St. Vitus’ Dance is the historical name for Sydenham Chorea, a neurological disorder primarily affecting children and adolescents. This condition is a type of chorea, a movement disorder characterized by involuntary, rapid, and jerky movements that appear random and non-rhythmic. The movements can affect the limbs, trunk, and facial muscles, often causing difficulties with voluntary actions like walking, writing, or speaking. Sydenham Chorea is not an inherited disease, but rather an acquired condition that arises due to a complex interplay between a common infection and an individual’s unique genetic makeup.
Sydenham Chorea: An Acquired Condition
Sydenham Chorea (SC) is an acquired disorder, meaning it is not caused by a gene passed directly from a parent to a child. The cause is rooted in an immune reaction that occurs after an infection with Group A \(\beta\)-hemolytic Streptococcus (GABHS), the bacteria responsible for strep throat and scarlet fever. Symptoms typically appear weeks or months after the initial streptococcal infection has cleared.
The mechanism driving this neurological complication is known as molecular mimicry, a process where the body’s immune system mistakenly attacks its own tissues. Antibodies produced to fight the GABHS bacteria recognize and attack structures within the brain, particularly in the basal ganglia. Specific proteins on the surface of the streptococcal bacteria share a similar structure with proteins found on neurons in the basal ganglia. The antibodies cross-react with these healthy brain cells, causing inflammation and damage. This autoimmune attack leads to the characteristic uncontrolled movements seen in Sydenham Chorea.
Understanding Genetic Susceptibility
While Sydenham Chorea is not inherited, genetics play a significant part in determining who develops the condition after a strep infection. This heightened risk is known as genetic susceptibility, which differs from inheriting the disease itself. The genes involved are primarily those related to the immune system, particularly the Human Leukocyte Antigen (HLA) complex.
HLA proteins are located on the surface of cells and are responsible for presenting foreign antigens to T-cells, initiating the immune response. Certain specific HLA Class II alleles, such as DRB107 and DQB10401-2, are associated with an increased risk for developing Sydenham Chorea. These HLA types make an individual’s immune system more likely to mount an aggressive, cross-reactive response against both the strep bacteria and the basal ganglia tissue. The presence of these genetic markers is not a guarantee of developing SC, but lowers the threshold for the autoimmune cascade to be triggered by the strep infection.
Distinguishing SC from Inherited Chorea Syndromes
Chorea is a symptom that can be caused by various conditions, some of which are passed directly through generations. The most recognized example of a truly hereditary chorea is Huntington’s Disease (HD). HD is caused by a single, dominant genetic mutation in the HTT gene, meaning a child needs only one copy of the faulty gene to develop the disease. This mutation directly causes the slow, progressive death of neurons in the brain, including those in the basal ganglia.
The genetic cause of HD is direct and predictable, manifesting regardless of any external trigger. In contrast, Sydenham Chorea requires two distinct elements: a genetic susceptibility and a specific environmental trigger, the GABHS infection. This fundamental difference—a direct genetic cause versus a genetic predisposition requiring an external trigger—separates acquired autoimmune choreas like SC from truly inherited chorea syndromes.