Congenital Myopathy with Fiber-Type Disproportion

A rare genetic myopathy presenting at birth with hypotonia and muscle weakness. Findings occurring later in life include short stature, progressive scoliosis, hip dislocation, and deformities of the feet.

Congenital Fiber-type Disproportion Myopathy.

The exact incidence of congenital myopathies is unknown, however, congenital myopathy with fiber-type disproportion (CMFTD) seems to account for approximately 20% of cases. Autosomal recessive transmission is the most common form; however, autosomal dominant inheritance and sporadic cases have been described. Genetic heterogeneity is probable.

CMFTD can be found in other forms of congenital myopathy and other diseases (e.g., Duchenne Muscular Dystrophy, Spinal Muscular Atrophy Syndrome, metabolic myopathies, central nervous system diseases such as leukodystrophies, Lowe Syndrome, and Moebius Syndrome); however, it also exists as a distinct diagnostic entity. Generalized hypotonia and weakness are noticeable at birth, and failure to thrive is common. The prognosis is often good because the disease is usually not progressive. In some cases, improvement with age has been reported. However, in approximately 25% of patients, the course is severe, with death resulting from respiratory failure in approximately 10% of patients. Skeletal involvement is common and may present with short stature, (kypho)scoliosis, contractures, joint laxity, and congenital hip dislocation. Bulbar palsy and ophthalmoplegia are less common and associated with a poor prognosis. Cardiac involvement (dilated cardiomyopathy, arrhythmias) is rare, but can be significant and require medical treatment or even heart transplant. In a minority of these patients, facial anomalies (e.g., high arched palate) and insulin-resistant diabetes mellitus have been reported. The diagnosis is confirmed by muscle biopsy, which commonly shows small, atrophic type I fibers and compensatory hypertrophic type II fibers. In CMFTD, type I fiber mean diameter should be at least 12% smaller than that of type II fibers, although some groups consider a difference of 25% more appropriate. Rarely, a small number of type I fibers shows signs of hypertrophy.

This disease requires a complete workup, including assessment of neurologic and motor milestones, family history, and past medical history. Respiratory function should be checked preoperatively (e.g., chest radiographs, arterial blood gas analysis, pulmonary function tests), and preoperative physiotherapy likely will be beneficial. Nevertheless, one should be prepared for prolonged postoperative mechanical ventilation. Regular and close monitoring of blood glucose in the perioperative period is recommended. Scoliosis not only may make airway management more difficult but also may lead to restrictive lung disease and cor pulmonale. A rapid-sequence induction technique should be considered in the presence of bulbar symptoms. Patient positioning may be difficult but must be done with great care because joint laxity with dislocation (hip) is a common problem in these patients. Because cardiac problems have been described in some of these patients, the threshold for a cardiac consult and/or echocardiography should be low. In case of cardiac anomalies, invasive monitoring is recommended. Regional anesthesia per se is not contraindicated, however, positioning problems and scoliosis can affect the success rate negatively (particularly for central neuraxial blockade).

The use of succinylcholine and volatile anesthetics has not been described in this disease. However, it seems prudent to avoid succinylcholine in muscular disorders. Maintain spontaneous ventilation until the airway has been secured.

Central Core Disease: Congenital myopathy with a specific histologic pattern and high susceptibility to malignant hyperthermia.

Myotubular Myopathy: Congenital muscle disease characterized by generalized hypotonia, muscle weakness, and central nuclei on muscle biopsy (myotube-like aspect).

Nemaline Rod Myopathy: A rare congenital and slowly progressive inherited neuromuscular disease usually apparent at birth and characterized by extreme hypotonia.