strabismus strabismus strabismus is a vision disorder in which the eyes do not properly align with each other when looking at an object. the eye that is pointed at an object can alternate. the condition may be present occasionally or constantly. if present during a large part of childhood, it may result in amblyopia, or lazy eyes, and loss of depth perception. if onset is during adulthood, it is more likely to result in double vision. strabismusother namesheterotropia, crossed eyes, squinta person with exotropia, an outward deviated eyepronunciation/strəˈbɪzməs/ specialtyophthalmology, optometrysymptomsnonaligned eyescomplicationsamblyopia, double visiontypesesotropia (eyes crossed); exotropia (eyes diverge); hypertropia (eyes vertically misaligned)causesmuscle dysfunction, farsightedness, problems in the brain, trauma, infectionsrisk factorspremature birth, cerebral palsy, family historydiagnostic methodobserving light reflected from the pupildifferential diagnosiscranial nerve disease, convergence insufficiencytreatmentglasses, surgeryfrequency~2% (children) strabismus can occur due to muscle dysfunction, farsightedness, problems in the brain, trauma or infections. risk factors include premature birth, cerebral palsy and a family history of the condition. types include esotropia, where the eyes are crossed ("cross eyed"); exotropia, where the eyes diverge ("lazy eyed" or "wall eyed"); and hypertropia or hypotropia where they are vertically misaligned. they can also be classified by whether the problem is present in all directions a person looks (comitant) or varies by direction (incomitant). diagnosis may be made by observing the light reflecting from the person's eyes and finding that it is not centered on the pupil. this is known as the hirschberg reflex. another condition that produces similar symptoms is a cranial nerve disease. treatment depends on the type of strabismus and the underlying cause. this may include the use of glasses and possibly surgery. some types benefit from early surgery. strabismus occurs in about 2% of children. the term comes from the ancient greek word στραβισμός (strabismós), meaning 'a squinting'. other terms for the condition include "squint" and "cast of the eye". "wall-eye" has been used when the eyes turn away from each other. signs and symptoms aligned vergenceesotropiaexotropia when observing a person with strabismus, the misalignment of the eyes may be quite apparent. a person with a constant eye turn of significant magnitude is very easy to notice. however, a small magnitude or intermittent strabismus can easily be missed upon casual observation. in any case, an eye care professional can conduct various tests, such as cover testing, to determine the full extent of the strabismus. symptoms of strabismus include double vision and eye strain. to avoid double vision, the brain may adapt by ignoring one eye. in this case, often no noticeable symptoms are seen other than a minor loss of depth perception. this deficit may not be noticeable in someone who has had strabismus since birth or early childhood, as they have likely learned to judge depth and distances using monocular cues. however, a constant unilateral strabismus causing constant suppression is a risk for amblyopia in children. small-angle and intermittent strabismus are more likely to cause disruptive visual symptoms. in addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading, and unstable or "jittery" vision. psychosocial effects fayum mummy portrait, c. 150–200 ad tommaso inghirami norma shearer jean-paul sartre marty feldman people of all ages who have noticeable strabismus may experience psychosocial difficulties. attention has also been drawn to potential socioeconomic impact resulting from cases of detectable strabismus. a socioeconomic consideration exists as well in the context of decisions regarding strabismus treatment, including efforts to re-establish binocular vision and the possibility of stereopsis recovery. one study has shown that strabismic children commonly exhibit behaviors marked by higher degrees of inhibition, anxiety, and emotional distress, often leading to outright emotional disorders. these disorders are often related to a negative perception of the child by peers. this is due not only to an altered aesthetic appearance, but also because of the inherent symbolic nature of the eye and gaze, and the vitally important role they play in an individual's life as social components. for some, these issues improved dramatically following strabismus surgery. notably, strabismus interferes with normal eye contact, often causing embarrassment, anger, and feelings of awkwardness, thereby affecting social communication in a fundamental way, with a possible negative effect on self esteem. children with strabismus, particularly those with exotropia, an outward turn, may be more likely to develop a mental health disorder than normal-sighted children. researchers have theorized that esotropia (an inward turn) was not found to be linked to a higher propensity for mental illness due to the age range of the participants, as well as the shorter follow-up time period; esotropic children were monitored to a mean age of 15.8 years, compared with 20.3 years for the exotropic group. a subsequent study with participants from the same area monitored people with congenital esotropia for a longer time period; results indicated that people who are esotropic were also more likely to develop mental illness of some sort upon reaching early adulthood, similar to those with constant exotropia, intermittent exotropia, or convergence insufficiency. the likelihood was 2.6 times that of controls. no apparent association with premature birth was observed, and no evidence was found linking later onset of mental illness to psychosocial stressors frequently encountered by those with strabismus. investigations have highlighted the impact that strabismus may typically have on quality of life. studies in which subjects were shown images of strabismic and non-strabismic persons showed a strong negative bias towards those visibly displaying the condition, clearly demonstrating the potential for future socioeconomic implications with regard to employability, as well as other psychosocial effects related to an individual's overall happiness. adult and child observers perceived a right heterotropia as more disturbing than a left heterotropia, and child observers perceived an esotropia as "worse" than an exotropia. successful surgical correction of strabismus, for adult as well as children, has been shown to have a significantly positive effect on psychological well-being. very little research exists regarding coping strategies employed by adult strabismics. one study