A determination of the prevalence of anisometropia has several difficulties. First of all, the measurement of refractive error may vary from one measurement to the next. Secondly, different criteria have been employed to define anisometropia, and the boundary between anisometropia and isometropia depend on their definition.

Several studies have found that anisometropia occurs more frequently and tends to be more severe for persons with high ametropia, and that this is particularly true for myopes. Anisometropia follows a U-shape distribution according to age: it is frequent in infants aged only a few weeks, is more rare in young children, comparatively more frequent in teenagers and young adults, and more prevalent after presbyopia sets in, progressively increasing into old age.

One study estimated that 6% of those between the ages of 6 and 18 have anisometropia.

Notwithstanding research performed on the biomechanical, structural and optical characteristics of anisometropic eyes, the underlying reasons for anisometropia are still poorly understood.

Anisometropic persons who have strabismus are mostly far-sighted, and almost all of these have (or have had) esotropia. However, there are indications that anisometropia influences the long-term outcome of a surgical correction of an inward squint, and vice versa. More specifically, for patients with esotropia who undergo strabismus surgery, anisometropia may be one of the risk factors for developing consecutive exotropia and poor binocular function may be a risk factor for anisometropia to develop or increase.

Strabismus can be seen in Down syndrome, Loeys-Dietz syndrome, cerebral palsy, and Edwards syndrome. The risk is increased among those with a family history of the condition.

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 congenital esotropia patients for a longer time period; results indicated that esotropic patients "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 patients 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 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.

Young children with strabismus normally suppress the visual field of one eye (or part of it), whereas adults who develop strabismus normally do not suppress and therefore suffer from double vision (diplopia). This also means that adults (and older children) have a higher risk of post-operative diplopia after undergoing strabismus surgery than young children. Patients who have undergone strabismus surgery at a young age often have monofixation syndrome (with peripheral binocular fusion and a central suppression scotoma).

"Congenital esotropia," or "infantile esotropia," is a specific sub-type of primary concomitant esotropia. It is a constant esotropia of large and consistent size with onset between birth and six months of age. It is not associated with hyperopia, so the exertion of accommodative effort will not significantly affect the angle of deviation. It is, however, associated with other ocular dysfunctions including oblique muscle over-actions, Dissociated Vertical Deviation (DVD,) Manifest Latent Nystagmus, and defective abduction, which develops as a consequence of the tendency of those with infantile esotropia to 'cross fixate.' Cross fixation involves the use of the right eye to look to the left and the left eye to look to the right; a visual pattern that will be 'natural' for the person with the large angle esotropia whose eye is already deviated towards the opposing side.

The origin of the condition is unknown, and its early onset means that the affected individual's potential for developing binocular vision is limited. The appropriate treatment approach remains a matter of some debate. Some ophthalmologists favour an early surgical approach as offering the best prospect of binocularity whilst others remain unconvinced that the prospects of achieving this result are good enough to justify the increased complexity and risk associated with operating on those under the age of one year.

Incomitant esotropias are conditions in which the esotropia varies in size with direction of gaze. They can occur in both childhood and adulthood, and arise as a result of neurological, mechanical or myogenic problems. These problems may directly affect the extra-ocular muscles themselves, and may also result from conditions affecting the nerve or blood supply to these muscles or the bony orbital structures surrounding them. Examples of conditions giving rise to an esotropia might include a VIth cranial nerve (or Abducens) palsy, Duane's syndrome or orbital injury.

This remains undetermined at the present time. A recent study by Major et al. reports that:

"Prematurity, family history or secondary ocular history, perinatal or gestational complications, systemic disorders, use of supplemental oxygen as a neonate, use of systemic medications, and male sex were found to be significant risk factors for infantile esotropia."

Further recent evidence indicates that a cause for "infantile strabismus" may lie with the input that is provided to the visual cortex. In particular, neonates who suffer injuries that, directly or indirectly, perturb binocular inputs into the primary visual cortex (V1) have a far higher risk of developing strabismus than other infants.

A paper published by Eltern für Impfaufklärung, a German Anti-Vaccination activist group, cites a study by The Robert Koch Institute (RKI), claiming significant correlation between children who received Vaccinations and the onset of cause of Spine, Face & Eye Asymmetry.

