What is the Difference Between ALS and SMA

The key difference between ALS and SMA is that ALS is a motor neuron disease that appears sporadic most of the time and is rarely inherited, while SMA is a motor neuron disease that is always inherited.

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are two different types of motor neuron diseases. Both these diseases are characterized by a loss of somatic motor neurons and innervations to voluntary skeletal muscles, leading to death by failures of respiratory function. ALS is mainly sporadic and can also be inherited, whereas SMA is always an inherited motor neuron disease. In addition, ALS is mainly observed in adults, while SMA is mainly observed in children and rarely in adults. Moreover, there is no proper cure for both these diseases at the moment.

CONTENTS

1. Overview and Key Difference
2. What is ALS 
3. What is SMA
4. Similarities – ALS and SMA
5. ALS vs SMA in Tabular Form
6. Summary – ALS vs SMA

What is ALS?

ALS is a rare neurological disease that mainly affects nerves that are responsible for controlling voluntary muscle movement. Voluntary muscles are the muscles that people use to move. ALS is the most common form of motor neuron disease. It has a lifetime risk of 1: 300. ALS normally has adult onset and is very aggressive. Most patients die 2 to three years of diagnosis. ALS appears sporadic in 90% of cases. But it can be inherited in 10% of cases.

When it is inherited, multiple mutations in different genes are causative agents, such as mutations in superoxide dismutase 1 (SOD1) gene, TARDBP (TDP43) gene, Fused in sarcoma (FUS) gene and Chromosome 9 open reading frame 72 (C9ORF72) gene. It is also inherited in an autosomal dominant inheritance pattern. The signs and symptoms of ALS may include difficulty walking, tripping and falling, weakness often in legs, feet, or ankles, clumsiness, slurred speech, trouble swallowing, muscle cramps, twitching in arms, shoulders, and tongue, crying, laughing, and yawning inappropriately and changes in the cognitive function and behaviour.

Amyotrophic lateral sclerosis can be diagnosed through an electromyogram (EMG), nerve conduction study, MRI, blood and urine study, spinal tap (lumbar puncture), and muscle biopsy. Furthermore, treatment options for amyotrophic lateral sclerosis may include medications like riluzole, edaravone, sodium phenylbutyrate, taurursodiol, and therapies such as breathing care, physical therapy, occupational therapy, speech therapy, nutritional therapy, psychological and social support.

What is SMA?

SMA is a hereditary motor neuron disorder that destroys the nerves in the brain stem and spinal cord, which are essential to control skeletal muscle activities such as speaking, breathing, and swallowing. This ultimately leads to muscle weakness and atrophy. SMA typically shows autosomal recessive inheritance patterns. The onset of this disease occurs before 6 months, and lethality by 2 years of age.

SMA is also the most common genetic cause of infant mortality, with an annual incidence of 1: 6000 to 1: 10000 live births. Moreover, spinal muscular atrophy is caused due to mutations in the survival motor neuron 1 (SMN1) gene, which leads to low levels of SMN protein. There are four types of SMA called SMA1 (severe form), SMA2 (intermediate form), SMA2 (mild form), and SMA4 (adult form). The symptoms of SMA may include a floppy or weak arms and legs, movement problems, twitching or shaking muscles, problems in bones and joints, swallowing problems, and breathing difficulties.

Spinal muscular atrophy can be diagnosed through blood tests, genetic tests, nerve conduction tests, and muscle biopsy. Furthermore, treatment options for spinal muscular atrophy may include disease-modifying therapy (giving the drugs stimulating SMN protein production such as Nusinersen) and gene replacement therapy (intravenous infusion of a drug called onasemnogene abeparvovec-xioi that replaces missing or faulty SMN1 gene).

What are the Similarities Between ALS and SMA?

• ALS and SMA are two motor neuron diseases.
• Both diseases primarily affect voluntary muscle movement.
• They are rare diseases.
• Both diseases can be inherited.
• They can cause severe lethality.
• Both diseases can be identified in adults.
• Both diseases can have similar symptoms, such as movement problems, swallowing problems, and breathing difficulties.
• They can be diagnosed through physical examination, blood tests, and nerve conduction.
• They are mainly treated through specific medicines.

What is the Difference Between ALS and SMA?

ALS is a motor neuron disease that appears sporadic most of the time and is inherited rarely, while SMA is a motor neuron disease that is always inherited. Thus, this is the key difference between ALS and SMA. Furthermore, ALS shows an autosomal dominant inheritance pattern when inherited, while SMA shows an autosomal recessive inheritance pattern when inherited.

The below infographic presents the differences between ALS and SMA in tabular form for side-by-side comparison.

Summary – ALS vs SMA

Motor neuron diseases are rare conditions that damage parts of the nervous system progressively. ALS and SMA are the two most common motor neuron disorders. ALS is a motor neuron disease that appears sporadic most of the time and is rarely inherited. It is autosomal dominant. SMA is a motor neuron disease that is always inherited. It is autosomal recessive. Both can lead to death by respiratory failure. So, this is the key difference between ALS and SMA.

Reference:

1. “Amyotrophic Lateral Sclerosis (ALS).” Mayo Clinic, Mayo Foundation for Medical Education and Research.
2. “Overview: Spinal muscular atrophy” NHS Choices, NHS.

Image Courtesy:

1. “ALS Disease Pathology and Proposed Disease Mechanisms” By Philip Van Damme, Wim Robberecht, and Ludo Van Den Bosch -“Modelling amyotrophic lateral sclerosis: progress and possibilities” Disease Models and Mechanisms. 10 (5): 537-549. doi:10.1242/dmm.029058 PMC: 5451175 PMID: 28468939 (CC BY 3.0) via Commons Wikimedia