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).

Between 2 and 5% of the population in western countries have amblyopia. In the U.K., 90% of visual health appointments in the child are concerning amblyopia.

Depending on the chosen criterion for diagnosis, between 1 and 4% of the children have amblyopia.

Hypertropia may be either congenital or acquired, and misalignment is due to imbalance in extraocular muscle function. The superior rectus, inferior rectus, superior oblique, and inferior oblique muscles affect the vertical movement of the eyes. These muscles may be either paretic, restrictive (fibrosis) or overactive effect of the muscles. Congenital cases may have developmental abnormality due to abnormal muscle structure, usually muscle atrophy / hypertrophy or rarely, absence of the muscle and incorrect placement.

Specific & common causes include:

- Superior oblique Palsy / Congenital fourth nerve palsy

- Inferior oblique overaction

- Brown's syndrome

- Duane's retraction syndrome

- Double elevator palsy

- Fibrosis of rectus muscle in Graves Disease (most commonly inferior rectus is involved)

- Surgical trauma to the vertical muscles (e.g. during scleral buckling surgery or cataract surgery causing iatrogenic trauma to the vertical muscles).

Sudden onset hypertropia in a middle aged or elderly adult may be due to compression of the trochlear nerve and mass effect from a tumor, requiring urgent brain imaging using MRI to localise any space occupying lesion. It could also be due to infarction of blood vessels supplying the nerve, due to diabetes and atherosclerosis. In other instances it may be due to an abnormality of neuromuscular transmission, i.e., Myasthenia Gravis.

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

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.).

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.

Deprivation amblyopia (amblyopia ex anopsia) results when the ocular media become opaque, such as is the case with congenital cataract or corneal haziness. These opacities prevent adequate visual input from reaching the eye, and disrupt development. If not treated in a timely fashion, amblyopia may persist even after the cause of the opacity is removed. Sometimes, drooping of the eyelid (ptosis) or some other problem causes the upper eyelid to physically occlude a child's vision, which may cause amblyopia quickly. Occlusion amblyopia may be a complication of a hemangioma that blocks some or all of the eye. Other possible causes of deprivation and occlusion amblyopia include obstruction in the vitreous and aphakia. Deprivation amblyopia accounts for less than 3% of all individuals affected by amblyopia.

Refractive errors such as hyperopia and Anisometropia may be associated abnormalities found in patients with vertical strabismus.

The vertical miscoordination between the two eyes may lead to

- Strabismic amblyopia, (due to deprivation / suppression of the deviating eye)

- cosmetic defect (most noticed by parents of a young child and in photographs)

- Face turn, depending on presence of binocular vision in a particular gaze

- diplopia or double vision - more seen in adults (maturity / plasticity of neural pathways) and suppression mechanisms of the brain in sorting out the images from the two eyes.

- cyclotropia, a cyclotorsional deviation of the eyes (rotation around the visual axis), particularly when the root cause is an oblique muscle paresis causing the hypertropia.

Among fifth and sixth grade children convergence insufficiency is 13%. In studies that used standardized definitions of Convergence insufficiency, investigators have reported a prevalence of 4.2% to 6% in school and clinic settings. The standard definition of Convergence insufficiency is exophoria greater at near than at distance, a receded near point of convergence, and reduced convergence amplitudes at near.

"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.

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.

Esotropia is a form of strabismus in which one or both eyes turns inward. The condition can be constantly present, or occur intermittently, and can give the affected individual a "cross-eyed" appearance. It is the opposite of exotropia and usually involves more severe axis deviation than esophoria. Esotropia is sometimes erroneously called "lazy eye", which describes the condition of amblyopia—a reduction in vision of one or both eyes that is not the result of any pathology of the eye and cannot be resolved by the use of corrective lenses. Amblyopia can, however, arise as a result of esotropia occurring in childhood: In order to relieve symptoms of diplopia or double vision, the child's brain will ignore or "suppress" the image from the esotropic eye, which when allowed to continue untreated will lead to the development of amblyopia. Treatment options for esotropia include glasses to correct refractive errors (see accommodative esotropia below), the use of prisms and/or orthoptic exercises and/or eye muscle surgery. The term is from Greek "eso" meaning "inward" and "trope" meaning "a turning".

