Bardet-Biedl Syndrome is a genetic disease that most often combines obesity, vision problems, finger abnormalities, and in some cases kidney and genital abnormalities. Learning difficulties are often present. Other malformations (of the heart, for example) may be associated, but more rarely.
The manifestations and severity of the syndrome vary considerably from person to person. How many people have Bardet-Biedl syndrome ? Is it present throughout the world ?
Bardet-Biedl syndrome is a rare disease whose prevalence (number of people affected in a population at a given time) is between 1 in 100,000 and 1 in 160,000 for the populations of Europe and North America. This syndrome is much more common in certain isolated populations such as the Bedouin populations of Kuwait where the prevalence is estimated at 1 in 13,500.
Who can be affected ?
Bardet-Biedl Syndrome affects both boys and girls and usually begins at birth.
What causes Bardet-Biedl syndrome ?
Bardet-Biedl syndrome is an inherited disease caused by a change (mutation) in a gene. To date, there are at least twenty-three different genes that may be responsible for this disease. These are genes BBS1 to BBS23.
Is Bardet-Biedl syndrome contagious ?
No, Bardet-Biedl syndrome is not contagious.
What are the symptoms of Bardet-Biedl syndrome ?
The clinical manifestations of Bardet-Biedl syndrome are multiple and vary considerably from person to person. Therefore, not all patients have all of the symptoms described below.
--> Visual disturbances
Almost all children with Bardet-Biedl Syndrome suffer from decreased vision, most often starting around the age of 5-6 years. It starts with a decrease and then a gradual loss of vision at night or when the light is a little dim. The child sees very little in the dark, but this may go unnoticed when he is small.
The field of vision gradually narrows at the sides (figure 1), giving the impression of looking through an increasingly narrow tube (so-called "tubular vision"). The quality of vision deteriorates greatly during adolescence. Sometimes, other ocular manifestations may be associated with it: blurred distance vision (myopia) or distorted vision (astigmatism), both of which can be corrected with glasses, opacification of the crystalline lens (cataract) leading to a progressive decrease in vision, the appearance of abnormal jerky eye movements (nystagmus), problems with colour distinction, etc. Eventually, central vision can also be affected, making the patient visually impaired.
The visual disturbances characteristic of Bardet-Biedl syndrome are due to an impairment of the retina, called retinopathy pigmentosa. The retina (figure 2) is the surface of the back of the eye that receives images, much like a photographic film, and transmits them to the brain in the form of electrical signals.
Despite a normal birth weight, affected children are almost always significantly overweight from the first year of life. This overweight often evolves into severe obesity, especially in the trunk area. Sometimes the height is smaller than average.
--> Abnormalities of toes and fingers
There are often, but not in all cases, malformations of the toes and fingers that are smaller than normal (brachydactyly). There are often six fingers and/or six toes instead of five, with the supernumerary finger(s) located next to the fifth finger (the little finger). This is called postaxial polydactyly. In addition, some children have two or more fingers joined together, i.e. not well separated, and connected by a membrane (webbed fingers or syndactyly).
--> Abnormalities of the genital organs
In boys, the genitals, penis and testicles, are often abnormally small (hypogonadism). In girls, malformations of the genital organs are also possible. The vagina may be closed by a partition, which causes the uterus to expand, often detected before birth (hydrometrocolpos).
--> Kidney and urinary tract deformities
Malformations of the kidneys and urinary tract are very common. They can be serious and, in a significant number of cases, lead to malfunction of the kidneys, whose function is to filter the blood and allow waste products to be eliminated through urine.
In people with Bardet-Biedl syndrome, the progressive reduction in kidney function, known as chronic kidney failure, may require the use of an artificial kidney and lead to a kidney transplant. One of the consequences of this kidney damage is an increase in blood pressure (hypertension), which occurs in more than half of adults. Even if there are no kidney malformations, problems can occur in regulating the amount of water in the body. This is manifested by an increase in the volume of urine (regardless of the volume of liquid absorbed), with a frequent need to urinate, as well as by intense thirst (this is called diabetes insipidus).
--> Intellectual deficiency and psychological disorders
The intellectual deficit is not always present. When it does exist, it is moderate or rarely severe. Most often, it is limited to learning difficulties that can be aggravated by vision problems. Affected children may also have a delay in language acquisition (they speak later than others), as well as phonation disorders (abnormal way of producing sounds). There may also be hearing loss (hearing loss), which is often mild and goes unnoticed.
Psychological or behavioural disorders may appear in some people in the course of their lives. These disorders include emotional instability, frequent tantrums, inappropriate or uninhibited behaviour (with no idea of what is or is not done), with more rarely obsessive-compulsive behaviour (i.e. repetitive actions, such as washing hands very frequently). However, these disorders only affect a few patients and it is not always clear whether they are directly linked to the syndrome.
--> Other manifestations
Diabetes (too high blood sugar levels) can also set in, as in many obese people. It initially manifests itself by intense tiredness, thirst and hunger, but over time it can lead to serious complications such as kidney failure, an increased risk of heart attacks and infections, nerve damage that can cause loss of feeling in the hands and feet, etc. It can be treated with medication.
Very rarely, incoordination of movements can occur, manifesting itself as abnormalities in staggering gait or poorly coordinated hand movements.
In rare cases there are malformations of the heart, in particular communication between the different chambers (atria or ventricles) or narrowing of the heart valves (valvular stenoses).
Long-term (chronic) constipation, when present, may be due to Hirschsprung's disease, which is a lack of contraction of the large intestine (colon) which leads to its dilation.
How can the symptoms be explained ?
The genes involved in this syndrome "control" the production of proteins that play a role in the cells' eyelashes. The cells have lashes that function like antennae, capturing and transmitting information about the state of their environment. When these cilia are defective (which is the case when genes are mutated), certain functions are also altered. In particular, cilia play an important role in vision and kidney function, which explains the visual deficit and possible kidney abnormalities in Bardet-Biedl syndrome.
A great deal of research is underway to understand the role of the cilia in all manifestations of the disease.
What is its evolution ?
If left untreated, the various manifestations can worsen, in particular kidney damage and obesity. Obesity, which is resistant to the usual diet and measures, can be complicated by diabetes and excess lipids in the blood (hyperlipidemia), heart and joint problems.
In addition, retinal abnormality leads to a severe reduction in vision, and even blindness between the ages of 15 and 30. Indeed, the field of vision gradually reduces and central vision can sometimes end up being reduced.
Early treatment can limit the worsening of symptoms.