Sequential muscle biopsy changes in a case of congenital myopathy

Congenital fiber type disproportion myopathy caused by LMNA mutations

A boy, who had shown muscle weakness and hypotonia from early childhood and fiber type disproportion (FTD) with no dystrophic changes on muscle biopsy, was initially diagnosed as having congenital fiber type disproportion (CFTD). Subsequently, he developed cardiac conduction blocks. We reconsidered the diagnosis as possible LMNA-myopathy and found a heterozygous mutation in the LMNA gene. This encouraged us to search for LMNA mutations on 80 patients who met the diagnostic criteria of CFTD with unknown cause. Two patients including the above index case had heterozygous in-frame deletion mutations of c.367_369delAAG and c.99_101delGGA in LMNA, respectively. Four of 23 muscular dystrophy patients with LMNA mutation also showed fiber type disproportion (FTD). Importantly, all FTD associated with LMNA-myopathy were caused by hypertrophy of type 2 fibers as compared with age-matched controls, whereas CFTD with mutations in ACTA1 or TPM3 showed selective type 1 fiber atrophy but no type 2 fiber hypertrophy. Although FTD is not a constant pathological feature of LMNA-myopathy, we should consider the possibility of LMNA-myopathy whenever a diagnosis of CFTD is made and take steps to prevent cardiac insufficiency.

A fatal case of cor pulmonale with undetected chronic hypoventilation in an infant with a known congenital myopathy

The authors of this paper wish to present a case of fatal cor pulmonale with right ventricular hypertrophy complicated by a congenital myopathy. It is our intention to demonstrate the importance of vigilant clinical assessment of children with a congenital myopathy, regardless of the exact etiology of their disease, or family history of disease severity. This case highlights the risk for fatal complications if hypoventilation and respiratory insufficiency go unrecognized in myopathic children. Consequently, we recommend respiratory and cardiac monitoring surveillance as well as appropriate referral to specialists in the management of such children.

Centronuclear (myotubular) myopathy

Centronuclear myopathy (CNM) is an inherited neuromuscular disorder characterised by clinical features of a congenital myopathy and centrally placed nuclei on muscle biopsy.

The incidence of X-linked myotubular myopathy is estimated at 2/100000 male births but epidemiological data for other forms are not currently available.

The clinical picture is highly variable. The X-linked form usually gives rise to a severe phenotype in males presenting at birth with marked weakness and hypotonia, external ophthalmoplegia and respiratory failure. Signs of antenatal onset comprise reduced foetal movements, polyhydramnios and thinning of the ribs on chest radiographs; birth asphyxia may be the present. Affected infants are often macrosomic, with length above the 90^th centile and large head circumference. Testes are frequently undescended. Both autosomal-recessive (AR) and autosomal-dominant (AD) forms differ from the X-linked form regarding age at onset, severity, clinical characteristics and prognosis. In general, AD forms have a later onset and milder course than the X-linked form, and the AR form is intermediate in both respects.

Mutations in the myotubularin (MTM1) gene on chromosome Xq28 have been identified in the majority of patients with the X-linked recessive form, whilst AD and AR forms have been associated with mutations in the dynamin 2 (DNM2) gene on chromosome 19p13.2 and the amphiphysin 2 (BIN1) gene on chromosome 2q14, respectively. Single cases with features of CNM have been associated with mutations in the skeletal muscle ryanodine receptor (RYR1) and the hJUMPY (MTMR14) genes.

Diagnosis is based on typical histopathological findings on muscle biopsy in combination with suggestive clinical features; muscle magnetic resonance imaging may complement clinical assessment and inform genetic testing in cases with equivocal features. Genetic counselling should be offered to all patients and families in whom a diagnosis of CNM has been made.

The main differential diagnoses include congenital myotonic dystrophy and other conditions with severe neonatal hypotonia.

Management of CNM is mainly supportive, based on a multidisciplinary approach. Whereas the X-linked form due to MTM1 mutations is often fatal in infancy, dominant forms due to DNM2 mutations and some cases of the recessive BIN1-related form appear to be associated with an overall more favourable prognosis.

Congenital neuromuscular disease with uniform type-1 fibers, presenting early stage dystrophic muscle pathology

We report two male siblings presenting with severe hypotonia, generalized muscle atrophy, multiple joint contractures and respiratory failure. The serum creatine kinase levels were within normal limits, 75 IU/l in the younger boy and 123 IU/l in the older one. Muscle biopsies at the age of 28 days in the younger boy and 48 days in the older one revealed dystrophic pathology with increased interstitial fibrous tissue, scattered basophilic fibers and an increased number of undeveloped type-2C fibers. Although the elder brother died from respiratory failure at 4 months of age, the younger child has been sustained with mechanical ventilation, and has been exhibiting non-progressive muscle symptoms. Upon re-biopsy of the younger sibling at the age of 3 years, neither basophilic regenerating fibers nor degenerating fibers were found. All muscle fibers were found to be extremely atrophic and behaved mostly like type-1 fibers, displaying the features of congenital neuromuscular disease with uniform type-1 fibers. Since early biopsies in congenital myopathies reveal numerous undifferentiated immature muscle fibers, it is difficult to make a definite diagnosis, unless we recognize disease-specific cytoplastic abnormalities of nemaline body formation and abnormalities of core structure.

