Botulinum toxin A for spinal muscular atrophy

Last updated Feb. 20, 2025, by Andrea Lobo, PhD
Fact-checked by José Lopes, PhD

What is botulinum toxin A for SMA?

Botulinum toxin A products are approved in the U.S. to treat spasticity — when muscles are abnormally stiff — and for multiple other indications.

In people with spinal muscular atrophy (SMA), they may be used to help manage spasticity and excessive drooling (sialorrhea) caused by jaw spasms and weakness in mouth and throat muscles that cause difficulty swallowing.

Allergan, now part of AbbVie, was the first company to market Botox (onabotulinumtoxinA) for eye muscle issues such as strabismus (eye misalignment). Approved Botox indications now also include overactive and dysfunctional bladder, urinary incontinence, headaches, spasticity, cervical dystonia (when neck muscles contract involuntarily), and sweating of the armpits.

Other botulinum toxin products available in the U.S. include Xeomin (incobotulinumtoxinA), which is approved to treat excessive drooling, spasticity, cervical dystonia, and blepharospasm (uncontrollable eyelid movements), and Dysport (abobotulinumtoxinA) for the treatment of spasticity and cervical dystonia.

For both spasticity and excessive drooling, botulinum toxin A products are approved for people ages 2 and older.

Therapy snapshot

How does botulinum toxin A work?

SMA is a rare disease mainly caused by mutations in the SMN1 gene, which provides instructions to produce the survival motor neuron protein (SMN). The loss of this protein leads to the dysfunction and loss of motor neurons, the nerve cells that control voluntary movement, resulting in muscle weakness and wasting. This can also lead to painful muscle spasms and spasticity, and gradually affect muscles needed to chew and swallow.

Obtained from a bacterium called Clostridium botulinum, this type of therapy works by preventing the release of a chemical messenger called acetylcholine into the connections between nerves and muscle cells. Acetylcholine normally triggers muscle contraction and sweat secretion.

These medications act locally and may be used to help muscles relax and to reduce the production of saliva.

How is botulinum toxin A administered?

Botulinum toxin is an injectable therapy, available as a dry powder, in vials containing 50 to 500 units (U), varying according to specific therapy. The powder is dissolved in saline and given as an injection at the target site.

For excessive drooling, for which Xeomin is indicated, the toxin is injected into the parotid salivary glands — located in front of and below the ears — and the submandibular glands, those located below the jaw.

In adults, the recommended dose per treatment session is 100 U, divided by the parotid and submandibular glands. In children, the treatment is approved for those weighing 12 kg (26.4 lbs) or more, and should be given at doses per gland ranging from 6 to 22.5 U, for up to 75 U in total.

For the treatment of spasticity, the dose depends on the toxin product, whether it is used to treat upper or lower limb spasticity, and whether it is used in adults or children. Generally, factors such as which muscles are affected, how severe spasticity is, and a patient’s treatment and adverse reaction history with botulinum toxins should guide dose selection.

Common side effects of botulinum toxin A

Side effects associated with the use of botulinum toxin A products, not in SMA specifically, depend on the indication the treatment is used for.

For the treatment of excessive drooling in adults, the most common side effects include:

• tooth extraction
• dry mouth
• diarrhea
• high blood pressure.

In children, the most common side effects include:

• headache
• bronchitis (inflammation of the lining of the bronchial tubes)
• nausea/vomiting.

In children ages 2 to 5 years treated for chronic excessive drooling, the common cold was the most frequent side effect in a clinical trial.

When used for the treatment of spasticity, the most commonly observed events in adults include:

• common cold
• upper respiratory tract infection
• muscle weakness
• pain in the extremities
• seizures
• falls
• dry mouth.

In children treated for spasticity, the most common side effects include:

• common cold
• bronchitis
• sore throat
• upper respiratory tract infection
• fever
• cough.

Spread of toxin effect

The prescribing information of all botulinum toxin A products carries a boxed warning that notes it may spread from the area of injection, producing effects consistent with the action of the toxin. Symptoms may occur hours to weeks after administration and may include lack of energy, muscle weakness, abnormal voice, slurred speech, and swallowing and breathing difficulties, which can be life-threatening.

The risk may be higher for children with spasticity, although it can also occur in adults, particularly those with preexisting conditions that increase risk. These symptoms may occur in patients receiving doses comparable to or lower than doses approved to treat spasticity. Therefore, patients and caregivers should immediately seek medical care if they experience swallowing, speech, or breathing difficulties.

Increased risks of clinically significant effects

People with amyotrophic lateral sclerosis, myasthenia gravis, or other neuromuscular disorders may be at increased risk of clinically significant side effects. These patients should be closely monitored when given botulinum toxin products.

Lack of interchangeability between botulinum toxin products

The potency units are specific for each type of botulinum toxin product. As a result, they cannot be compared to or converted into units of other toxin products.

Allergic reactions

Serious allergic reactions have been reported in patients treated with botulinum toxin products. These reactions may include breathing difficulties, swelling, skin rash, and anaphylaxis (a life-threatening allergic reaction that stops breathing and makes blood pressure drop suddenly). If such a reaction occurs, treatment should be stopped and appropriate medical therapy immediately given.

Swallowing and breathing problems

Treatment with botulinum toxin A products may result in swallowing and respiratory difficulties, particularly in patients with preexisting conditions that affect swallowing or breathing. In most cases, these effects occur when the injection area is close to weakened muscles involved in swallowing or breathing.

Patients treated with botulinum toxin products may require immediate medical attention should they experience problems with swallowing, speech, or breathing.

Use in pregnancy and breastfeeding

There are no data regarding fetal risk when using botulinum toxin A products in pregnant women. However, in animal studies, the administration of these products led to reduced fetal weight and bone formation, early embryonic death, and stillbirth at clinically relevant doses, associated with maternal toxicity.

These treatments should only be used during pregnancy if the potential benefit justifies the potential risk to the fetus.

There are also no data showing the presence of the toxin in human milk, or its effects on milk production and breastfed infants. The health benefits of breastfeeding should be considered along with the mother’s need for treatment and any potential adverse effects on the infant.

SMA News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.