It may be acquired from:

- Diseases. Some of the diseases that present nystagmus as a pathological sign:

- Aniridia

- Toxic or metabolic reasons could be the result of the following:

- Central nervous system (CNS) disorders, such as with a cerebellar problem, the nystagmus can be in any direction "including" horizontal. Purely vertical nystagmus is usually central in origin, but it is also a frequent adverse effect of high phenytoin toxicity. Causes include:

Nystagmus is a relatively common clinical condition, affecting one in several thousand people. A survey conducted in Oxfordshire, United Kingdom found that by the age of two, one in every 670 children had manifested nystagmus. Authors of another study in the United Kingdom estimated an incidence of 24 in 10,000 (~0.240 %), noting an apparently higher rate amongst white Europeans than in individuals of Asian origin.

Suppression may treated with vision therapy, though there is a wide range of opinions on long-term effectiveness between eye care professionals, with little scientific evidence of long-term improvement of suppression, if the underlying cause is not addressed (strabismus, amblyopia, etc.).

The causes of exotropia are not fully understood. There are six muscles that control eye movement, four that move the eye up and down and two that move it left and right. All these muscles must be coordinated and working properly in order for the brain to see a single image. When one or more of these muscles doesn't work properly, some form of strabismus may occur. Strabismus is more common in children with disorders that affect the brain such as cerebral palsy, Down syndrome, hydrocephalus, and brain tumors. One study has found that children with exotropia are three times more likely to develop a psychiatric disorder in comparison with the general population.

Controversy has arisen regarding the selection and planning of surgical procedures, the timing of surgery and about what constitutes a favourable outcome.

1. Selection and planning

Some ophthalmologists, notably Ing and Helveston, favour a prescribed approach often involving multiple surgical episodes whereas others prefer to aim for full alignment of the eyes in one procedure and let the number of muscles operated upon during this procedure be determined by the size of the squint.

2. Timing and outcome

This debate relates to the technical anatomical difficulties of operating on the very young versus the possibility of an increased potential for binocularity associated with early surgery. Infants are often operated upon at the age of six to nine months of age and in some cases even earlier at three or four months of age. Some emphasize the importance of intervening early such as to keep the duration of the patient's abnormal visual experience to a minimum. Advocates of early surgery believe that those who have their surgery before the age of one are more likely to be able to use both eyes together post-operatively.

A Dutch study (ELISSS) compared early with late surgery in a prospective, controlled, non-randomized, multicenter trial and reported that:

"Children operated early had better gross stereopsis at age six as compared to children operated late. They had been operated more frequently, however, and a substantial number of children in both [originally-recruited] groups had not been operated at all."

Other studies also report better results with early surgery, notably Birch and Stager and Murray et al. but do not comment on the number of operations undertaken. A recent study on 38 children concluded that surgery for infantile esotropia is most likely to result in measureable stereopsis if patient age at alignment is not more than 16 months.

Another study found that for children with infantile esotropia early surgery decreases the risk of dissociated vertical deviation developing after surgery.

Aside the strabismus itself, there are other aspects or conditions that appear to improve after surgery or botulinum toxin eye alignment. Study outcomes have indicated that after surgery the child catches up in development of fine-motor skills (such as grasping a toy and handling a bottle) and of large-muscle skills (such as sitting, standing, and walking) in case a developmental delay was present before. Evidence also indicates that as of the age of six, strabismic children become less accepted by their peers, leaving them potentially exposed to social exclusion starting at this age unless their eye positioning is corrected by this time ("see also:" Psychosocial effects of strabismus).

Diplopia has a diverse range of ophthalmologic, infectious, autoimmune, neurological, and neoplastic causes.

A comprehensive eye examination including an ocular motility (i.e., eye movement) evaluation and an evaluation of the internal ocular structures will allow an eye doctor to accurately diagnose the exotropia. Although glasses and/or patching therapy, exercises, or prisms may reduce or help control the outward-turning eye in some children, surgery is often required.

There is a common form of exotropia known as "convergence insufficiency" that responds well to orthoptic vision therapy including exercises. This disorder is characterized by an inability of the eyes to work together when used for near viewing, such as reading. Instead of the eyes focusing together on the near object, one deviates outward.

