Spinal muscular atrophy (SMA)a genetic disease, is characterized by the loss of motor neurons, or nerve cells that control the movement of voluntary muscles.

There are several types of SMA, which differ by the age of onset and severity of symptoms. SMA types 0 to 4 are caused by mutations in the SMN1 gene. This gene provides instructions to build a protein that is essential for the survival of motor neurons.

What is SMARD1?

SMA with respiratory distress type 1 (SMARD1) is caused by mutations in a different gene, namely IGHMBP2, which provides instructions for making a protein that is involved in DNA replication. (Before a cell divides to form new cells, DNA has to be replicated.)

SMARD1 is inherited in an autosomal recessive pattern. This means a child will develop this disease only if both copies of the gene, one inherited from the mother and the other from the father, are mutated.

SMARD1 is also sometimes referred to as severe infantile axonal neuropathy with respiratory failure and distal SMA type 1, among other names. 

Symptoms

SMARD1 is characterized by muscle weakness and respiratory failure, usually beginning between six weeks and six months of life. The first noticeable signs of SMARD1 are often breathing difficulties.

Breathing problems — respiratory distress — are caused by the paralysis of the diaphragm, the thin sheet of muscles that separates the chest cavity from the abdomen and that is essential for breathing.  These children have difficulties with breathing, especially when inhaling, their breathing is usually noisy, and their cries are weak. They typically, and suddenly, progress to respiratory failure, meaning they are not able to breathe without breathing support.

Muscle weakness usually becomes apparent shortly after respiratory failure. The weakness starts in distal muscles, or those that are furthest from the center of the body, such as the muscles of hands and feet, and continues to spread to all other muscles. But this steady decline in muscle strength stops typically within two years and stabilizes.

Muscular weakness causes problems with swallowing, crawling, sitting, standing, and walking. Some children develop an abnormal side-to-side (scoliosis) or back to front (kyphosis) curvature of the spine.

Other symptoms of SMARD1 include reduced pain sensitivity, loss of bladder and bowel control, excessive sweating, and an irregular heartbeat (arrhythmia).

Diagnosis

Because SMARD1 is a very rare condition, it is difficult to diagnose, and a diagnosis can take a long time. A definite diagnosis is only possible with genetic testing to identify mutations in the IGHMBP2 gene. More than 60 different mutations on this gene are linked to SMARD1.

Treatment

Currently, there is no cure for SMARD1. Disease management focusses on reducing discomfort and maintaining quality of life.

Respiratory failure can be managed with breathing aids.

Occupational therapists and physiotherapists can help families and caregivers with choices in adaptations and equipment that may facilitate movement.

Outlook

The life expectancy of children with SMARD1 is affected, but it is difficult to predict because the progression of the disease differs among patients. Appropriate breathing support is likely to improve life expectancy.

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