Spinal Muscular Atrophy: the second genetic cause of infant death

• Spinal muscular atrophy is a rare and serious genetic neuromuscular disease and the second genetic cause of infant death; early diagnosis is key to treatment.

Santo Domingo, November: Spinal muscular atrophy (SMA) is a neuromuscular disease of genetic origin. It is considered a rare and serious disease, mainly characterised by a progressive loss of muscle strength and deterioration of lung function due to the degeneration of motor neurons in the spinal cord.

The incidence of SMA is 1 in every 6,000–10,000 live births; 1 in 40 people are carriers, and among Hispanics the frequency is 1:77.

This disease is the second most common fatal autosomal recessive disease, meaning both parents must carry the gene responsible for it. There are different types of SMA depending on the age at which symptoms start and the gross motor milestones reached. In type I, children can hold up their heads; in type II, they can sit; in type 3, they can walk independently but may later lose this ability; and type 4 begins in adulthood.

In the Dominican Republic, the vast majority of patients have SMA type 2.

Early detection and treatment

Early, timely detection matters because of the treatment options available: the earlier the diagnosis is made and therapies begin, the better the outcomes for patients’ quality of life. Innovative drugs have been developed that change the natural course of the disease. 

According to Dr. María Gómez Peña, a paediatric neurologist, each child has different symptoms, but there are early signs to watch for, for example:

• Children with suspected early-onset SMA (before 6 months) often have difficulty holding up their heads, appear floppy (hypotonia) and weak, and show a characteristic sign: continuous trembling movement of the tongue (tongue fasciculations). This history and clinical evidence call for genetic testing.

• Children aged 6 to 18 months manage to sit but do not walk, and as the disease progresses they lose the ability to sit on their own for no apparent reason, together with a fine tremor that parents describe as nervousness and trembling of the tongue. 

• Young children who are already walking and begin to have difficulty, such as frequent tripping, sudden falls, trouble getting up from the floor and climbing stairs, or holding and carrying objects easily, may have SMA.

• In adolescence, changes in the spine can be seen and scoliosis often develops. 

These signs are very important in raising clinical suspicion of the disease, especially in children with no history of problems during pregnancy or birth.

In the Dominican Republic, the most common reason for consultation is delay in reaching age-appropriate motor milestones, followed by distal hand tremor and loss of motor milestones, according to Dr. Gómez Peña.

To determine the type of SMA, the age at which symptoms started and the motor function reached so far are taken into account.

SMA type I: known as Werdnig–Hoffmann disease, this is the most common and severe type. It begins in the first months of life. Patients can hold up their heads but cannot sit without support; without specific treatment they do not survive beyond 2 years.

SMA type II: symptoms appear between 6 and 18 months of age. Children can sit without support and some can stand without support, but they cannot walk independently. They have muscle weakness and may develop scoliosis. They reach adulthood.

SMA type III: known as Kugelberg–Welander disease. It is less severe and starts after 18 months of age. Patients can walk but have frequent falls, weakness and loss of the ability to walk. They reach adulthood.

SMA type IV: begins in adulthood and has a milder course; patients achieve and keep independent walking, though some lose it in their third or fourth decade of life.

Currently, a blood test is recommended to diagnose the disease, to determine whether the child lacks the SMN1 gene and how many copies of SMN2 they have. Motor neurons need a protein called SMN (Survival Motor Neuron) to work properly. When both parents pass on a mutation or deletion of the SMN1 gene to their child, the child’s body cannot make this protein, and the motor milestones reached will depend on the number of copies of the SMN2 gene.

“Parents play a crucial role in the early detection of SMA, as they can contribute significantly to early diagnosis by closely observing their children’s development and any changes in their health. Familiarity among health staff and therapists with the initial signs, such as muscle weakness, hypotonia, distal tremor and fasciculations, can facilitate early intervention, increasing the chances of comprehensive management and improving treatment outcomes. Open communication between parents and health professionals is needed to ensure children’s well-being,” added the specialist.

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