Spinal and bulbar muscular atrophy (SBMA), also known as Kennedy’s disease, is a rare genetic disorder. It affects certain nerve cells in the spinal cord and the brainstem. “Bulbar” refers to a bulb-shaped region in the lower part of the brain that controls voluntary muscle movement.

Over time, SBMA makes it hard for you to control voluntary movements, affecting mobility, speech, and swallowing. The areas most affected by SBMA are the arms and legs, as well as facial and throat muscles.

SBMA occurs as the result of a defect on the X chromosome. The onset is usually after age 30. The disease progresses slowly, and there’s currently no cure.

Medications can help with pain and muscle spasms. Various types of physical therapy can help you manage your symptoms. People with SBMA can have an average lifespan, but usually require mobility aids and long-term care.

Read on to find out more about why this disease occurs, what symptoms look like, and what we know so far about treating it.

Most people who develop SBMA are assigned male at birth, or have XY chromosomes. Usually they experience onset of the disease between the ages of 30 and 50. However, SBMA can also show up in the teen years and later in life.

Early symptoms usually include falling down, and struggling to walk or control muscles.

Other hallmark symptoms include:

As SBMA progresses, muscles in the hands and face (including the tongue) may twitch. Eventually, the nerves that control certain muscles start to die, leaving the muscles unable to contract. This causes numbness and weakness.

In some cases, one side of the body is more affected than the other.

Sex vs. gender in research articles

A lot of research into SBMA refers to participants as “female” or “male” instead of as women or men. This is because most of these studies focus on chromosomes, one component of birth sex, and a key factor in understanding SBMA.

“Female” and “male” refer to biological sex categories. Each is associated with specific genitalia, chromosomes, primary and secondary sex characteristics. Intersex people have characteristics of more than one sex.

“Women” and “men” refer to society’s dominant gender categories, although there are many others. It is traditionally assumed people assigned female at birth are women, and people assigned male at birth are men. This is true most of the time, but not always.

Biological sex is not the same as gender, nor does it determine someone’s gender. We use the phrase “assigned male/female at birth” to make space for this reality, while also acknowledging that certain biological characteristics can inform hereditary conditions.

SBMA is an inherited condition, caused by an X chromosome defect.

The mutation in a section of DNA is called a CAG trinucleotide repeat. Our DNA is made up of building blocks called “nucleotides.” The CAG trinucleotide repeat happens when too many cytosine, adenine, and guanine nucleotides occur in a row.

The affected DNA gene codes for the androgen receptor (AR), a protein involved in the processing of male hormones. When the gene has extra nucleotides in the form of a CAG trinucleotide repeat, the androgen receptors it codes for are defective. They can’t efficiently transport androgens. This results in some motor neurons being unable to work.

The role of sex in SBMA

The reason SBMA almost exclusively affects people assigned male at birth (often those with XY chromosomes) is that androgen function is involved in the disease. The androgen testosterone is the dominant male hormone.

In rare cases of people assigned female at birth (often those with XX chromosomes) who have SBMA, symptoms are usually mild. Many don’t have high enough testosterone levels to activate SBMA’s androgen receptor mutation.

People assigned female at birth can be asymptomatic carriers of the disease, and have a 50 percent chance of passing it to their offspring. If a male child (XY chromosomes) gets the gene, they would develop active SBMA. If a female child (XX chromosomes) gets the gene, they would be a carrier.

SBMA research limitations

There isn’t much SBMA research incorporating people with trans, nonbinary, or other gender identities as participants.

However, some interesting findings include:

  • One 2016 study of SBMA in a trans woman found that longterm estrogen therapy (which lowers testosterone and other androgen levels) wasn’t successful in preventing the disease.
  • A 2018 study explains that people assigned female at birth, or those with XX chromosomes, may manifest the disease if carrying the gene and exposed to high testosterone levels.

When diagnosing SBMA, doctors will usually start by doing the following:

  • consider your symptoms
  • conduct a physical examination
  • ask about your medical history
  • ask about your family’s medical history

Doctors may also order various lab tests. This includes a blood test checking for levels of the enzyme creatine kinase, which is released by atrophied muscles into the bloodstream.

The primary tool to diagnose the neuromuscular disorder is molecular genetic testing. This is a special blood test that looks for CAG trinucleotide repeat expansion in the AR gene.

