A 9-YEAR-OLD boy presented with an 18-month history of progressively unsteady and ungainly gait and poor performance in school sports that involved running. There were no neonatal problems, and developmental milestones were normal. His gait problems were first noticed when he started school, but, as his father and paternal grandfather also had an "odd walk," his parents were unconcerned. In the 18 months before his presentation, his gait gradually became more uncoordinated and unsteady. He dragged his feet and held onto the stair rails when climbing stairs. He had a recent history of pain in the right groin and down the back of his right leg.
On physical examination, he had a stiff back with kyphoscoliosis and marked limitation of straight leg raising. No other abnormal neurological signs were found. A diagnosis of juvenile ankylosing spondylitis (AS) was considered, and a radiograph (Figure 1), and magnetic resonance imaging scans (Figure