Without prismatic glasses the head is turned.
Certain persons have eyes which move ceaselessly; their eyes are livened up(led) by ceaseless oscillations; they can exist from the birth or appear later mostly in neurological pathologies.
The most frequent are the nystagmus where eyes move in a horizontal way; it can be vertical, or rotational.
Spontaneously the child turns(shoots) the head, to the right or to the left; he tilts the head forward or behind. You should not prevent him from it, because he so finds a position of the head where his eyes move least, and consequently where he sees best. The principle of the treatment(processing) by prisms or by surgery rests(bases) on this observation.
– The child tends to get closer too much(many) to his sheet(leaf) or to the screen; this position allows him to back off its nystagmus and to see better.
– The balance sheet(assessment) of the eye motility (balance sheet(assessment) of orthoptics) looks for the dominant eye, and for the fixing eye which decreases the eye oscillations. A penalization of the not fixing eye gets a lesser nystagmus. Penalization proposed by the ophthalmologist or the orthoptist.
– Prisms allow the child to keep the straight(right) head while taking advantage of the best visual direction.
– Sometimes we find a position of the eyes where the nystagmus is locked; a surgery of the eye muscles is then possible.