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Understanding Spinal-Bulbar Muscular Atrophy (SBMA)

Posted on February 11, 2021
Medically reviewed by
Amit M. Shelat, D.O.
Article written by
Bethany J. Sanstrum, Ph.D.

Spinal muscular atrophy (SMA) is a motor neuron disease caused by genetic mutations passed on from one’s parents. Among the many types of SMA is spinal-bulbar muscular atrophy (SBMA), also called Kennedy’s disease, bulbospinal muscular atrophy, and X-linked spinal and bulbar muscular atrophy.

A person with SBMA may first notice symptoms such as hand tremors and muscle twitching. As the condition progresses, they may develop muscle weakness in their limbs that limits mobility. They may also develop weakness in the face and tongue that causes difficulty with speech and swallowing.

How Is SBMA Different From Other Types of SMA?

SBMA differs from more common types of SMA in two notable ways: It’s diagnosed primarily in men, and symptoms don’t start showing until adulthood.

Prevalence in Men

SBMA is prevalent in men because the mutated gene responsible for the condition is recessive and located on the X chromosome. Males have only one X chromosome, which they inherit from their mothers. So, they only need to inherit one mutated copy of the gene to develop SBMA.

Females, on the other hand, have two X chromosomes and are therefore less often affected by SBMA. A healthy gene on one X chromosome may protect from a mutated gene on the other X chromosome. However, female carriers who have one copy of this mutation can experience a mild form of SBMA.

Average Age of Diagnosis

SBMA is a very slowly progressing disease that develops over a period of decades. It’s considered a late-onset form of SMA and is most often diagnosed between the ages of 30 and 50.

How Is SBMA Diagnosed?

Your doctor or health care team may perform a standard physical exam to diagnose a generalized neuromuscular disorder. A blood test for an enzyme called creatine kinase, which is found in deteriorating muscles, can then determine the presence of muscle damage. Genetic testing can help determine the specific cause of the muscle impairment and confirm a diagnosis of SBMA.

Life Expectancy and Disease Prognosis With SBMA

Most people with SBMA typically have a normal life span, as the condition begins late in life and progresses relatively slowly compared to other forms of SMA.

A person with SBMA may require a wheelchair at later stages of the disease. Further weakness of facial and tongue muscles in late disease phases can also result in more debilitating symptoms, including recurrent aspiration pneumonia, which can become life-threatening without intervention and preventative treatment.

However, with earlier diagnoses and thorough supportive care, rehabilitation of muscle weakness is possible and can help prevent or delay muscle atrophy or wasting. These factors are vital to increasing the amount of time a person can walk as the disease progresses.

Causes of SBMA

SBMA is caused by a mutated androgen receptor gene with an error called trinucleotide repeat. The gene is located on the X chromosome, which means that males only need to inherit one copy of the gene to develop SBMA.

Normally, the androgen receptor processes androgens — male hormones — to maintain healthy muscles. However, the trinucleotide repeat causes the mutated gene to create a misshapen protein that may clump together and interfere with normal cell function in the brain and spinal cord.

Symptoms of SBMA

SBMA is primarily characterized by muscle weakness, which is commonly found in other forms of SMA. The first signs of disease include:

  • Difficulty walking long distances
  • Trouble climbing stairs
  • Muscle cramps
  • Muscle twitching

The dysfunctions of the bulbar muscles (muscles of the mouth and throat) can lead to many symptoms. Specifically, symptoms of late-phase SBMA include:

  • Dysarthria (weakening of the muscles used for speaking)
  • Dysphagia (difficulty swallowing)
  • Frequent aspiration pneumonia
  • Weakened throat muscles leading to breathing impairment during sleep

Additionally, some men with SBMA experience a reduction in androgen processing (androgen insensitivity). The changes in normal hormone functioning may result in the development of enlarged breasts (gynecomastia) or reduced fertility due to testicular atrophy or shrinking.

Since women do not produce or use androgens to the same degree as men, they do not experience hormonal symptoms with SBMA. However, female carriers can develop muscle cramps or twitching later in life, resulting from carrying a mutant copy of the androgen receptor gene.

Treatments for SBMA

Currently, the bulk of the treatments for SBMA focus on supportive physical therapy or rehabilitation for muscle weakness. There currently are no cures or medications approved by the U.S. Food and Drug Administration for the condition. However, other treatment options are available for people, depending on their symptoms, that can help make SBMA more manageable.

Physical Therapy

Physical therapy, or PT, is a treatment option for improving mobility, preventing loss of muscle strength, and preserving independence. Types of PT with benefits specific to SBMA include:

  • Strength-based exercises to prevent muscle weakness (hypotonia)
  • Stretching exercises to prevent contractures (joint tightening or shortening)
  • Weight-bearing exercises to improve balance and mobility and prevent osteoporosis (bone fractures)

Read more about physical therapy for SMA.

Occupational Therapy

Occupational therapists help people with movement disorders or muscle weakness retain their physical ability to perform everyday tasks and activities, such as walking, writing, using technology, dressing, and showering. Together with physical therapy, occupational therapy can further help people with SBMA maintain their independence by introducing assistive devices, making helpful modifications to the home, and promoting self-care practices.

Read more about occupational therapy for SMA.

Breathing Support

Noninvasive breathing support can be very important for people with late-stage SBMA, during which the risk for obstructive sleep apnea or hypoventilation (weak breathing) increases. Certain types of at-home ventilators can help improve the sleep of people who need breathing assistance, thereby prolonging their strength and health without surgery.

Read more about breathing support for SMA.

Find the Support You Need

Join mySMAteam, the social network for people with SMBA and all types of spinal muscular atrophy and their loved ones. On mySMAteam, members come together to ask questions, give advice, and share their stories with others who understand life with SMA.

Are you or a loved one newly diagnosed and looking for answers? Share your experience in the comments below or start a conversation on mySMAteam.

Amit M. Shelat, D.O. is a fellow of the American Academy of Neurology and the American College of Physicians. Review provided by VeriMed Healthcare Network. Learn more about him here.
Bethany J. Sanstrum, Ph.D. holds a doctorate in cell and molecular biology with a specialization in neuroscience from the University of Hawaii at Manoa. Learn more about her here.

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