRG1, a candidate gene for FSHD, in skeletal muscle. The muscles of the FRG-1 mice did not show any plasmamembrane defect suggesting a novel pathogenetic mechanism for FSHD. Here, we study structure and function of muscle fibres from three lines of mice overexpressing FRG1 at different levels: FRG1-low, FRG1-med, FRG1-high. Cross-sectional area (CSA), specific force (Po/CSA) and maximum shortening velocity (V(o)) of identified types of muscle fibres from FRG1-low and FRG1-med mice were analysed and found to be lower than in WT mice. Fast fibres and especially type 2B fibres (the fastest type) were preferentially involved in the dystrophic process showing a much larger force deficit than type 1 (slow) fibres. Consistent with the latter observation, the MHC isoform distribution of several muscles of the three FRG1 lines showed a shift towards slower MHC isoforms in comparison to WT muscle. Moreover, fast muscles showed a more evident histological deterioration, a larger atrophy and a higher percentage of centrally nucleated fibres than the soleus, the slowest muscle in mice. Interestingly, loss in CSA, Po/CSA and V(o) of single muscle fibres and MHC isoform shift towards a slower phenotype can be considered early signs of muscular dystrophy (MD). They were, in fact, found also in FRG1-low mice which did not show any impairment of function in vivo and of muscle size in vitro and in soleus muscles, which had a completely preserved morphology. This study provides a detailed characterization of structure and function of muscle fibres in a novel murine model of one of the main human MDs and suggests that fundamental features of the dystrophic process, common to most MDs, such as the intrinsic loss of contractile strength of muscle fibres, the preferential involvement of fast fibres and the shift towards a slow muscle phenotype can occur independently from obvious alterations of the plasma membrane.

Hereditary skeletal muscle diseases in the horse. A review

Since riders nowadays are expecting the highest level of performance from their horses, muscular disorders therefore represent a major problem for the equine athlete. A lot of research has been done to identify muscular disorders and their etiopathogenesis. Both acquired and inherited forms of muscle diseases have been described. In this review only the latter forms will be mentioned. Major signs of all muscle disorders are muscular stiffness, cramping or pain, muscular fasciculations, muscular atrophy and exercise intolerance. Muscle biopsies can help to identify the cause of rhabdomyolysis or muscular atrophy. However, especially in hereditary muscular diseases, a lot of questions are still to be answered. Increasing knowledge of the etiopathogenesis and newer diagnostic tests may lead to a more accurate diagnosis of the individual diseases in future.

Scoliosis in Steinert syndrome: a case report

BACKGROUND CONTEXT

Steinert syndrome is described as an autosomal dominant condition characterized by progressive muscular wasting, myotonia, musculoskeletal manifestations and rare spinal defects. Little is reported about spinal deformity associated with this syndrome.

PURPOSE

We present a patient with Steinert syndrome complicated by scoliosis. In the literature on muscular dystrophy, other than Duchenne, little mention is given to the problem of scoliosis in general and its treatment in particular.

STUDY DESIGN

A case report of a patient with Steinert syndrome associated with thoracic scoliosis and hypokyphosis is presented.

METHODS

A 17-year-old boy presented with King type II right thoracic scoliosis (T5-T11, Cobb angle of 40 degrees) and hypokyphosis--10 degrees. He was treated with posterior stabilization and instrumentation at level T3-L2 with a postoperative correction of the scoliotic curve to 20 degrees. Histopathologic examination of the muscles confirmed the diagnosis of Steinert myotonic dystrophy.

RESULTS

At 30-month follow-up, the patient was clinically pain free and well balanced. Plain radiographs showed solid spine fusion with no loss of deformity correction.

CONCLUSIONS

Scoliosis in Steinert syndrome shares the characteristic of an arthrogrypotic neuromuscular curve and demands the extensive soft tissue release for optimal surgical correction. Intraoperative observations included profound tissue bleeding, abnormally tough soft tissues and a difficult recovery from anaesthesia.

Mice lacking the myotonic dystrophy protein kinase develop a late onset progressive myopathy

Myotonic dystrophy (DM) is an autosomal dominant disorder resulting from the expansion of a CTG repeat in the 3' untranslated region of a putative protein kinase (DMPK). To elucidate the role of DMPK in DM pathogenesis we have developed Dmpk deficient (Dmpk-/-) mice. Dmpk-/-mice develop a late-onset, progressive skeletal myopathy that shares some pathological features with DM. Muscles from mature mice show variation in fibre size, increased fibre degeneration and fibrosis. Adult Dmpk-/-mice show ultrastructural changes in muscle and a 50% decrease in force generation compared to young mice. Our results indicate that DMPK may be necessary for the maintenance of skeletal muscle structure and function and suggest that a decrease in DMPK levels may contribute to DM pathology.

