Congenital Myopathies

Genetic Causes of Weakness in Newborns

Newborn baby boy crying
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Neurological diseases are especially heart-wrenching when they impact children.  This often results from a genetic disorder that can impact the child from birth.  Sometimes such mutations lead to problems with muscles, which leads to weakness. 

It may be possible to detect such problems even before birth.  Fetal movements may be decreased or absent if the child’s muscles are not developing normally.

  After the child is born, parents may notice diminished ability to feed.  Sometimes the signs are even more dramatic, as the newborn may be too weak to breathe and may have poor muscle tone (the baby may appear to be “floppy”).  In general, the child is able to move his or her eyes normally. 

Central Core Disease

Central core disease is so called because when muscles are viewed under the microscope, there are clearly defined regions in that lack normal cell components such as mitochondria or sarcoplasmic reticulum. 

The disease results from a mutation in the ryanodine receptor gene (RYR1).  Although the disease is inherited in an autosomal dominant fashion, which normally would mean that symptoms would also be present in a parent, the gene is not normally fully expressed—even if someone has the active form of the mutation, sometimes their symptoms are mild. 

Central core disease can also impact children later in life, leading to delayed motor development.

  For example, the child may not walk until they are age 3 or 4. Sometimes the onset may come on even later, in adulthood, though , in this case ,the symptoms are usually milder.  Sometimes they only become apparent after receiving an anesthetic medication, which causes a severe reaction in people with this disorder.

 

Multicore Disease

Multicore disease also causes decreased muscle tone, and usually becomes symptomatic when someone is in infancy or early childhood.  Motor milestones such as walking may be delayed, and when the child does walk they may appear to be waddling, and may fall frequently.  Sometimes the weakness may impede the child’s ability to breathe normally, especially at night.  The disorder gets its name from having small regions of decreased mitochondria within muscle fibers.  Unlike central core disease, these regions do not extend the entire length of the fiber.

Nemaline Myopathy

Nemaline myopathy is usually inherited in an autosomal dominant fashion, though there is an autosomal recessive form that causes symptoms to come on during infancy.  Several genes have been implicated, including the nebulin gene (NEM2), the alpha-actingene (ACTA1), or the tropomyosin gene (TPM2).  

Nemaline myopathy can come on in different ways.  The most severe form is in neonates.  The delivery of the child is usually difficult, and the child will appear blue at birth due to difficulty breathing.

  Due to weakness, the child will usually not feed very well, and won’t move spontaneously very often.  Because the child may be too weak to cough and protect its lungs, serious lung infections lead to high early mortality. 

An intermediate form of nemaline myopathy leads to weakness of thigh muscles in infants.  Motor milestones are delayed, and many children with this disorder will be wheelchair-bound by the age of 10.  Unlike other myopathies, the facial muscles may be severely affected. 

There are also milder forms of nemaline myopathy that cause less severe weakness in infancy, childhood or even adulthood.  In the mildest of these forms, motor development may be normal, though some weakness comes on later in life.

The diagnosis of nemaline myopathy is usually first made by muscle biopsy.  Small rods called nemaline bodies appear in muscle fibers. 

Myotubular (Centronuclear) Myopathy

Myotubular myopathy is inherited in many different ways.  The most common is an X-linked recessive pattern, meaning that it appears most commonly in boys whose mother carried the gene but did not have symptoms.   This type of myotubular myopathy usually begins to cause problems in utero.  Fetal movements are reduced and the delivery is difficult.  The head may be enlarged.  After birth, the child’s muscle tone is low and so they appear floppy.  They may be too weak to fully open their eyes.  They may need to be helped with mechanical ventilation in order to survive. Again, there are less severe variations of this disease as well. 

Source: 

Nima Mowzoon, Disorders of Muscle, Neurology Board Review: An Illustrated Study Guide, 2007 

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