Congenital Myopathies

Congenital myopathies are a heterogeneous group of inherited muscle disorders defined by characteristic structural abnormalities on muscle biopsy and onset typically at birth or early infancy. The pooled prevalence across all subtypes is approximately 1.5 per 100,000 in the general population and 2.7 per 100,000 in children, with a recent population-based study from Sweden reporting a birth prevalence of 14.9 per 100,000 live births. Unlike congenital muscular dystrophies, which show dystrophic changes (necrosis, regeneration, fibrosis), congenital myopathies are characterized by specific structural abnormalities within myofibers—cores, rods, central nuclei, or fiber-type disproportion—without prominent degenerative features. Historically considered static or slowly progressive, it is now recognized that the clinical spectrum ranges from fatal neonatal presentations to mild adult-onset weakness. Advances in next-generation sequencing have expanded the genetic landscape enormously, with over 30 causative genes now identified. Although no disease-modifying therapies are currently approved, emerging gene therapy approaches and an expanding understanding of pathomechanisms offer hope for future treatments.

Classification by Pathologic Hallmark

The traditional classification of congenital myopathies is based on the predominant structural abnormality identified on muscle biopsy. Four major subgroups are recognized, though significant overlap exists between categories, and mutations in the same gene (particularly RYR1) can produce different histological patterns.

Core Myopathies

Core myopathies are defined by well-circumscribed areas of sarcomere disorganization and mitochondrial depletion within muscle fibers, appearing as "cores" on oxidative enzyme stains (NADH-TR, SDH, COX). The primary genes involved encode proteins critical to excitation-contraction coupling and calcium homeostasis at the sarcoplasmic reticulum.

Central core disease (CCD) is the most common congenital myopathy overall and is predominantly caused by dominant mutations in RYR1 (ryanodine receptor 1, chromosome 19q13.2). CCD is characterized by single, well-defined cores running the length of type 1 fibers. Clinical features include proximal weakness that is typically mild-to-moderate and stable or only slowly progressive, congenital hip dislocation, scoliosis, and susceptibility to malignant hyperthermia (MH). The RYR1 gene comprises 106 exons and encodes the skeletal muscle ryanodine receptor, which is the principal calcium release channel of the sarcoplasmic reticulum. Dominant missense mutations in the C-terminal region tend to produce a "leaky channel" phenotype with gain-of-function characteristics, while recessive loss-of-function mutations cause more severe presentations and can produce cores, minicores, or centronuclear pathology.

Multiminicore disease (MmD) is characterized by multiple, poorly circumscribed, short areas of sarcomere disorganization (minicores) distributed across both fiber types. The classic form is caused by recessive mutations in SELENON (formerly SEPN1, encoding selenoprotein N), with an additional subset caused by recessive RYR1 mutations. SELENON-related MmD presents with prominent axial weakness, early-onset rigid spine, severe progressive scoliosis, and disproportionate respiratory insufficiency relative to limb weakness. RYR1-related MmD shows more limb-predominant weakness, ophthalmoplegia, and variable MH susceptibility. Additional genes associated with multiminicore pathology include MEGF10 and TTN.

Nemaline Myopathies

Nemaline myopathies (NM) are defined by the presence of nemaline rods (also called nemaline bodies)—electron-dense, rod-shaped protein aggregates derived from the Z-disc that stain red-purple with the modified Gomori trichrome technique. At least 13 genes have been implicated, all encoding components of the thin filament of the sarcomere. The two most common are NEB (nebulin, autosomal recessive, ~50% of genetically confirmed cases) and ACTA1 (skeletal muscle α-actin, autosomal dominant or de novo, ~15–25% of cases but responsible for >50% of severe forms).