Relationship of primary mitochondrial respiratory chain dysfunction to fiber type abnormalities in skeletal muscle

Variation in the size and relative proportion of type 1 and type 2 muscle fibers can occur in a number of conditions, including structural myopathies, neuropathies, and various syndromes. In most cases, the pathogenesis of such fiber type changes is unknown and the etiology is heterogeneous. Skeletal muscle mitochondrial respiratory chain analysis was performed in 10 children aged 3 weeks to 5 years with abnormalities in muscle fiber type, size, and proportion. Five children were classified as having definite, four as probable, and one as possible mitochondrial disease. Type 1 fiber predominance was the most common histological finding (six of 10). On light microscopy, four cases had subtle concomitants of a mitochondriopathy, including mildly increased glycogen, lipid, and/or succinate dehydrogenase staining, and one case had more prominent evidence of underlying mitochondrial disease with marked subsarcolemmal staining. Most cases (nine of 10) had abnormal mitochondrial morphology on electron microscopy. All were found to have mitochondrial electron transport chain (ETC) abnormalities and met diagnostic criteria for mitochondrial disease. We did not ascertain any patients who had isolated fiber type abnormalities and normal respiratory chain analysis during the period of study. We conclude that mitochondrial ETC disorders may represent an etiology of at least a subset of muscle fiber type abnormalities. To establish an etiologic diagnosis and to determine the frequency of such changes in mitochondrial disease, we suggest analysis of ETC function in individuals with fiber type changes in skeletal muscle, even in the absence of light histological features suggestive of mitochondrial disorders.

X-linked myotubular and centronuclear myopathies

Recent work has significantly enhanced our understanding of the centronuclear myopathies and, in particular, myotubular myopathy. These myopathies share similar morphologic appearances with other diseases, namely the presence of hypotrophic myofibers with prominent internalized or centrally placed nuclei. Early workers suggested that this alteration represented an arrest in myofiber maturation, while other hypotheses implicated either failure in myofiber maturation or neurogenic causes. Despite similarities in morphology, distinct patterns of inheritance and some differences in clinical features have been recognized among cases. A severe form, known as X-linked myotubular myopathy (XLMTM), presents at or near birth. Affected males have profound global hypotonia and weakness, accompanied by respiratory difficulties that often require ventilation. Most of these patients die in infancy or early childhood, but some survive into later childhood or even adulthood. The responsible gene (MTM1) has been cloned; it encodes a phosphoinositide lipid phosphatase known as myotubularin that appears to be important in muscle maintenance. In autosomal recessive centronuclear myopathy (AR CNM), the onset of weakness typically occurs in infancy or early childhood. Some investigators have divided AR CNM into 3 subgroups: 1) an early-onset form with ophthalmoparesis, 2) an early-onset form without ophthalmoparesis, and 3) a late-onset form without ophthalmoparesis. Clinically, autosomal dominant CNM (AD CNM) is relatively mild and usually presents in adults with a diffuse weakness that is slowly progressive and may be accompanied by muscle hypertrophy. Overall, the autosomal disorders are not as clinically uniform as XLMTM, which has made their genetic characterization more difficult. Currently the responsible gene(s) remain unknown. This review will explore the historical evolution in understanding of these myopathies and give an update on their histopathologic features, genetics and pathogenesis.

Clinical and histologic changes in the follow-up of a congenital myopathy

A 19-year-old woman was born with congenital hypotonia, generalized weakness, and dysmorphic features. A muscle biopsy performed at age 18 months found that type I fibers were smaller and more numerous than type II fibers, and she was diagnosed with congenital fiber type disproportion. She grew up with moderate motor impairment, but after a stationary period her weakness progressed gradually and she developed a severe ophthalmoplegia. When she was 18 years old a second muscle biopsy still indicated the predominance of type I fibers but also the presence of central nuclei and strong oxidative enzyme activity in the center of most of the fibers; this was compatible with centronuclear myopathy. The diagnostic reconsideration raises questions about the pathogenesis of these diseases and the recognition of congenital fiber type disproportion as a distinct nosologic entity.

Floppy infant syndrome

Floppiness/hypotonia is a common neurologic symptom in infancy. A variety of neuromuscular disorders and central nervous system (CNS) disorders cause floppy infant syndrome (FIS). CNS disorders are the much more common causes of the syndrome than neuromuscular disorders. On long-term follow up, cerebral palsy and mental retardation turn out to be the 2 most common causes of FIS. This review focuses on neuromuscular causes of FIS. With the advent of molecular diagnosis, a few conditions can be diagnosed by DNA analysis of the peripheral lymphocytes (myotonic dystrophy, spinal muscular atrophy); however, for the most part, electrodiagnostic studies and muscle biopsy remain as essential diagnostic tools for FIS. Immunohistochemical study of the biopsied muscle also improves diagnostic capability. Management for most conditions remains supportive.

