Congenital Myopathy

Key Points:

  • Congenital mypopathies are rare disorders presenting in infancy or childhood with diffuse muscle hypotonia 
  • Gross motor delay, poor coordination, and fascial weakness are common findings 
  • Orthopaedic considerations include cavus and equinovarus foot deformities, joint contractures, hip dysplasia and scoliosis 
  • There is no definitive treatment for any subtype at this time 
  • Orthopaedic management should be part of a multi-disciplinary team including pulmonology, cardiology, nutrition, and physical therapy 

Description:

Congenital myopathies are rare conditions representing a wide range of disorders in muscle development and function. Over twenty genes have been associated with congenital myopathy subtypes. (Dastgir 2018)  The most severe forms of these diseases can present in utero. More commonly, congenital myopathies present in infancy or childhood with hypotonia and developmental delay.  Facial and respiratory muscle weaknesses are common.  Treatment at this time is largely supportive care and best provided through a multidisciplinary approach. 

Epidemiology:

As a group, congenital myopathies are rare and tend to be less severe than congenital muscular dystrophies.  The underlying genetic mutations distinguish each type of congenital myopathy. Multiple mutations have been associated with each type of myopathy. The most common types include central core myopathy, nemaline myopathies, and centronuclear myopathies. These disorders have been shown to be inherited in x-linked, autosomal recessive and autosomal dominant forms.  
 

Clinical Findings:

Patients typically present either at birth or early in development with findings of hypotonia, myopathic facies and developmental delay. (Lewis 2011) The most severe forms of congenital myopathies present with reduced fetal movement in utero and may require ventilator assistance immediately after birth. Less severe forms can present in early adulthood and manifest as respiratory muscle weakness.  These diseases tend to be slowly progressive. 
 
The differential diagnosis for a congenital myopathy includes spinal muscular atrophy, muscular dystrophy, a metabolic disorder or toxic exposure. (Lewis 2011, Dastgir 2018, Burnette 2016)  
 
Patients with central core myopathies typically exhibit hypotonia at birth and may be described as “floppy”. (Burnette 2016, Dastgir 2018) Developmental hip dysplasia is common with hip dislocation being a common finding on orthopaedic exam. (Amato 2016) Motor milestones tend to be delayed and children may fall frequently and have difficulty with coordination. Patients may have a cavus foot deformity, diffuse muscle weakness on exam and diminished deep tendon reflexes. (Burnette 2016, Amato 2016) There is a strong association with malignant hyperthermia. (Robinson 2006) Unlike patients with nemaline and centronuclear myopathies, the facial and extraocular muscles tend to be spared. (Burnette 2016)  
 
Nemaline myopathies also result in diffuse muscle weakness as well as fascial and bulbar muscle weakness. (Dastgir 2018) Orthopaedic findings can include kyphoscoliosis and cavus feet. (Burnette 2016) Severe forms of nemaline myopathy can result in cardiomyopathy and respiratory failure.  
 
Centronuclear myopathies can present either early in life or in a delayed fashion. The most common form is a severe, infantile myopathy characterized by significant facial and limb weakness. (Burnette 2016) Respiratory failure in infancy is common. These infants can have associated muscle contractures in the lower extremities affecting the hips, knees and ankles. A milder form of disease can present later in life with ptosis, facial weakness and limb weakness.  These patients can develop equinovarus foot deformities that may require stretching and/or bracing.

Imaging Studies:

Although muscle biopsy is the mainstay for diagnosis, MRI and ultrasound have been used to evaluate skeletal muscle changes in patients with congenital mypothies. (Dastgir 2018) Different myopathy subtypes can affect different groups of muscles. These imaging studies can help determine which muscle groups are most affected by the disease process. (Jarraya 2012, Quijano-Roy 2011)  
 
For patients with scoliosis, standard scoliosis radiographs can be obtained and used to document the severity and progression of the curvature.