A new inherited muscular disorder in Japanese quails (Coturnix coturnix japonica)

Thirteen adult mutant (LWC strain) Japanese quails (Coturnix coturnix japonica), between the ages of 8 and 60 weeks were examined for a progressive muscular disorder. The disorder, inherited as an autosomal dominant trait, was clinically apparent as early as 28 days of age; it was characterized by generalized myotonia, muscle stiffness, and muscle weakness. Affected birds were identified by their inability to lift their wings vertically upward and by their inability to right themselves when placed on their dorsum. Electromyographic studies in two mutant quails showed high-frequency repetitive discharges comparable to those of myotonic runs. These discharges persisted after nerve resection. The distinctive histopathologic changes in the various muscles examined were ring fibers, sarcoplasmic masses, and internal migration of sarcolemmal nuclei. A slight decrease in the size of type IIB muscle fibers and a slight increase in the size of type IIA fibers were observed in the M. pectoralis thoracicus of affected quails. In older affected birds, inter- and intrafascicular fatty infiltration with replacement of type IIB fibers by fat cells was seen in the pectoral muscles. Single fiber necrosis, nonspecific lymphorrages, and variations in the muscle fiber size and shape were also noted. The typical muscle lesions and multisystem involvement, which was manifested by testicular degeneration and atrophy in the male LWC specimens and bilateral lenticular cataracts in 6 of 13 affected mutant quails, suggest resemblance of this new inherited muscular disorder to myotonic dystrophy in man.

Skeletal muscle changes associated with equine myotonic dystrophy

A progressive neuromuscular disorder in young horses, clinically apparent as early as 1 month of age, is characterized by generalized myotonia, muscle stiffness, muscle weakness and atrophy. Myotonia is identified by percussion dimpling and myotonic EMG discharges. Changes in one case included testicular hypoplasia, cataract formation, and glucose intolerance, indicating a systemic involvement. Pathologic changes in skeletal muscles from three affected foals were examined. Sarcoplasmic masses, ringed fibers, internal positioning of sarcolemmal nuclei, and nuclear rowing were among the primary histologic changes noted. Variation in fiber diameter size, especially atrophy, and type I predominance were also prominent changes. A neurogenic involvement was indicated by type grouping changes in several muscles.

Chronic progressive external ophthalmoplegia. II. A qualitative and quantitative electronmicroscopy study of skeletal muscles

This study quantifies the major electron microscopic changes in limb muscle biopsies from 31 out of 34 patients with the syndrome of chronic progressive external ophthalmoplegia. Patients were divided into three clinical groups -- A) 10 sporadic cases with muscle weakness only; B) 9 familial cases with muscle weakness only; C) 15 cases with muscle weakness and one or more of the following features: pigmentary retinopathy, cerebellar ataxia, pyramidal signs and peripheral neuropathy. Electron microscopic mitochondrial abnormalities were found in all groups (8 patients from group A, 3 from group B, 14 from group C). Quantitative measurements of certain muscle fibre constituents, using a point-counting technique, revealed decreased myofibril volume-fractions and increased volume-fractions of mitochondria, glycogen and lipid in some biopsies from each group. Mitochondrial volume-fractions correlated positively with lipid content, the proportion of type 1 fibres, and the percentage of fibres with increased oxidative enzyme activity. The three groups defined clinically showed no significant differences in terms of the relative proportions of these measured constituents.

Immaturity of muscle fibers in the congenital form of myotonic dystrophy: its consequences and its origin

Skeletal muscle maturation is impaired in children with congenital myotonic dystrophy. This immaturity is characterized at the light microscopy level by an abnormal presence of myotubes, small fascicles of muscle fibers, thin myofibers, and delayed muscle fiber type differentiation with a peripheral halo lacking mitochondrial oxidative enzyme activity. At an ultrastructural level, the characteristics are a paucity of myofibrils with a peripheral rim devoid of mitochondria and myofibrils in the fibers. In time the muscle is able to gain a certain degree of maturity as shown in one of our cases who had two successive muscle biopsies. The muscle, however, never becomes normal but retains discrepancies in fiber size and fiber type distribution and shows some fiber necrosis. Maturation of the motoneurons is normal, which may explain necrosis of immature muscle fibers. In an experimental study carried out to look for evidence of a circulatory factor in mothers of children with congenital myotonic dystrophy, it was found that sera from these mothers administered intra-peritoneally to newborn rats does in fact impair muscle maturation, whereas rats injected similarly with sera from control women showed normal muscle maturation.

A myopathy associated with muscle hypertonicity in the Cavalier King Charles Spaniel

Clinical signs of electrically silent muscle hypertonicity are described in five Cavalier King Charles dogs. Biopsies of the biceps femoris and triceps muscles, when examined with the electron microscope, revealed evidence of sarcotubular and mitochondrial abnormalities. These included enlargement of the sarcoplasmic reticulum, hydropic degeneration of mitochondria, tubular proliferations in the vicinity of the triads and vacuolar invagination of mitochondria. The exact nature of these findings is not clear and it is suggested that utilization of tracer techniques would help to explain them.

