Congenital myopathies are a diverse group of skeletal muscle disorders that are genetic in origin, often apparent at birth, are usually slowly or non-progressive, and have overlapping clinical features.1-3 Until recently, the classification of congenital myopathies into distinct groups was most frequently based on the histopathologic features of the muscle.4

Congenital fiber type disproportion myopathy (CFTD) is a congenital myopathy in which the chief pathologic features are small type 1 fibers and type 1 fiber predominance (T1FP).5 Type 1 fibers are considered small when they are more than 12% smaller than type 2 fibers. CFTD is often associated with high arch palate, kyphoscoliosis, contracture, and a slightly increased serum creatine kinase (CK) level.

The chief pathologic feature of congenital myopathy with type 1 fiber predominance (CMT1P) is only T1FP without other structural abnormalities.6,7 In this report, the clinical characteristics of CFTD and CMT1P were investigated.

Nearly four dozen congenital myopathies have been described, but the majority of these disorders are rare and considered to be nosologically questionable entities.2 Three firmly established clinical disease entities are central core disease, nemaline myopathy, and centronuclear myopathy. Less common diseases, including congenital fiber type disproportion (CFTD), multicore disease, reducing body myopathy, fingerprint myopathy, and cytoplasmic or inclusion body myopathy, are not as well delineated but are generally accepted as recognizable entities.

T1FP is defined as type 1 fibers composing more than 55% of the total.8 T1FP occurs commonly in conjunction with more specific structural alterations as seen in many of the congenital myopathies.5,9-13

Brook6 and Dubowitz7 introduced the possibility of a distinct entity for children with T1FP without other structural abnormalities found using light or electron microscopy. It has been called "congenital myopathy with type 1 fiber predominance (CMT1P)". Kyriakides et al.14 reported that 9 cases of CMT1P showed hyponia, mild proximal weakness, delayed motor milestones, and normal or mildly elevated serum CK. It is not clear whether "CMT1P" is related to the uniform type 1 fiber entity described by Oh.15 In the present study, four of the 6 patients with CMT1P demonstrated delayed motor milestones and non-progressive proximal lower extremity weakness. One patient showed congenital hypotonia. Another patient (Patient No. 7) showed mild right proximal lower extremity weakness with hip pain. High arched palate, kyphoscoliosis, and contractures were present in one patient. There were no cardiac problems or family histories of neuromuscular disorders. All but one patient (Patient No. 5) with CMT1P showed normal cognitive function. In this study, most patients with CMT1P had mild weakness and normal or slightly elevated CK levels. Considering the clinical, electrophysiological, and morphological findings, the cases lacked distinctive features to support the diagnosis of any other congenital myopathies with specific structural alterations. The diagnosis of hypothyroid myopathy was excluded on laboratory grounds. The clinical manifestations and laboratory findings of these patients with CMT1P were similar to previous reports.14

CFTD is characterized by congenital hypotonia and delayed motor milestones, and is often associated with congenital dislocation of the hip joint, high arched palate, kyphoscoliosis and contractures. Central nervous system abnormalities have been described in some cases of CFTD.16 This disorder has also been associated with hyperinsulinemia and peripheral insulin resistance.17 Electromyographic changes have been different in reported cases of CFTD. In some cases, the electromyography has been described as normal or myopathic but not conclusively diagnostic.18 Other cases have shown neurogenic electromyographic findings such as fibrillation, giant motor unit action potentials, and reduced interference patterns. The serum CK level is normal or mildly elevated. Associated cardiomyopathies have rarely been reported.19 Intelligence is usually normal. The weakness of muscles involved may include those of the legs, arm, trunk, neck or face, but pharyngeal and ocular groups are spared. Despite normal sensation, deep tendon reflexes are usually diminished or absent. In this study, three patients with CFTD demonstrated slowly progressing muscle weakness involving the legs, arm, neck or face, and showed muscle fiber type disproportion in the muscle biopsy. Congenital hypotonia, delayed motor milestones, high arched palate, kyphoscoliosis, and contractures were present in different combinations in this study's patients. Two of the 3 patients with CFTD in this study had cardiac abnormalities. The serum CK level showed normal ranges and the electromyography was described as myopathic in all the patients. The clinical manifestations and laboratory findings of the patients with CFTD were similar to previous reports. The possibility of other congenital myopathies and other diseases, such as congenital myotonic dystrophy, infantile acid maltase deficiency, and Krabbe's disease, was excluded by pathologic, clinical, and laboratory findings.

In this study, the clinical characteristics between CFTD and CMT1P were similar, but the frequency of dysmorphic features was relatively less in CMT1P than in CFTD. Long term observational studies of CMT1P are needed to determine if it will change into another form of congenital myopathy or if CMT1P is a distinct clinical entity.