Hypomyelination with brainstem and spinal cord involvement and leg spasticity (HBSL) is a condition that affects the (central nervous system). In particular, the condition affects nerves in specific regions (called tracts) within the spinal cord and the brainstem, which is the part of the brain that connects to the spinal cord. HBSL is a form of leukodystrophy, which is a group of conditions that involve abnormalities of the nervous system’s white matter. The white matter consists of nerve fibers covered by a fatty substance, called , that insulates the fibers and promotes the rapid transmission of nerve impulses. In HBSL, the nervous system has a reduced ability to form myelin (hypomyelination).
In HBSL, early development of motor skills (such as rolling over and sitting) may be normal, but movement problems typically begin within the infant’s first year. However, in some individuals, these problems do not appear until adolescence. The characteristic feature of HBSL is muscle stiffness (spasticity) in the legs that worsens over time. Most people with HBSL are unable to walk independently. Other neurological problems in affected individuals can include abnormal side-to-side movements of the eyes (nystagmus), weak muscle tone (hypotonia) in the torso, and mild intellectual disability.
Distinct changes in the brains of people with HBSL can be seen using magnetic resonance imaging (MRI). These characteristic abnormalities typically involve specific regions (called tracts) within the and spinal cord, especially the pyramidal tract, lateral corticospinal tract, and the dorsal column.