Spasmus nutans is an eye movement disorder that develops in the first two years of life, and then usually disappears before age 3-4. No one knows what causes it -- the medical term for one of these mysterious conditions is "idiopathic" -- but patients usually demonstrate three characteristics:

Nystagmus (eyes shaking), which is high velocity and small amplitude
Torticollis, which may be a head turn or tilt that the patient often uses
Head bobbing or nodding

Here is a video from a patient with spasmus nutans whose mother kindly allowed me to share:

Because spasmus nutans can rarely be mimicked by a serious neurologic condition, including a tumor, patients should receive an MRI scan of their brain. Fortunately, for patients with the classic findings of spasmus nutans and a negative MRI, the prognosis is very good.