"Consecutive exotropia" is an exotropia that arises after an initial esotropia. Most often it results from surgical overcorrection of the initial esotropia. It can be addressed with further surgery or with vision therapy; vision therapy has shown promising results if the consecutive exotropia is intermittent, alternating and of small magnitude. (Consecutive exotropia may however also spontaneously develop from esotropia, without surgery or botulinum toxin treatment.)

Because of the risks of surgery, and because about 35% of people require at least one more surgery, many people try vision therapy first. This consists of visual exercises. Although vision therapy is generally not covered by American health insurance companies, many large insurers such as Aetna have recently begun offering full or partial coverage in response to recent studies.

Strabismus surgery is sometimes recommended if the exotropia is present for more than half of each day or if the frequency is increasing over time. It is also indicated if a child has significant exotropia when reading or viewing near objects or if there is evidence that the eyes are losing their ability to work as a single unit (binocular vision). If none of these criteria are met, surgery may be postponed pending simple observation with or without some form of eyeglass and/or patching therapy. In very mild cases, there is a chance that the exotropia will diminish with time. The long-term success of surgical treatment for conditions such as intermittent exotropia is not well proven, and surgery can often result in a worsening of symptoms due to overcorrection. Evidence from the available literature suggests that unilateral surgery was more effective than bilateral surgery for individuals affected with intermittent exotropia.

The surgical procedure for the correction of exotropia involves making a small incision in the tissue covering the eye in order to reach the eye muscles. The appropriate muscles are then repositioned in order to allow the eye to move properly. The procedure is usually done under general anesthesia. Recovery time is rapid, and most people are able to resume normal activities within a few days. Following surgery, corrective eyeglasses may be needed and, in many cases, further surgery is required later to keep the eyes straight.

When a child requires surgery, the procedure is usually performed before the child attains school age. This is easier for the child and gives the eyes a better chance to work together. As with all surgery, there are some risks. However, strabismus surgery is usually a safe and effective treatment.

Temporary binocular diplopia can be caused by alcohol intoxication or head injuries, such as concussion (if temporary double vision does not resolve quickly, one should see an optometrist or ophthalmologist immediately). It can also be a side effect of benzodiazepines or opioids, particularly if used in larger doses for recreation, the anti-epileptic drugs Phenytoin and Zonisamide, and the anti-convulsant drug Lamotrigine, as well as the hypnotic drug Zolpidem and the dissociative drugs Ketamine and Dextromethorphan. Temporary diplopia can also be caused by tired and/or strained eye muscles or voluntarily. If diplopia appears with other symptoms such as fatigue and acute or chronic pain, the patient should see an ophthalmologist immediately.

Heterophoria is an eye condition in which the directions that the eyes are pointing at rest position, when "not" performing binocular fusion, are not the same as each other, or, "not straight". There can be esophoria, where the eyes tend to cross inward in the absence of fusion; exophoria, in which they diverge; or hyperphoria, in which one eye points up or down relative to the other. Phorias are known as 'latent squint' because the tendency of the eyes to deviate is kept latent by fusion. A person with two normal eyes has single vision (usually) because of the combined use of the sensory and motor systems. The motor system acts to point both eyes at the target of interest; any offset is detected visually (and the motor system corrects it). Heterophoria only occurs during dissociation of the left eye and right eye, when fusion of the eyes is absent. If you cover one eye (e.g. with your hand) you remove the sensory information about the eye's position in the orbit. Without this, there is no stimulus to binocular fusion, and the eye will move to a position of "rest". The difference between this position, and where it would be were the eye uncovered, is the heterophoria. The opposite of heterophoria, where the eyes are straight when relaxed and not fusing, is called orthophoria.

In contrast, fixation disparity is a very small deviation of the pointing directions of the eyes that is present while performing binocular fusion.

Heterophoria is usually asymptomatic. This is when it is said to be "compensated". When fusional reserve is used to compensate for heterophoria, it is known as compensating vergence. In severe cases, when the heterophoria is not overcome by fusional vergence, sign and symptoms appear. This is called decompensated heterophoria.

Heterophoria may lead to squint or also known as strabismus.

DVD typically becomes apparent between 18 months and three years of age, however, the difficulties of achieving the prolonged occlusion required for accurate detection in the very young, make it possible that onset is generally earlier than these figures suggest.