The clinical spectrum spans six severity grades with significant overlap:

• Severe neonatal form: Profound hypotonia at birth, absent spontaneous movements, severe respiratory failure, feeding difficulties; usually fatal in infancy; most commonly caused by ACTA1 mutations
• Intermediate form: Neonatal onset with moderate weakness and respiratory insufficiency; independent ambulation may be achieved but respiratory support is usually required; may progress
• Typical congenital form: The most common (~50% of NM cases); onset in the first year with generalized weakness, facial weakness, feeding difficulties; most achieve ambulation; respiratory status varies
• Childhood-onset form: Weakness manifesting between ages 1 and 15 years; typically slower progression
• Adult-onset form: Rare; slowly progressive proximal weakness beginning after age 20; must distinguish from sporadic late-onset nemaline myopathy (SLONM), which is acquired and potentially treatable
• Amish nemaline myopathy: Caused by a founder mutation in TNNT1 (troponin T type 1); severe tremor, chest rigidity, and respiratory failure; fatal by age 2 years

Additional NM genes include TPM2 (tropomyosin 2), TPM3 (tropomyosin 3), TNNT1, CFL2, KBTBD13, KLHL40, KLHL41, LMOD3, and MYPN. Mutations in KBTBD13 produce a distinctive phenotype with slowness of movement and relative sparing of facial muscles.

Centronuclear Myopathies

Centronuclear myopathies (CNM) are characterized by an abnormally high proportion of muscle fibers with centrally placed nuclei (normally <3% of fibers have central nuclei). Three main genetic forms are recognized:

X-linked myotubular myopathy (XLMTM) is caused by mutations in MTM1 (myotubularin, chromosome Xq28) and is the most severe form. Affected males present at birth with profound hypotonia, generalized weakness, respiratory failure requiring mechanical ventilation, and ophthalmoplegia. Prenatal features include reduced fetal movements and polyhydramnios. Affected infants are often macrosomic with large head circumference and undescended testes. Biopsy shows small, round fibers with central nuclei resembling fetal myotubes. Most truncating MTM1 mutations cause early lethality, though some missense mutations allow survival into adulthood. The estimated incidence is 2 per 100,000 male births.

Autosomal dominant CNM is caused by mutations in DNM2 (dynamin 2, chromosome 19p13.2). This is the mildest form, with onset typically in adolescence or adulthood (range 12–74 years). Patients present with slowly progressive distal or generalized weakness, ptosis, and ophthalmoplegia. Cataracts and neutropenia may occur. DNM2 mutations can also cause Charcot-Marie-Tooth disease type 2M, and some patients have overlapping myopathy and neuropathy.

Autosomal recessive CNM is associated with BIN1 (amphiphysin 2, chromosome 2q14) and, less commonly, RYR1 and TTN. BIN1-related CNM presents with mild-to-moderate neonatal hypotonia that may be unrecognized, with relatively favorable prognosis. RYR1-related CNM can be severe and is associated with ophthalmoplegia.

Congenital Fiber-Type Disproportion (CFTD)

CFTD is defined by type 1 fiber smallness (≥12% smaller mean diameter than type 2 fibers) with type 1 fiber predominance but no other specific structural abnormality (no rods, cores, or central nuclei). It is considered a diagnosis of exclusion, as fiber-type disproportion can accompany many other congenital myopathies. Causative genes include TPM3 (~20–25% of cases), ACTA1 (~6%), RYR1 (~10–20%), SELENON, TPM2, and MYH7. Clinical features include neonatal-onset hypotonia, proximal weakness, facial weakness, respiratory compromise (~30%), and feeding difficulties (~30%). The prognosis is generally favorable: over 90% of patients have stable or improving weakness, and most achieve ambulation. However, up to 25% with severe neonatal presentation die in infancy or childhood.

Key Genes and Proteins

Clinical Features

The Hypotonic Infant

Congenital myopathies are a major cause of the "floppy infant" presentation. Neonatal hypotonia manifests as reduced spontaneous movements, a frog-leg posture, head lag, and slip-through on vertical suspension. Feeding difficulties, a weak cry, and respiratory distress are common accompanying features. Prenatal clues include reduced fetal movements, polyhydramnios (from impaired fetal swallowing), and thin ribs on postnatal chest radiographs (reflecting chronic in utero respiratory weakness).

