"Saint Vitus' dance" is a kind of chorea, the medical term for a neurological disorder causing involuntary, quick, spasmodic movements of the body's muscles. (Chorea comes from the ancient Greek choreia = dance.) In recent medical practice Saint Vitus' Dance, which affects the feet, hands, and face, has come to be known as Sydenham's chorea. It often afflicts children who have had rheumatic fever or other streptococcal infections.
Saint Vitus was an early Christian martyr whose feast day was celebrated, in some parts of Europe during the later Middle Ages, by dancing before the saint's statue. (Perhaps coincidentally, his feast day is June 15.) Vitus became the patron saint of dancers and epilectics.
The grotesque pseudo-dancing of "A deafmute idiot
with goggle eyes" helps set the scene of this
in which half-crazed people stumble through a hallucinatory
cityscape, surrendering to mad random jerks of thought and
imagination. Trapped within a dysfunctional body, the boy also
introduces the reader to the Homeric condition of Circean
enchantment, in which rational individuals are plunged into
groteque animal corporeality.