According to the National Organization for Rare Disorders, SBMA is diagnosed in people with more than 36 CAG trinucleotide repeats in the AR gene.

SBMA shares some of the same major symptoms as other neuromuscular disorders. These include:

While early symptoms make look alike for these conditions, there are key differences between them.

Difference with ALS

One of the main differences between SBMA and ALS is the speed of progression:

Unlike SBMA, which is always genetic, only a minority of people with ALS get the disease through inheritance. The majority of cases are sporadic. ALS can affect people no matter their sex or gender.

In diagnostic testing for SBMA and ALS, a muscle biopsy or electromyography (EMG) is used to distinguish between the conditions.

Difference with ALD

ALD progresses slowly, like SBMA, but it tends to develop at a younger age than SBMA.

Additionally, ALD often leads to cognitive decline. SBMA rarely causes any impaired thinking or memory loss.

Difference with myasthenia gravis

Myasthenia gravis also affects the voluntary muscles of the face and limbs, but it’s an autoimmune disease in addition to a neuromuscular one. The body’s immune system mistakenly attacks healthy neurons, disrupting communication between nerves and muscles.

Myasthenia gravis tends to affect the eye muscles in particular. Symptoms can fluctuate in severity within a single day. It most commonly affects people assigned female at birth under age 40, and those assigned male at birth older than 60.

There’s currently no cure for spinal and bulbar muscular atrophy, and no medications have been proven to slow or reverse disease progression. However, treatments exist to manage symptoms and reduce their severity.

It’s important to understand that SBMA is a progressive disease. That is, it will continue to worsen over time. Most people with SBMA will eventually need live-in caregivers.

Medications and ongoing research

Here’s some important findings on SBMA management:

  • Studies of anti-androgen drugs to block the impact of the AR gene have produced mixed results. There’s not enough research yet to support using these drugs to treat the disease.
  • According to the Muscular Dystrophy Association, a new class of drugs, called selective androgen receptor modulators (SARMs), is being explored. Experts hope these drugs may one day be able to prevent or lessen the severity of SBMA.
  • Testosterone treatment, commonly used to treat gynecomastia and erectile dysfunction (two SBMA symptoms), can actually worsen disease. For those with SBMA experiencing gynecomastia, breast reduction surgery is another option.
  • A small 2013 study of the drug clenbuterol appeared to improve stamina levels in people with SBMA. Clenbuterol has steroid-like properties and is used primarily to help people with breathing difficulties, such as asthma.

Medications to treat tremors and muscle cramps are often prescribed to treat symptoms. These may lessen pain and discomfort, but don’t affect the course of SBMA.

Physical therapy and lifestyle adjustments

In many cases, certain adjustments in your daily life can make managing SBMA a little easier.

Because swallowing can become more difficult, cutting food into smaller pieces can help prevent choking. Consulting with a dysphagia specialist or speech-language pathologist (SLP) can help you practice safe and effective ways to swallow.

Swallowing problems are often factors in early mortality among people with SBMA, due to complications such as pneumonia or asphyxiation. If breathing becomes difficult, respiratory therapy may be helpful. In addition, ventilation aids, such as a bi-level positive air pressure device, can make breathing a little easier.

Because the leg muscles can become weak over time, a cane or walker may be necessary to walk safely and avoid fall-related injuries. Eventually a mobility aid (such as a scooter or wheelchair) may be essential. Physical therapy may help prolong the time before a person with SBMA is required to use a walker or wheelchair.

Spinal and bulbar muscular atrophy is a rare genetic disorder that affects certain nerves that control voluntary movement. Caused by an X chromosome defect, it almost always affects people assigned male at birth (those with XY chromosomes). The disease’s genetic cause can be identified in blood tests.

Over time, SBMA can make swallowing and speaking difficult, as well as affect mobility and muscle control. Eventually, people with SBMA will require long-term care to help them with daily activities, including eating and getting dressed.

Medications can treat pain or muscle spasms, but won’t slow the disease. Physical therapy may prolong mobility, assist in safe swallowing, and help those with SBMA hold onto speech capabilities longer.

Unlike ALS — which causes similar symptoms — SBMA progresses slowly and often allows for an average lifespan.

Talk with your doctor about risk factors for you and your family, and any symptoms you may be experiencing.