Use of high doses of opioid drugs such as morphine, oxycodone, heroin, or hydrocodone can cause ptosis. Pregabalin (Lyrica), an anticonvulsant drug, has also been known to cause mild ptosis.

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.

The symptoms and signs associated with convergence insufficiency are related to prolonged, visually demanding, near-centered tasks. They may include, but are not limited to, diplopia (double vision), asthenopia (eye strain), transient blurred vision, difficulty sustaining near-visual function, abnormal fatigue,

headache, and abnormal postural adaptation, among others. In some cases, difficulty with making eye contact have been noted as a complaint amongst sufferers.

Note that some Internet resources confuse convergence and divergence dysfunction, reversing them.

Ptosis occurs due to dysfunction of the muscles that raise the eyelid or their nerve supply (oculomotor nerve for levator palpebrae superioris and sympathetic nerves for superior tarsal muscle). It can affect one eye or both eyes and is more common in the elderly, as muscles in the eyelids may begin to deteriorate. One can, however, be born with ptosis. Congenital ptosis is hereditary in three main forms. Causes of congenital ptosis remain unknown. Ptosis may be caused by damage/trauma to the muscle which raises the eyelid, damage to the superior cervical sympathetic ganglion or damage to the nerve (3rd cranial nerve (oculomotor nerve)) which controls this muscle. Such damage could be a sign or symptom of an underlying disease such as diabetes mellitus, a brain tumor, a pancoast tumor (apex of lung) and diseases which may cause weakness in muscles or nerve damage, such as myasthenia gravis or Oculopharyngeal muscular dystrophy. Exposure to the toxins in some snake venoms, such as that of the black mamba, may also cause this effect.

Ptosis can be caused by the aponeurosis of the levator muscle, nerve abnormalities, trauma, inflammation or lesions of the lid or orbit. Dysfunctions of the levators may occur as a result of autoimmune antibodies attacking and eliminating the neurotransmitter.

Ptosis may be due to a myogenic, neurogenic, aponeurotic, mechanical or traumatic cause and it usually occurs isolated, but may be associated with various other conditions, like immunological, degenerative, or hereditary disorders, tumors, or infections

Acquired ptosis is most commonly caused by aponeurotic ptosis. This can occur as a result of senescence, dehiscence or disinsertion of the levator aponeurosis. Moreover, chronic inflammation or intraocular surgery can lead to the same effect. Also, wearing contact lenses for long periods of time is thought to have a certain impact on the development of this condition.

Congenital neurogenic ptosis is believed to be caused by the Horner syndrome. In this case, a mild ptosis may be associated with ipsilateral ptosis, iris and areola hypopigmentation and anhidrosis due to the paresis of the Mueller muscle. Acquired Horner syndrome may result after trauma, neoplastic insult, or even vascular disease.

Ptosis due to trauma can ensue after an eyelid laceration with transection of the upper eyelid elevators or disruption of the neural input.

Other causes of ptosis include eyelid neoplasms, neurofibromas or the cicatrization after inflammation or surgery. Mild ptosis may occur with aging.

A drooping eyelid can be one of the first signals of a third nerve palsy due to a cerebral aneurysm, that otherwise is asymptomatic and referred to as an oculomotor nerve palsy.

When this magnification difference becomes excessive the effect can cause diplopia, suppression, disorientation, eyestrain, headache, and dizziness and balance disorders.

Retinal image size is determined by many factors. The size and position of the object being viewed affects the characteristics of the light entering the system. Corrective lenses affect these characteristics and are used commonly to correct refractive error. The optics of the eye including its refractive power and axial length also play a major role in retinal image size.

Aniseikonia can occur naturally or be induced by the correction of a refractive error, usually anisometropia (having significantly different refractive errors between each eye) or antimetropia (being myopic (nearsighted) in one eye and hyperopic (farsighted) in the other.) Meridional aniseikonia occurs when these refractive differences only occur in one meridian (see astigmatism). Refractive surgery can cause aniseikonia in much the same way that it is caused by glasses and contacts.