Dissociated vertical deviation (DVD) is an eye condition which occurs in association with a squint, typically infantile esotropia. The exact cause is unknown, although it is logical to assume it is from faulty innervation of eye muscles.

Refractive surgery causes only minimal size differences, similar to contact lenses. In a study performed on 53 children who had amblyopia due to anisometropia, surgical correction of the anisometropia followed by strabismus surgery if required led to improved visual acuity and even to stereopsis in many of the children ("see:" Refractive surgery#Children).

When the fusional vergence system can no longer hold back heterophoria, the phoria manifests. In this condition, the eyes deviate from the fixating position.

Esophoria is an eye condition involving inward deviation of the eye, usually due to extra-ocular muscle imbalance. It is a type of heterophoria.

Causes include:

- Refractive errors

- Divergence insufficiency

- Convergence excess; this can be due to nerve, muscle, congenital or mechanical anomalies.

Unlike esotropia, fusion is possible and therefore diplopia is uncommon.

Pseudomyopia refers to an intermittent and temporary shift in refractive error of the eye towards myopia, in which the focusing of light in front of the retina is due to a transient spasm of the ciliary muscle causing an increase in the refractive power of the eye. It may be either organic, through stimulation of the parasympathetic nervous system, or functional in origin, through eye strain or fatigue of ocular systems. It is common in young adults who have active accommodation, and classically occurs after a change in visual requirements, such as students preparing for an exam, or a change in occupation.

The major symptom is intermittent blurring of distance vision particularly noticeable after prolonged periods of near work, and symptoms of asthenopia. The vision may clear temporarily using concave (minus) lenses. The diagnosis is done by cycloplegic refraction using a strong cycloplegic like atropine or homatropine eye drops. Accommodative amplitude and facility may be reduced as a result of the ciliary muscle spasm.

Treatment is dependent on the underlying aetiology. Organic causes may include systemic or ocular medications, brain stem injury, or active ocular inflammation such as uveitis. Functional pseudomyopia is managed though modification of working conditions, an updated refraction, typically involving a reduction of a myopic prescription to some lower myopic prescription, or through appropriate ocular exercises.

Most patients are diagnosed by the age of 10 years and Duane's is more common in girls (60 percent of the cases) than boys (40 percent of the cases). A French study reports that this syndrome accounts for 1.9% of the population of strabismic patients, 53.5% of patients are female, is unilateral in 78% of cases, and the left eye (71.9%) is affected more frequently than the right. Around 10–20% of cases are familial; these are more likely to be bilateral than non-familial Duane syndrome. Duane syndrome has no particular race predilection.

The optokinetic response is a combination of a slow-phase and fast-phase eye movements. It is seen when an individual follows a moving object with their eyes, which then moves out of the field of vision at which point their eye moves back to the position it was in when it first saw the object. The reflex develops at about 6 months of age.

Optokinetic nystagmus (OKN) is nystagmus that occurs in response to a rotation movement. It is present normally. The optokinetic response allows the eye to follow objects in motion when the head remains stationary (e.g., observing individual telephone poles on the side of the road as one travels by them in a car, or observing stationary objects while walking past them).

Because the nerve emerges near the bottom of the brain, it is often the first nerve compressed when there is any rise in intracranial pressure. Different presentations of the condition, or associations with other conditions, can help to localize the site of the lesion along the VIth cranial nerve pathway.

The most common causes of VIth nerve palsy in adults are:

- More common: Vasculopathic (diabetes, hypertension, atherosclerosis), trauma, idiopathic.

- Less common: Increased intracranial pressure, giant cell arteritis, cavernous sinus mass (e.g. meningioma, Brain stem Glioblastoma aneurysm, metastasis), multiple sclerosis, sarcoidosis/vasculitis, postmyelography, lumbar puncture, stroke (usually not isolated), Chiari Malformation, hydrocephalus, intracranial hypertension, tuberculosis meningitis.

In children, Harley reports typical causes as traumatic, neoplastic (most commonly brainstem glioma), as well as idiopathic. Sixth nerve palsy causes the eyes to deviate inward (see: Pathophysiology of strabismus). Vallee et al. report that benign and rapidly recovering isolated VIth nerve palsy can occur in childhood, sometimes precipitated by ear, nose and throat infections.