Spinal muscular atrophy and Duchenne muscular dystrophy
This morning, I had a conversation with a mother of a 9-year-old who has a differential diagnosis of a neuromuscular disorder. She was seeking clarification on the differences between spinal muscular atrophy and Duchenne muscular dystrophy. Both Spinal muscular atrophy (SMA) and Duchenne muscular dystrophy (DMD) are genetic disorders that primarily affect muscle strength and function, but there are significant differences between these conditions.
During our conversation, I noticed that the mother was using the term 'Spinal Muscular Dystrophy' instead of 'Spinal Muscular Atrophy.' I took the opportunity to educate her that SMA is not traditionally classified as a dystrophy. Atrophy and Dystrophy are medical terms that describe muscle conditions, but the causes and implications of these terms are different. The main difference between the two is the cause of muscle weakness and wasting. Atrophy is caused by a lack of use or stimulation, while dystrophy is caused by a genetic mutation. SMA is caused by a loss of motor neurons in the spinal cord and brainstem, which leads to a communication breakdown between the motor neurons and muscles and results in muscle weakness and wasting. DMD, on the other hand, is caused by genetic mutations that affect the muscle fibers themselves, leading to their degeneration and weakness. To add spice to the discussion of dystrophy, atrophy often mingles with dystrophy. I tried to educate the mother on these differences; however, I realized that not everyone needs to be familiar with these subtleties in medical terminology. It is more important for lay people to focus on the more important aspects of the conditions and leave the complicated terminology to those in the medical field. So, I decided to ignore her using 'spinal muscular dystrophy' instead of 'spinal muscular atrophy.'
Phenotypically, the primary difference between SMA and DMD lies in the location of muscle weakness. SMA primarily affects the muscles in the spinal cord and causes weakness and wasting in the limbs, neck, and torso. DMD, in contrast, primarily affects the muscles of the hips, pelvis, legs, and torso and leads to a more rapid progression of muscle weakness and loss.
Another key difference between SMA and DMD is the age of onset. SMA primarily affects the muscles in the spinal cord, causing weakness and wasting in the limbs, neck, and torso. DMD, on the other hand, primarily affects the muscles of the hips, pelvis, legs, and torso, leading to a more rapid progression of muscle weakness and loss. Another key difference between SMA and DMD is the age of onset. SMA typically begins in infancy or early childhood, while DMD usually begins in early childhood and progresses rapidly. The earlier onset of DMD means that children with this condition may experience more severe symptoms and complications at a younger age.
The genetic cause of SMA and DMD also differs. SMA is caused by a genetic defect in the survival motor neuron (SMN) gene, which produces a protein that helps maintain and repair nerve cells. Mutations in the SMN gene can lead to 4 different types of SMA and other diseases, such as Charcot-Marie-Tooth disease and hereditary spastic paraplegia. DMD is caused by a defect in the dystrophin (DMD) gene, which produces a protein that is critical for maintaining muscle health. Mutations in the dystrophin gene can also lead to phenotypes other than DMD, including Becker muscular dystrophy, a less severe form of muscular dystrophy.
In terms of progression, SMA can range from mild to severe and can be progressive, while DMD is a rapidly progressive disease. The life expectancy of individuals with SMA and DMD also differs. SMA can range from a normal lifespan to a shortened lifespan depending on the severity, while DMD is generally fatal in early adulthood and individuals usually live into their 20s or 30s, though there are exceptional cases, e.g., Tom Sulfaro of Michigan (51 this year), where patients have outlived all life expectancy predictions. The mother seemed to be very upset when I told her about the life expectancy of the people with these diseases, but when I told her about Tol Sulfaro, she seemed like she found a ray of hope!
SMA and DMD are two distinct medical conditions that primarily affect muscle strength and function but differ in terms of the location and extent of muscle weakness, age of onset, genetic cause, progression, and life expectancy. Understanding these differences is crucial for individuals and families affected by these conditions to provide the best possible care and support.
27 July 2022