Passage: In the late nineteenth century, the neurophysiologist Vladimir Bekhterev of Russia in 1893, Adolph Strümpell of Germany in 1897, and Pierre Marie of France in 1898 were the first to give adequate descriptions which permitted an accurate diagnosis of AS prior to severe spinal deformity. For this reason, AS is also known as Bekhterev Disease, Bechterew's Disease or Marie–Strümpell Disease.

Question: What medical condition were they trying to provide accurate descriptions of?
AS