Etiology:

It is estimated that approximately 1 in 26,000 children are born with a congenital myopathy. This number may be an underestimate given the growing number of genetic mutations that have been associated with these conditions and the range of clinical presentations. (Dastgir 2018) Congenital myopathy was first described in 1956. (Magee 1956) Since that time, multiple types of congenital myopathies have been identified, each associated with one or multiple genetic mutations. (Dastgir 2018)  
 
The name of each subtype reflects the characteristic changes seen on biopsy samples from affected individuals. Central core myopathy is typically associated with mutations in the ryanodine receptor. Histologic analysis of muscle specimens demonstrate dye-free cores in the muscle fibers of affected patients. (Amato 2016) Patients with centronuclear myopathies have central nuclei within the muscle fibers seen on histologic analysis. Depending on the underlying mutation, centronuclear myopathy can be inhereited in x-linked recessive, autosomal recessive or autosomal dominant patterns. (Amato 2016) Nemaline myopathies have been associated with a range of mutations including nebulin, actin and tropomysin.  Muscle biopsy shows an accumulation of nemaline rods within muscle fibers. (Amato 2016) 

Treatment:

The current mainstay of treatment is supportive care. There are no current treatments for the underlying genetic mutations. Some small studies have shown benefit in patients with centronuclear myopathy who receive pyridostigmine. (Robb 2011) Gene therapy studies in animal models have been promising for certain subtypes of congenital myopathy, but there are no current gene therapies approved for human use. (Dastgir 2018) Supportive care, especially with respiratory or ventilator assistance, is provided to patients with respiratory muscle weakness and failure.  Due to facial and bulbar muscle weakness, patients may require assistance through either feeding therapy or placement of a gastrostomy tube. (Dastgir 2018)  
 
Contractures and foot deformities should be managed with stretching and bracing. There are currently no large studies that support surgical release of contractures in these patients. Concern exists that surgical treatment in the setting of a congenital myopathy may worsen the patient’s underlying muscle weakness.1 Therefore, physical and occupational therapy and the use of assistive devices are promoted. For patients who develop scoliosis, decision for bracing and or surgical intervention should be made not only based on curve magnitude, but also in coordination with pulmonology, cardiology and anesthesia. These patients are at a high risk for respiratory compromise, cardiomyopathy and may be at risk for malignant hyperthermia. The decision to proceed with surgical management should be made with consideration of the patient’s underlying health issues, quality of life, and surgical risk. 

Complications:

Complications are related to the specific type of orthopaedic intervention chosen for each patient as well as to the patient’s related medical issues. Surgical intervention should be decided upon with caution given the risks of malignant hyperthermia as well as underlying respiratory muscle weakness. (Burnette 2016) These patients are also prone to gastroesophageal reflux and gastric dysmotility, which can complicate the perioperative period. (Dastgir 2018)

References:

  1. Dastgir J, Gonorazky HD, Strober JB, Chrestian N, Dowling JJ: Congenital Myopathies, in Swaiman KF, Ashwal S, Ferriero DM, et al. (eds): Swaiman's pediatric neurology : principles and practice, ed Sixth edition. Edinburgh ; New York: Elsevier,, 2018, pp xxvi, 1403 pages. 
  2. Lewis D: Weakness and Hypotonia, in Marcdante KJ, Nelson WE (eds): Nelson essentials of pediatrics, ed 6th. Philadelphia, PA: Saunders/Elsevier,, 2011, pp XIX, 831 p. 
  3. Burnette WB: Hypotonic (Floppy) Infant, in Daroff RB, Hitchen M, Chovan J (eds): Bradley’s neurology in clinical practice, ed Seventh edition. London, England: Elsevier, 2016, pp 1 online resource (2,847 pages. 
  4. Amato AA: Disorders of Skeletal Muscle, in Daroff RB, Hitchen M, Chovan J (eds): Bradley's neurology in clinical practice, ed Seventh edition. London, England: Elsevier,, 2016, pp 1 online resource (2,847 pages. 
  5. Robinson R, Carpenter D, Shaw MA, Halsall J, Hopkins P: Mutations in RYR1 in malignant hyperthermia and central core disease. Hum Mutat 2006;27:977-989. 
  6. Jarraya M, Quijano-Roy S, Monnier N, et al.: Whole-Body muscle MRI in a series of patients with congenital myopathy related to TPM2 gene mutations. Neuromuscul Disord 2012;22 Suppl 2:S137-147. 
  7. Quijano-Roy S, Carlier RY, Fischer D: Muscle imaging in congenital myopathies. Semin Pediatr Neurol 2011;18:221-229. 
  8. Magee KR, Shy GM: A new congenital non-progressive myopathy. Brain 1956;79:610-621. 
  9. Robb SA, Sewry CA, Dowling JJ, et al.: Impaired neuromuscular transmission and response to acetylcholinesterase inhibitors in centronuclear myopathies. Neuromuscul Disord 2011;21:379-386. 

Top Contributors:

Elizabeth W. Hubbard MD