categorized coping methods into three subcategories: avoidance (refraining from participation in an activity), distraction (deflecting attention from the condition), and adjustment (approaching an activity differently). the authors of the study suggested that individuals with strabismus may benefit from psychosocial support such as interpersonal skills training. no studies have evaluated whether psychosocial interventions have had any benefits on individuals undergoing strabismus surgery. pathophysiology the extraocular muscles control the position of the eyes. thus, a problem with the muscles or the nerves controlling them can cause paralytic strabismus. the extraocular muscles are controlled by cranial nerves iii, iv, and vi. an impairment of cranial nerve iii causes the associated eye to deviate down and out and may or may not affect the size of the pupil. impairment of cranial nerve iv, which can be congenital, causes the associated eye to drift up and perhaps slightly inward. sixth nerve palsy causes the associated eye to deviate inward and has many causes due to the relatively long path of the nerve. increased cranial pressure can compress the nerve as it runs between the clivus and brain stem. also, if the doctor is not careful, twisting of the baby's neck during forceps delivery can damage cranial nerve vi. evidence indicates a cause for strabismus may lie with the input provided to the visual cortex. this allows for strabismus to occur without the direct impairment of any cranial nerves or extraocular muscles. strabismus may cause amblyopia due to the brain ignoring one eye. amblyopia is the failure of one or both eyes to achieve normal visual acuity despite normal structural health. during the first seven to eight years of life, the brain learns how to interpret the signals that come from an eye through a process called visual development. development may be interrupted by strabismus if the child always fixates with one eye and rarely or never fixates with the other. to avoid double vision, the signal from the deviated eye is suppressed, and the constant suppression of one eye causes a failure of the visual development in that eye. also, amblyopia may cause strabismus. if a great difference in clarity occurs between the images from the right and left eyes, input may be insufficient to correctly reposition the eyes. other causes of a visual difference between right and left eyes, such as asymmetrical cataracts, refractive error, or other eye disease, can also cause or worsen strabismus. accommodative esotropia is a form of strabismus caused by refractive error in one or both eyes. due to the near triad, when a person engages accommodation to focus on a near object, an increase in the signal sent by cranial nerve iii to the medial rectus muscles results, drawing the eyes inward; this is called the accommodation reflex. if the accommodation needed is more than the usual amount, such as with people with significant hyperopia, the extra convergence can cause the eyes to cross. a child with accommodative esotropia affecting the right eye diagnosis during an eye examination, a test such as cover testing or the hirschberg test is used in the diagnosis and measurement of strabismus and its impact on vision. retinal birefringence scanning can be used for screening of young children for eye misalignment. a cochrane review to examine different types of diagnosis test found only one study. this study used a photoscreener which was found to have high specificity (accurate in identifying those without the condition) but low sensitivity (inaccurate in identifying those with the condition). several classifications are made when diagnosing strabismus. latency strabismus can be manifest (-tropia) or latent (-phoria). a manifest deviation, or heterotropia (which may be eso-, exo-, hyper-, hypo-, cyclotropia or a combination of these), is present while the person views a target binocularly, with no occlusion of either eye. the person is unable to align the gaze of each eye to achieve fusion. a latent deviation, or heterophoria (eso-, exo-, hyper-, hypo-, cyclophoria or a combination of these), is only present after binocular vision has been interrupted, typically by covering one eye. this type of person can typically maintain fusion despite the misalignment that occurs when the positioning system is relaxed. intermittent strabismus is a combination of both of these types, where the person can achieve fusion, but occasionally or frequently falters to the point of a manifest deviation. onset strabismus may also be classified based on time of onset, either congenital, acquired, or secondary to another pathological process. many infants are born with their eyes slightly misaligned, and this is typically outgrown by six to 12 months of age. acquired and secondary strabismus develop later. the onset of accommodative esotropia, an overconvergence of the eyes due to the effort of accommodation, is mostly in early childhood. acquired non-accommodative strabismus and secondary strabismus are developed after normal binocular vision has developed. in adults with previously normal alignment, the onset of strabismus usually results in double vision. any disease that causes vision loss may also cause strabismus, but it can also result from any severe and/or traumatic injury to the affected eye. sensory strabismus is strabismus due to vision loss or impairment, leading to horizontal, vertical or torsional misalignment or to a combination thereof, with the eye with poorer vision drifting slightly over time. most often, the outcome is horizontal misalignment. its direction depends on the person's age at which the damage occurs: people whose vision is lost or impaired at birth are more likely to develop esotropia, whereas people with acquired vision loss or impairment mostly develop exotropia. in the extreme, complete blindness in one eye generally leads to the blind eye reverting to an anatomical position of rest. although many possible causes of strabismus are known, among them severe and/or traumatic injuries to the affected eye, in many cases no specific cause can be identified. this last is typically the case when strabismus is present since early childhood. results of a u.s. cohort study indicate that the incidence of adult-onset strabismus increases with age, especially after the sixth decade of life, and peaks in the eighth decade of life, and that the lifetime risk of being diagnosed with adult-onset strabismus is approximately 4%. laterality strabismus may be classified as unilateral if the one eye consistently deviates, or alternating if either of the eyes can be seen to deviate. alternation of the strabismus may occur spontaneously, with or without subjective