What is the most likely diagnosis for a 6-year-old girl with Down syndrome who presents with ataxic gait, dizziness, and positive Babinski reflex?

The diagnosis of atlantoaxial instability in a 6-year-old girl with Down syndrome presenting with ataxic gait, dizziness, and a positive Babinski reflex is particularly relevant due to the anatomical and physiological considerations associated with Down syndrome. Children with Down syndrome are predisposed to atlantoaxial instability due to bony abnormalities of the cervical spine that may occur, primarily affecting the first and second cervical vertebrae (the Atlas and Axis).

Ataxic gait and dizziness are indicative of neurological compromise, which can arise from compression of the spinal cord or brainstem in the context of atlantoaxial instability. The presence of a positive Babinski reflex suggests upper motor neuron involvement, which can occur if there is cord compression. In this case, the symptoms align well with the potential neurological consequences of the instability, making atlantoaxial instability the most likely diagnosis.

Spinal stenosis is less common in this age group, particularly in the context of Down syndrome, and it would typically present with different symptoms. Multiple sclerosis, while possible in older children, is unlikely in a young child and particularly in one with known Down syndrome. Psychogenic factors could contribute to similar symptoms but would not typically be the primary diagnosis in the context of clear neuro