Differential Diagnosis of the Floppy Infant

The differential diagnosis of neonatal hypotonia is broad and extends beyond the congenital myopathies. A systematic approach is essential:

• Central causes: Hypoxic-ischemic encephalopathy, chromosomal abnormalities (Down syndrome, Prader-Willi syndrome), brain malformations, metabolic encephalopathies; central hypotonia is typically accompanied by reduced alertness, seizures, or dysmorphic features
• Anterior horn cell: Spinal muscular atrophy (SMA)—the most common genetic cause of infant death; distinguished by tongue fasciculations, absent reflexes, and markedly reduced CMAP amplitudes on NCS; genetic testing for SMN1 deletions is now part of newborn screening in many regions
• Peripheral nerve: Congenital hypomyelinating neuropathies; distinguished by very slow motor nerve conduction velocities
• Neuromuscular junction: Congenital myasthenic syndromes (CMS); distinguished by fatigable weakness, decremental response on RNS, and response to acetylcholinesterase inhibitors; genetic overlap exists (e.g., DOK7, RAPSN)
• Congenital muscular dystrophies: Distinguished by elevated CK, dystrophic biopsy changes, and CNS or ocular involvement in some forms
• Metabolic myopathies: Pompe disease (acid maltase deficiency)—screened on newborn dried blood spot; mitochondrial myopathies—elevated lactate, multisystem involvement

Muscle MRI Patterns

Muscle MRI is increasingly used as a complementary diagnostic tool in congenital myopathies, offering characteristic patterns of fatty replacement and relative muscle sparing that can guide genetic testing. T1-weighted sequences detect fatty infiltration, while STIR or T2 fat-suppressed sequences detect muscle edema (less common in congenital myopathies than in inflammatory or dystrophic disease). Distinctive patterns include:

• RYR1 myopathy: Selective involvement of the sartorius, adductor magnus, and vasti with relative sparing of the rectus femoris and gracilis in the thighs; in the lower legs, peroneal and soleus involvement with tibialis anterior sparing
• SELENON myopathy: Relative sparing of limb muscles with prominent involvement of the sartorius and semitendinosus; medial gastrocnemius involvement in the lower legs
• NEB-related NM: Selective involvement of the tibialis anterior in the lower legs with soleus sparing; in the thighs, preferential quadriceps involvement
• DNM2-related CNM: Characteristic distal-predominant involvement, especially of the medial gastrocnemius and soleus in the lower legs

Diagnostic Approach

Laboratory Studies

Serum creatine kinase (CK) is typically normal or only mildly elevated (<5× upper limit of normal) in congenital myopathies, in contrast to the markedly elevated CK seen in most muscular dystrophies. A normal CK does not exclude myopathy. In the appropriate clinical context, a CK >10× normal should prompt consideration of alternative diagnoses (congenital muscular dystrophy, metabolic myopathy, or inflammatory myopathy).

Electromyography

Needle EMG findings in congenital myopathies range from entirely normal to a myopathic pattern (short-duration, low-amplitude, polyphasic motor unit potentials with rapid recruitment). Fibrillation potentials and myotonic discharges may occasionally be seen. EMG may show mixed myopathic and neurogenic features, particularly in severe cases with chronic reinnervation. Repetitive nerve stimulation is usually normal, though mild jitter on single-fiber EMG has been reported in some RYR1, MTM1, and TPM2/TPM3-related myopathies, suggesting subclinical neuromuscular junction dysfunction. Nerve conduction studies are normal unless there is a coexisting neuropathy (e.g., DNM2 mutations can cause both myopathy and CMT).

Muscle Biopsy

Muscle biopsy remains essential when genetic testing is inconclusive and is the definitive method for identifying the specific structural abnormality:

• Core myopathies: Central cores (single, well-defined areas of myofibrillar disorganization running the fiber length) or minicores (multiple short areas) on oxidative enzyme stains (NADH-TR, SDH, COX); cores lack mitochondria and oxidative enzyme activity
• Nemaline myopathies: Nemaline rods appear as red-purple inclusions on modified Gomori trichrome stain; electron microscopy confirms electron-dense, rod-shaped structures arising from the Z-disc; rods may be subsarcolemmal, intranuclear (ACTA1), or diffuse
• Centronuclear myopathies: Centrally placed nuclei in >25% of fibers (normally <3%); fiber smallness and type 1 predominance; in XLMTM, fibers resemble fetal myotubes; radial sarcoplasmic strands may be seen on NADH staining in DNM2-related forms
• CFTD: Type 1 fiber smallness (≥12% smaller than type 2 fibers) with type 1 predominance; no other specific structural abnormality

Open biopsy is preferred over needle biopsy to reduce sampling error, and tissue should be processed frozen (not paraffin-embedded) for proper histochemical analysis. Biopsy should target a mildly-to-moderately weak muscle (MRC grade 4) to avoid both false-negative results from normal-strength muscle and end-stage changes from severely affected muscle.

