Congenital Myopathy

Definition And Overview

Congenital myopathies are a diverse group of rare, inherited genetic developmental disorders of the muscle.
They arise due to genetic defects in the contractile apparatus of the muscle.
They are defined by distinctive histochemical or ultrastructural changes on skeletal muscle biopsy.
These disorders typically result in a benign congenital hypotonia with generally a good outlook for a normal life span.
The muscle weakness is usually static or slowly progressive, differentiating it from muscular dystrophies which show ongoing progressive muscle degeneration.

Patients mostly present in the neonatal or early infantile period.
They typically present as a floppy infant with profound generalized hypotonia.
The muscle weakness follows a predominantly proximal pattern.
Deep tendon reflexes are generally normal, sluggish, or decreased.
History may reveal decreased fetal movements and polyhydramnios during the antenatal period.

Facial weakness is a prominent feature.
Affected children often have a typical myopathic facies.
This includes an elongated face, open mouth, tented upper lip, and high arched or cleft palate.
Sucking and swallowing difficulties are common in the early months of life.
Extraocular muscle involvement may present as ptosis and ophthalmoparesis, though this varies by subtype.

Respiratory insufficiency is common due to weakness of the respiratory muscles.
Muscle bulk is generally poor.
Congenital contractures and skeletal deformities, such as early-onset scoliosis and rigid spine, may be present.
Congenital hip dislocation and distal joint laxity can also occur.

Congenital myopathies are classified based on the predominant morphological findings on muscle biopsy.

Myopathy Subtype	Histological Feature	Genetic Inheritance	Specific Clinical Clues
Centronuclear Myopathy	Central nuclei in all muscle fibers; small fibers.	X-linked recessive (MTM1), Autosomal Recessive (RYR1), Autosomal Dominant (DNM2).	Severe form with neonatal death. Prominent ptosis, ophthalmoparesis, and facial weakness.
Nemaline Myopathy	Nemaline bodies or rod-shaped inclusions on Gomori trichrome stain.	Autosomal Recessive (NEB, most common), Autosomal Dominant (ACTA1).	Bulbar dysfunction and lower facial weakness. Ptosis and extraocular weakness are not prominent.
Core Myopathies	Poorly defined fibers with short cores; areas devoid of mitochondria.	Autosomal Dominant (RYR1), Autosomal Recessive (SEPN1).	Most common type. High risk of malignant hyperthermia. Neonatal weakness is prominent.
Fiber-Type Disproportion	Predominant type 1 fibers with small type 2 fibers.	Autosomal Dominant, Autosomal Recessive, X-linked.	Face, ocular, and neck muscles involved. Associated with chest deformity and scoliosis.

This form is characterized by large vesicular nuclei placed centrally in a single row within the muscle fibers.
The X-linked recessive form is the most severe.
Approximately 75 percent of severely affected neonates with the X-linked disease die within the first few weeks or months of life.
Patients show severe axial and appendicular hypotonia, along with ophthalmoparesis and ptosis.
Cardiac involvement is generally absent, except in cases with SPEG mutations.

This is the most common variant among the congenital myopathies.
Biopsy reveals abnormal inclusion-like structures composed of alpha-actinin and actin.
Bulbar dysfunction is prominent.
The face can resemble congenital myotonic dystrophy.

These encompass Central Core Disease and Multiminicore Disease.
Patients with Central Core Disease inherit the condition in an autosomal dominant pattern.
A critical complication in patients with RYR1 mutations is the susceptibility to malignant hyperthermia.
Malignant hyperthermia can be triggered by halogenated anesthetic agents and depolarizing muscle relaxants.

This can present as an isolated myopathy or coexist with other muscle pathologies.
Clinically, it resembles nemaline myopathy with mild congenital contractures and developmental delay.
The diagnosis requires specific measurement demonstrating disproportionately small type 1 fibers.

Serum creatine kinase (CK) levels are usually normal or only mildly elevated.
Electromyography (EMG) reveals a characteristic myopathic pattern.
This myopathic pattern consists of short duration, small amplitude, polyphasic motor unit potentials.
Nerve conduction studies are generally normal, which helps rule out peripheral neuropathies.

Skeletal muscle biopsy remains a cornerstone for definitive diagnosis.
Advanced histochemical stains, immunohistochemistry, and electron microscopy are used to identify the specific subcellular structural defects.
Molecular genetic testing helps confirm the diagnosis and classify the exact subtype.
Identifying the specific genetic mutation is crucial for genetic counseling and determining the risk of malignant hyperthermia.

Differentiation is based on clinical presentation and specific physical findings.

Differential Diagnosis	Distinguishing Clinical Features
Congenital Myotonic Dystrophy	Ptosis and myopathic facies are present. Extraocular weakness is absent. Mothers often have clinical myotonia.
Congenital Myasthenic Syndrome	Characterized by fluctuating weakness. Extremity weakness is less severe than in congenital myopathies.
Mitochondrial Disease	Ptosis and ophthalmoparesis are present. Lower facial weakness is generally not obvious.
Spinal Muscular Atrophy (SMA)	Profound hypotonia and areflexia. Tongue fasciculations are present. Facial muscles are usually spared.
Congenital Muscular Dystrophy	Elevated serum CK levels. Extraocular muscles are usually spared. May have associated brain malformations.

There is currently no definitive medical cure for congenital myopathies.
Management relies heavily on a supportive and multidisciplinary approach.
Rehabilitation through regular physiotherapy and occupational therapy helps prevent contractures and maintains joint mobility.
Nutritional support is vital. Children with severe bulbar weakness may require feeding via a nasogastric tube or gastrostomy.
Respiratory care is paramount. Prompt treatment of recurrent chest infections is necessary.
Patients should receive the annual influenza vaccination.
Regular orthopedic surveillance is needed to manage complications like scoliosis and hip dislocation.
Pre-anesthetic screening and the use of dantrolene are mandatory for patients with core myopathies to prevent malignant hyperthermia.
Genetic counseling must be offered to the parents for future pregnancies.