"Ragged-red" fibres in myotonic dystrophy

Twenty-five muscle biopsies (18 from the left biceps and 7 from the left quadriceps) of 25 patients suffering from myotonic dystrophy (MyD) were studied, 13 of which showed "ragged-red" fibres (RRFs); all the RRFs, which were type I fibres, were found in biceps muscles, while none of the quadriceps muscles showed RRFs. The incidence of RRFs varied from 0.5% to 20.0% (average 4.2%). On electron microscopy, RRFs contained enlarged mitochondria, usually in subsarcolemmal clusters, including dense granular matrix materials, concentrically whired membranous cristae, and paracrystalline inclusions, consistent with those of previously reported cases of mitochondrial myopathy, suggesting that RRFs observed in biopsies from patients with MyD are due to abnormal mitochondria. The biopsy findings indicative of MyD including pyknotic nuclear clumps, moth-eaten fibres, ring fibres, type I fibre atrophy, and type I fibre predominance, were much more common findings in biceps muscles than quadriceps muscles, and in biopsies with RRFs than those without RRFs. From our observations, it is possible that RRFs in biopsied muscles from patients with MyD are not incidental observations but are intimately associated with the pathogenesis of this disorder, and that RRFs may be a special form of pathological reaction in which accumulation of abnormal mitochondria occurs.

Hyperinsulinemia in myotonic dystrophy: identity of the maternal factor causing the neonatal myotonic dystrophy syndrome

An environmental factor acting on the fetus is thought to cause a neonatal syndrome characterized by marked muscular hypotonia, lack of respiratory drive and feeding difficulties, in some infants born to mothers with myotonic dystrophy. Mortality is high, especially amongst those babies born prematurely, but muscle strength and tone improve rapidly in survivors. Nevertheless, most survivors have physical deformities and mental retardation and are thought to develop myotonic dystrophy later. We propose that alterations in maternal insulin secretion (usual in myotonic dystrophy subjects) alter fetal blood glucose and amino acid levels and retard growth and maturation of fetal skeletal muscle. This leads to severe muscular hypotonia in affected infants. Also, we suggest that infants who die during the perinatal period may not have inherited the defective autosomal dominant gene that causes myotonic dystrophy.

Morphologic and morphometric analyses of muscle in the neonatal myotonic dystrophy syndrome

Autopsy studies of three premature siblings who died soon after birth with the neonatal myotonic dystrophy syndrome revealed pulmonary hypoplasia and congenital pleural effusions. Neither of these findings has been described previously in this condition. New ultrastructural findings include focal diaphragmatic myofiber degeneration and necrosis, which were attributed to over-stretching of the fetal diaphragm. In addition, abnormally small stores of free and intravesicular glycogen were observed in skeletal muscle fibers. The morphometric features of control fetal and neonatal skeletal muscle were recorded for comparison with muscle fiber measurements in the three infants. Fiber diameters in the latter were much smaller than expected for body weights. The morphologic and morphometric findings support the concept that fetal muscle maturation is severely retarded in this syndrome.

Role of mechanical damage in pathogenesis of proximal myopathy in man

No adequate explanation has as yet been provided for the predominant proximal and symmetrical distribution of skeletal muscle weakness and wasting in human myopathies. One obvious difference between proximal and distal muscles is that in their postural antigravity role the former are involved in eccentric contractions to a much greater extent than are the latter. Recent physiological studies have shown that eccentric contractions produce considerable muscle damage in normal healthy subjects. The damage starts in individual sarcomeres but becomes more extensive over 1-2 days, the progression probably being due to the stronger sarcomeres stretching the weaker, damaged sarcomeres during normal activity and/or to the enzyme degradation of myofibrillar proteins when muscle damage results in Ca++ inflow. As a muscle becomes weaker and unable to meet the functional demands made upon it the likelihood of accidental stretch becomes greater. The vicious circle of weakness, stretch, damage, and further weakness may be the reason why the proximal muscles, which normally function eccentrically to some degree, are the most severely affected in a wide range of myopathic disorders.

Skeletal muscle pathology in ovine congenital progressive muscular dystrophy. 1. Histopathology and histochemistry

The histopathological lesions of ovine congenital progressive muscular dystrophy (CPMD) were characterized by myofiber hypertrophy, focal myofibrillar degeneration, formation of peripheral and central sarcoplasmic masses devoid of myofibrils and internal nuclei often in chains. Progressive loss of myofibrils was associated with atrophy of the fiber and eventual collapse of the sarcolemma. The process was polyphasic, consequently in mature lesions there was great variation in fiber diameter. Split fibers were common but ring fibers occurred rarely. Myofiber loss was associated with fatty or fibrous tissue replacement. Only type I (red, slow twitch, oxidative) fibers were affected and there was no histological evidence of effective regeneration. Ovine CPMD has many histopathological features in common with dystrophia myotonica in humans.