SBMA is a rare inherited neuromuscular condition. It is also sometimes known as:

• X-linked motor neuron disease
• X-linked recessive bulbospinal neuropathy
• X-linked SBMA

It is an adult onset form of SMA that mainly affects men. It is estimated that about 1 in 40,000 people have the genetic fault that causes the condition and, because it is relatively rare, SBMA is often initially misdiagnosed or goes undiagnosed for years.

SBMA causes progressive weakening and wasting of the muscles, particularly in the arms and legs. This is due to the degeneration of lower motor neurons within the spinal cord and brainstem. It also causes hormonal changes. Symptoms and effects of SBMA are described in more detail in the information published by the MND Associations and Kennedy’s Disease UK (see below under Support and Resources).

The affected gene is:

• The Androgen Receptor (AR) gene, on the X sex chromosome¹‾².

Further information about the AR Gene >

SBMA has an X-linked Recessive inheritance pattern.

Kennedy’s Disease UK

• Phone: 01604 250505
• Website >

MND Association

• Phone: 0808 802 6262
• Website >

1. La Spada et al. (1991) ‘Androgen receptor gene mutations in X-linked spinal and bulbar muscular atrophy’, Nature, 352, pp. 77–79.

2. https://www.omim.org/entry/313200 (last accessed 21st August 2024)