Genetic Testing

Next-generation sequencing (NGS) gene panels targeting congenital myopathies have become first-line diagnostic tools, increasingly preceding biopsy in pediatric patients. Panels typically include 30–60+ genes associated with congenital myopathies and overlap conditions. Key considerations include:

• Panels are preferred over whole-exome or whole-genome sequencing as initial testing because they report variants of uncertain significance that can guide further investigation
• NEB is technically challenging to sequence due to a triplicated region; some panels have reduced sensitivity for NEB variants
• Variants of uncertain significance are common and require correlation with clinical phenotype, family segregation, and sometimes muscle biopsy for interpretation
• Array-based comparative genomic hybridization can detect large deletions/duplications missed by standard sequencing
• De novo dominant mutations (especially ACTA1, DNM2) can occur in sporadic cases with no family history

Comparison of Major Congenital Myopathy Subtypes

Respiratory Management

Respiratory insufficiency is a leading cause of morbidity and mortality across all congenital myopathy subtypes and requires proactive, systematic monitoring. Key principles include:

• Screening: All patients with congenital myopathies should undergo baseline pulmonary function testing (forced vital capacity [FVC], maximal inspiratory and expiratory pressures [MIP, MEP]) and overnight pulse oximetry or polysomnography. In nonambulatory patients or those with prominent axial weakness (especially SELENON-related disease), screening for sleep-disordered breathing should begin early
• Monitoring frequency: Pulmonary function tests every 6–12 months; more frequently during periods of growth or clinical decline. Morning headaches, excessive daytime somnolence, and orthopnea are red flags for nocturnal hypoventilation
• Noninvasive ventilation (NIV): Bilevel positive airway pressure (BiPAP) in spontaneous/timed mode is the mainstay. Indications include FVC <50% predicted, nocturnal desaturation, hypercapnia (pCO₂ >45 mmHg), or symptomatic sleep-disordered breathing. NIV is initiated nocturnally and extended to daytime as needed
• Cough assistance: Mechanical insufflation-exsufflation ("cough assist") for patients with peak cough flow <270 L/min; critical during respiratory infections
• Tracheostomy: Required in severe cases (most XLMTM patients and severe neonatal NM) when NIV is insufficient for adequate ventilation
• Vaccinations: Influenza, pneumococcal, and RSV (palivizumab in eligible infants) vaccines are recommended to reduce respiratory infection burden

Multidisciplinary Management

No disease-modifying therapies are currently approved for any congenital myopathy subtype. Management is supportive and requires a multidisciplinary team approach:

• Neurology: Diagnosis, genetic counseling, longitudinal monitoring of motor function, coordination of multidisciplinary care
• Pulmonology: Respiratory monitoring, NIV titration, cough assistance, management of acute respiratory illness
• Orthopedics: Scoliosis surveillance (especially SELENON-related disease, which often requires spinal fusion); contracture management; hip surveillance for subluxation/dislocation
• Physical and occupational therapy: Submaximal exercise programs to maintain function without overexertion; assistive devices; mobility aids; stretching to prevent contractures
• Nutrition and gastroenterology: Feeding support for bulbar weakness; nasogastric or gastrostomy tube for severe dysphagia; monitoring for failure to thrive and aspiration
• Cardiology: Periodic echocardiography and ECG, particularly for TTN, ACTA1, and MYH7-related myopathies
• Anesthesia: Malignant hyperthermia precautions for all RYR1-related myopathies; avoidance of volatile anesthetics and succinylcholine; preoperative respiratory optimization

Distinction from Congenital Muscular Dystrophies

Congenital muscular dystrophies (CMDs) and congenital myopathies share neonatal-onset weakness and hypotonia but differ in pathology, progression, and associated features. The key distinctions are:

Emerging Therapies

While no approved treatments exist for congenital myopathies, the therapeutic pipeline is expanding:

