awareness of the alternation. alternation may also be triggered by various tests during an eye exam. unilateral strabismus has been observed to result from a severe or traumatic injury to the affected eye. direction and latency horizontal deviations are classified into two varieties, using prefixes: eso- describes inward or convergent deviations towards the midline, while exo- describes outward or divergent misalignment. vertical deviations are also classified into two varieties, using prefixes: hyper- is the term for an eye whose gaze is directed higher than the fellow eye, while hypo- refers to an eye whose gaze is directed lower. finally, the prefix cyclo- refers to torsional strabismus, which occurs when the eyes rotate around the anterior–posterior axis to become misaligned and is quite rare. these five directional prefixes are combined with -tropia (if manifest) or -phoria (if latent) to describe various types of strabismus. for example, a constant left hypertropia exists when a person's left eye is always aimed higher than the right. a person with an intermittent right esotropia has a right eye that occasionally drifts toward the person's nose, but at other times is able to align with the gaze of the left eye. a person with a mild exophoria can maintain fusion during normal circumstances, but when the system is disrupted, the relaxed posture of the eyes is slightly divergent. other considerations strabismus can be further classified as follows: paretic strabismus is due to paralysis of one or several extraocular muscles. nonparetic strabismus is not due to paralysis of extraocular muscles. comitant (or concomitant) strabismus is a deviation that is the same magnitude regardless of gaze position. noncomitant (or incomitant) strabismus has a magnitude that varies as the person shifts his or her gaze up, down, or to the sides. nonparetic strabismus is generally concomitant. most types of infant and childhood strabismus are comitant. paretic strabismus can be either comitant or noncomitant. incomitant strabismus is almost always caused by a limitation of ocular rotations that is due to a restriction of extraocular eye movement (ocular restriction) or due to extraocular muscle paresis. incomitant strabismus cannot be fully corrected by prism glasses, because the eyes would require different degrees of prismatic correction dependent on the direction of the gaze. incomitant strabismus of the eso- or exo-type are classified as "alphabet patterns": they are denoted as a- or v- or more rarely λ-, y- or x-pattern depending on the extent of convergence or divergence when the gaze moves upward or downward. these letters of the alphabet denote ocular motility pattern that have a similarity to the respective letter: in the a-pattern there is (relatively speaking) more convergence when the gaze is directed upwards and more divergence when it is directed downwards, in the v-pattern it is the contrary, in the λ-, y- and x-patterns there is little or no strabismus in the middle position but relatively more divergence in one or both of the upward and downward positions, depending on the "shape" of the letter. types of incomitant strabismus include: duane syndrome, horizontal gaze palsy, and congenital fibrosis of the extraocular muscles. when the misalignment of the eyes is large and obvious, the strabismus is called large-angle, referring to the angle of deviation between the lines of sight of the eyes. less severe eye turns are called small-angle strabismus. the degree of strabismus can vary based on whether the person is viewing a distant or near target. strabismus that sets in after eye alignment had been surgically corrected is called consecutive strabismus. differential diagnosis pseudostrabismus is the false appearance of strabismus. it generally occurs in infants and toddlers whose bridge of the nose is wide and flat, causing the appearance of esotropia due to less sclera being visible nasally. with age, the bridge of the child's nose narrows and the folds in the corner of the eyes become less prominent. retinoblastoma may also result in abnormal light reflection from the eye.strabismus correction surgery management main article: management of strabismus strabismus is usually treated with a combination of eyeglasses, vision therapy, and surgery, depending on the underlying reason for the misalignment. as with other binocular vision disorders, the primary goal is comfortable, single, clear, normal binocular vision at all distances and directions of gaze. whereas amblyopia (lazy eye), if minor and detected early, can often be corrected with use of an eye patch on the dominant eye or vision therapy, the use of eye patches is unlikely to change the angle of strabismus. glasses in cases of accommodative esotropia, the eyes turn inward due to the effort of focusing far-sighted eyes, and the treatment of this type of strabismus necessarily involves refractive correction, which is usually done via corrective glasses or contact lenses, and in these cases surgical alignment is considered only if such correction does not resolve the eye turn. in case of strong anisometropia, contact lenses may be preferable to spectacles because they avoid the problem of visual disparities due to size differences (aniseikonia) which is otherwise caused by spectacles in which the refractive power is very different for the two eyes. in a few cases of strabismic children with anisometropic amblyopia, a balancing of the refractive error eyes via refractive surgery has been performed before strabismus surgery was undertaken. early treatment of strabismus when the person is a baby may reduce the chance of developing amblyopia and depth perception problems. however, a review of randomized controlled trials concluded that the use of corrective glasses to prevent strabismus is not supported by existing research. most children eventually recover from amblyopia if they have had the benefit of patches and corrective glasses. amblyopia has long been considered to remain permanent if not treated within a critical period, namely before the age of about seven years; however, recent discoveries give reason to challenge this view and to adapt the earlier notion of a critical period to account for stereopsis recovery in adults. eyes that remain misaligned can still develop visual problems. although not a cure for strabismus, prism lenses can also be used to provide some temporary comfort and to prevent double vision from occurring. glasses affect the position by changing the person's reaction to focusing. prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position. surgery strabismus surgery does not remove the need for a child to wear glasses. currently it is unknown whether there are any differences for completing strabismus surgery before or after amblyopia therapy in children. strabismus surgery attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles. the procedure can typically be performed in about an hour, and requires about six to eight weeks for recovery. adjustable sutures may be used to permit refinement of the eye alignment in the early postoperative period. it is unclear if there are differences between adjustable versus non-adjustable sutures as it has not been sufficiently studied. an alternative to the classical procedure is minimally invasive strabismus surgery (miss) that uses smaller incisions than usual. medication medication is used for strabismus in certain circumstances. in 1989, the us fda approved botulinum toxin therapy for strabismus in people over 12 years old. most commonly used in adults, the technique is also used for treating children, in particular children affected by infantile esotropia. the toxin is injected in the stronger muscle, causing temporary and partial paralysis. the treatment may need to be repeated three to four months later once the paralysis wears off. common side effects are double vision, droopy eyelid, overcorrection, and no effect. the side effects typically resolve also within three to four months. botulinum toxin therapy has been reported to be similarly successful as strabismus surgery for people with binocular vision and less successful than surgery for those who have no binocular vision. prognosis when strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated eye. even with therapy for amblyopia, stereoblindness may occur. the appearance of strabismus may also be a cosmetic problem. one study reported 85% of adult with strabismus "reported that they had problems with work, school, and sports because of their strabismus." the same study also reported 70% said strabismus "had a negative effect on their self-image." a second operation is sometimes required to straighten the eyes. other animals a balinese cat exhibiting esotropia siamese cats and related breeds are prone to having crossed eyes. research suggests this is a behavioral compensation for developmental abnormalities in the routing of nerves in the optic chiasm. strabismus may also occur in dogs, most often due to imbalanced muscle tone of the muscles surrounding the eye. some breeds such as the shar pei are genetically predisposed to the condition. treatment may involve surgery or therapy to strengthen the muscles.