The term Bardet-Biedl syndrome, or BBS, is actually a collection of disorders with a number of common features. At least 14 types can be distinguished on the basis of specific gene mutations that have been identified. The clinical distinctions between these are often subtle.
All BB syndromes seem to have in common: retinal degeneration, obesity, some mental handicap, extra digits on the hands and feet, and genital abnormalities. Kidney disease is common often leading to life threatening renal failure. Some patients have diabetes, liver scarring, and high cholesterol.
The retinal disease begins in the first decade of life with night blindness. An electroretinogram or ERG, which measures the ability of the retina to respond to light, is abnormal from early childhood. Degeneration of the rods and cones occurs rapidly and many patients are legally blind by the age of 20 years. An examination by an eye doctor can see changes in the color of the retina and sometimes the optic nerve becomes pale indicating that connections with retinal cells are defective.
Abnormal weight gain begins in early childhood and with it hypertension, hypercholesterolemia and diabetes. Extra fingers and toes are common features, but the digits may also be fused or shortened. Mental capacity varies widely from severe to mild. Emotional immaturity may occur. Motor difficulties such as unsteadiness and clumsiness are often present. The kidneys may have cysts and kidney failure is frequent along with scarring of the liver and various heart malformations which are less frequent. Sometimes the genitalia are underdeveloped and abnormally formed.
Bardet-Biedl syndromes are inherited in an autosomal recessive pattern. the presence of two mutations is necessary for the disorders to be expressed which requires the contribution of a mutation from each parent. The risk that both parents are such carriers increases if mother and father are related. In such cases, the risk to each child for inheriting BBS is 25%.
Since BBS is a complex, multisystem disease, no simple treatment is available. Diagnosis is a team effort which may require multiple specialists. Ultimately, the findings of severe retinal degeneration, with extra digits, obesity, genital anomalies, and a mental deficit to some degree suggest the diagnosis. The prognosis is guarded primarily because kidney failure is so life-threatening.