Congenital myopathies/dystrophies

The congenital myopathies and congenital muscular dystrophies are a group of relatively infrequent neuromuscular disorders. Ultimate understanding of these disorders, however, will undoubtedly shed considerable light on skeletal muscle development and function. Three classical congenital myopathies are central core disease, nemaline myopathy, and centronuclear myopathy. The congenital muscular dystrophies are often distinguished by whether or not they are associated with clinically evident cerebral involvement.

The myopathology of floppy and hypotonic infants in Singapore

AIMS

This study attempts to determine the type and relative frequency of muscle diseases contributing to floppy and hypotonic infants in Singapore.

METHODS

Eighty consecutive muscle biopsies in the Department of Pathology, National University of Singapore, in the period 1978-2000, in which a clinical diagnosis of floppy or hypotonic infant was made, were reviewed.

RESULTS

The commonest cause of severe hypotonia in infancy was spinal muscular atrophy, which accounted for 33% of cases followed by congenital muscular dystrophy (13%). Eight cases (10%) of infantile type II glycogenosis (Pompe's disease) were encountered. There were seven cases of congenital myopathy, of which four were centronuclear myopathy, and one each of central core myopathy, nemaline myopathy and congenital fibre type disproportion. One case of centronuclear myopathy was associated with type I fibre smallness. Type II atrophy, which is generally considered a non-specific change, was encountered in five cases. Of interest is the relatively large number of muscle biopsies (29%) in which no significant pathological features were encountered at the light microscopic, histochemical as well as ultra-structural level.

CONCLUSIONS

The study has revealed a great variety of pathology affecting the muscle of children presenting as floppy infants or with hypotonia. The muscle diseases included spinal muscular atrophy, congenital muscular dystrophies, congenital myopathies and metabolic myopathies. However, 23 (29%) cases showed no significant pathology. For this group of floppy and hypotonic infants further studies are needed.

Congenital Fiber Type Disproportion—30 Years On

Thirty years ago, M. H. Brooke coined the term "congenital fiber type disproportion" (CFTD) to describe 12 children who had clinical features of a congenital myopathy and relative type 1 fiber hypotrophy on muscle biopsy. It is now clear that this histological pattern can accompany a wide range of neurological disorders, leading to disillusionment with CFTD as a distinct nosological entity. To determine whether the CFTD has clinical utility as a diagnostic entity, we have reviewed the literature for cases of type 1 fiber hypotrophy and have used strict exclusion criteria to identify 67 cases of CFTD. Most patients presented at birth with weakness and hypotonia, had normal intelligence, and followed a static or improving clinical course. In 43% of families, more than 1 individual was affected. Failure to thrive was common and 25% of patients had contractures or spinal deformities. Bulbar weakness and ophthalmoplegia were less common and cardiac involvement was rare. Twenty-five percent followed a severe course and 10% had died at the time of reporting, all from respiratory failure. Ophthalmoplegia and facial and bulbar weakness were significantly associated with a poorer prognosis. The relatively homogeneous phenotype supports the retention of CFTD as a distinct diagnostic entity and familial occurrence suggests a genetic basis. Regarding the diagnosis of CFTD, we found no strong evidence that the minimum difference between type 1 and type 2 fiber sizes should be increased from 12% to 25%. We also list the other reported causes of relative type 1 fiber hypotrophy to aid their exclusion from CFTD.

Congenital myopathies at their molecular dawning

The introduction and application of molecular techniques have commenced to influence and alter the nosology of congenital myopathies. Long-known entities such as nemaline myopathies, core diseases, and desmin-related myopathies have now been found to be caused by unequivocal mutations. Several of these mutations and their genes have been identified by analyzing aggregates of proteins within muscle fibers as a morphological hallmark as in desminopathy and actinopathy, the latter a subtype among the nemaline myopathies. Immunohistochemistry has played a crucial role in recognizing this new group of protein aggregate myopathies within the spectrum of congenital myopathies. It is to be expected that other congenital myopathies marked by inclusion bodies may turn out to be such protein aggregate myopathies, depending on analysis of individual proteins within these protein aggregates and their association with putative gene mutations.

Congenital fibre type disproportion a time-locked diagnosis: a clinical and morphological follow-up study

This is the first report that describes the clinical and histological findings in a patient suffering from pure congenital fibre type disproportion (CFTD), who had two biopsies with an interval of 16 years. Additionally, we compared the clinical signs and symptoms of the present case to those of 35 CFTD cases reviewed from the literature. From this we conclude that smallness of type I fibre diameter is not a characteristic feature during the course of pure CFTD. This suggests that CFTD is a time-locked diagnosis.