• Gene replacement therapy for XLMTM: The ASPIRO trial (phase 1/2) evaluated resamirigene bilparvovec (AT132), an AAV8-delivered MTM1 gene replacement therapy, in boys with XLMTM requiring ventilator support. Treated patients showed significant improvements in respiratory function (with >50% achieving ventilator independence) and motor milestones. However, fatal hepatotoxicity (cholestatic liver failure) occurred in 4 participants, leading to a partial clinical hold. The trial continues at lower doses with enhanced hepatic monitoring, and data continue to be analyzed for the risk-benefit profile
• Tamoxifen for CNM: Preclinical studies have shown that tamoxifen can improve muscle function in animal models of centronuclear myopathy by modulating dynamin 2 levels; early-phase clinical investigation is underway
• Dynamin 2 reduction strategies: Antisense oligonucleotides targeting DNM2 have shown promise in animal models of both XLMTM and DNM2-related CNM, as dynamin 2 overactivity contributes to pathology in multiple CNM subtypes
• Pyridostigmine for NMJ dysfunction: Subclinical neuromuscular junction dysfunction has been demonstrated in some RYR1, MTM1, and TPM2/TPM3-related myopathies; pyridostigmine may provide modest symptomatic benefit in selected patients with documented jitter or decrement
• RYR1-targeted therapies: N-acetylcysteine (antioxidant) and dantrolene (RYR1 antagonist) are being explored for RYR1-related myopathies in clinical and preclinical studies. A phase 2 trial of N-acetylcysteine showed trends toward improved oxidative stress markers in RYR1 myopathy
• Broader gene therapy approaches: CRISPR-based gene editing and dual-AAV vector strategies are in preclinical development for NEB-related nemaline myopathy and other subtypes, though the large size of NEB (363 exons) poses significant delivery challenges

Genetic Counseling

Genetic counseling is a critical component of managing congenital myopathies and should be offered to all families with a confirmed or suspected genetic diagnosis. Key considerations include:

• Inheritance patterns: AD (central core disease, some ACTA1-NM, DNM2-CNM); AR (most NEB-NM, SELENON-MmD, BIN1-CNM); X-linked (MTM1-XLMTM); counseling should clarify recurrence risk, carrier testing for parents and siblings, and implications for reproductive planning
• De novo mutations: Common in ACTA1 and DNM2; the absence of affected family members does not exclude a dominant genetic condition, but the recurrence risk for future siblings is low (typically <1% unless germline mosaicism is present)
• Carrier testing: In AR conditions, both parents are typically asymptomatic carriers with a 25% recurrence risk per pregnancy. In XLMTM, the mother is the carrier (often asymptomatic, though some female carriers develop mild myopathy due to skewed X-inactivation) with a 50% risk for each son
• Prenatal and preimplantation diagnosis: Available when the causative mutation is known; chorionic villus sampling or amniocentesis for prenatal diagnosis; preimplantation genetic testing with in vitro fertilization is an option for families wishing to avoid affected pregnancies
• Family screening: At-risk relatives of patients with RYR1 mutations should be offered genetic testing and counseled regarding MH susceptibility, even if asymptomatic for myopathy

Prognosis

Prognosis varies widely depending on the genetic subtype, severity of respiratory involvement, and specific mutation. In general:

• Favorable: Central core disease (mild, stable course; normal lifespan with MH avoidance); CFTD (>90% stable or improving); AD-DNM2 centronuclear myopathy (slowly progressive but compatible with prolonged survival); typical congenital nemaline myopathy (most achieve ambulation)
• Intermediate: Multiminicore disease (SELENON-related: progressive scoliosis and respiratory decline requiring intervention but often compatible with long survival); intermediate nemaline myopathy; BIN1-related CNM
• Poor: Severe neonatal nemaline myopathy (often fatal in infancy); X-linked myotubular myopathy (high early mortality, though survivors may have prolonged ventilator-dependent life); Amish nemaline myopathy (fatal by age 2)

Long-term outcomes are heavily influenced by the quality of respiratory care and multidisciplinary management. Even in more severe subtypes, proactive respiratory support and nutritional optimization can significantly improve survival and quality of life.

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