The most common and severe form of Sandhoff disease begins in infancy. Infants with this disorder typically appear normal until the age of 3 to 6 months, when development slows and muscles used for movement weaken. Affected infants lose motor skills such as turning over, sitting, and crawling. As the disease progresses, infants develop seizures, vision and hearing loss, mental retardation, and paralysis. An eye abnormality called a cherry-red spot, which can be identified with an eye examination, is characteristic of this disorder. Some infants with Sandhoff disease may also have enlarged organs (organomegaly) or bone abnormalities. Children with the severe form of this disorder usually live only into early childhood.

Other forms of Sandhoff disease are very rare. Signs and symptoms can begin in childhood, adolescence, or adulthood and are usually milder than those seen with the infantile form of Sandhoff disease. As in the infantile form, mental abilities and coordination are affected. Characteristic features include muscle weakness, loss of muscle coordination (ataxia) and other problems with movement, speech problems, and mental illness. These signs and symptoms vary widely among people with late-onset forms of Sandhoff disease.

Mutations in the HEXB gene cause Sandhoff disease. The HEXB gene provides instructions for making a protein that is part of two critical enzymes in the nervous system. These enzymes, beta-hexosaminidase A and beta-hexosaminidase B, function in nerve cells to break down fatty substances, complex sugars, and molecules that are linked to sugars. In particular, beta-hexosaminidase A breaks down a fatty compound called GM2 ganglioside. Mutations in the HEXB gene disrupt the activity of these enzymes, preventing the breakdown of GM2 ganglioside and other molecules. As a result, these compounds can accumulate to toxic levels within cells. Progressive damage caused by the buildup of GM2 ganglioside leads to the destruction of nerve cells, which causes the signs and symptoms of Sandhoff disease. The condition is inherited in an autosomal recessive pattern.

Sandhoff disease is a rare disorder; its frequency varies among populations. This condition appears to be more common in the Creole population of northern Argentina; the Metis Indians in Saskatchewan, Canada; and people from Lebanon.