Sydenham’s chorea is caused primarily by an autoimmune reaction that follows an infection with a specific type of bacteria called group A streptococcus. This bacterium is the same one responsible for strep throat and certain skin infections. When a person, often a child or adolescent, contracts this infection, their immune system produces antibodies to fight off the bacteria. However, in some cases, these antibodies mistakenly target parts of the brain, particularly a region called the basal ganglia, which controls movement. This misdirected immune attack leads to the involuntary, jerky movements characteristic of Sydenham’s chorea.
The process begins with a streptococcal infection, which triggers an immune response. Normally, antibodies produced by the immune system recognize and attack the bacteria. But in Sydenham’s chorea, the antibodies cross-react with neurons in the basal ganglia due to molecular mimicry—where bacterial proteins resemble proteins in the brain. This causes inflammation and dysfunction in the basal ganglia neurons, disrupting normal motor control and resulting in rapid, irregular, purposeless movements, emotional instability, and sometimes muscle weakness.
This autoimmune response does not occur immediately after the infection. Instead, Sydenham’s chorea typically appears weeks to months after the initial streptococcal infection, often as part of a broader condition called acute rheumatic fever. Rheumatic fever itself is an inflammatory disease that can affect the heart, joints, skin, and brain. Sydenham’s chorea is considered one of the major neurological manifestations of this disease.
The basal ganglia, especially the caudate nucleus and putamen, are the main brain areas affected. These structures are crucial for coordinating smooth, controlled movements. When antibodies attack these regions, they interfere with the normal signaling pathways involving neurotransmitters such as dopamine, which plays a key role in movement regulation. This disruption leads to the characteristic involuntary movements seen in Sydenham’s chorea.
In addition to movement symptoms, patients may experience emotional lability, meaning rapid and unpredictable changes in mood. This is also linked to the basal ganglia’s role in regulating not only motor functions but also certain emotional and cognitive processes.
The exact reason why some individuals develop Sydenham’s chorea after a streptococcal infection while others do not is not fully understood. Genetic susceptibility likely plays a role, as some people’s immune systems may be more prone to producing these cross-reactive antibodies. Environmental factors and the severity or strain of the streptococcal infection might also influence the risk.
In summary, Sydenham’s chorea is caused by an autoimmune reaction triggered by a group A streptococcal infection. The immune system’s antibodies mistakenly attack the basal ganglia in the brain, leading to involuntary movements and emotional changes. This condition is a delayed complication of rheumatic fever and highlights the complex interplay between infection, immunity, and brain function.
