A form of strabismus in which one or both eyes are deflected inward, towards the nose. It can be constant or periodic, occur in childhood or adulthood. The causes can be congenital defects, ametropia, neurological diseases or an imbalance of the oculomotor muscles. Symptoms include double vision, eye strain, loss of stereoscopic perception; In children, if left untreated, amblyopia may develop. Treatment may include correction with glasses, orthoptic exercises, prismatic lenses, or eye muscle surgery.