One cause of significant anisometropia and subsequent aniseikonia has been aphakia. Aphakic patients do not have a crystalline lens. The crystalline lens is often removed because of opacities called cataracts. The absence of this lens left the patient highly hyperopic (farsighted) in that eye. For some patients the removal was only performed on one eye, resulting in the anisometropia / aniseikonia. Today, this is rarely a problem because when the lens is removed in cataract surgery, an intraocular lens, or IOL is left in its place.

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.

Sometimes asthenopia can be due to specific visual problems—for example, uncorrected refraction errors or binocular vision problems such as accommodative insufficiency or heterophoria. It is often caused by the viewing of monitors such as those of computers or phones for prolonged periods of time.

There is no good evidence for any preventive actions, since it appears this is a natural response to aging changes in the vitreous. Posterior vitreous detachment (PVD) has been estimated to occur in over 75 per cent of the population over age 65, that PVD is essentially a harmless condition (although with some disturbing symptoms), and that it does not normally threaten sight. However, since epiretinal membrane appears to be a protective response to PVD, where inflammation, exudative fluid, and scar tissue is formed, it is possible that NSAIDs may reduce the inflammation response. Usually there are flashing light experiences and the emergence of floaters in the eye that herald changes in the vitreous before the epiretinal membrane forms g

The eye, like any other optical system, suffers from a number of specific optical aberrations. The optical quality of the eye is limited by optical aberrations, diffraction and scatter. Correction of spherocylindrical refractive errors has been possible for nearly two centuries following Airy's development of methods to measure and correct ocular astigmatism. It has only recently become possible to measure the aberrations of the eye and with the advent of refractive surgery it might be possible to correct certain types of irregular astigmatism.

The appearance of visual complaints such as halos, glare and monocular diplopia after corneal refractive surgery has long been correlated with the induction of optical aberrations. Several mechanisms may explain the increase in the amount of higher-order aberrations with conventional eximer laser refractive procedures: a change in corneal shape toward oblateness or prolateness (after myopic and hyperopic ablations respectively), insufficient optical zone size and imperfect centration. These adverse effects are particularly noticeable when the pupil is large.

Eye strain, also known as asthenopia, is an eye condition that manifests through nonspecific symptoms such as fatigue, pain in or around the eyes, blurred vision, headache, and occasional double vision. Symptoms often occur after reading, computer work, or other close activities that involve tedious visual tasks.

When concentrating on a visually intense task, such as continuously focusing on a book or computer monitor, the ciliary muscle tightens. This can cause the eyes to get irritated and uncomfortable. Giving the eyes a chance to focus on a distant object at least once an hour usually alleviates the problem.

A CRT computer monitor with a low refresh rate (<70Hz) or a CRT television can cause similar problems because the image has a visible flicker. Aging CRTs also often go slightly out of focus, and this can cause eye strain. LCDs do not go out of focus but are also susceptible to flicker if the backlight for the LCD uses PWM for dimming. This causes the backlight to turn on and off for shorter intervals as the display becomes dimmer, creating noticeable flickering which causes eye fatigue.

A page or photograph with the same image twice slightly displaced (from a printing mishap, or a camera moving during the shot) can cause eye strain by the brain misinterpreting the image fault as diplopia and trying in vain to adjust the sideways movements of the two eyeballs to fuse the two images into one. The word is from Greek "asthen-opia: ἀσθεν-ωπία" = "weak-eye-condition".

Eye strain can happen with a blurred image (including images deliberately partly blurred for censorship), due to the ciliary muscle tightening trying in vain to focus the blurring out.

A rostral lesion within the midbrain may affect the convergence center thus causing bilateral divergence of the eyes which is known as the WEBINO syndrome (Wall Eyed Bilateral INO) as each eye looks at the opposite "wall".

If the lesion affects the PPRF (or the abducens nucleus) and the MLF on the same side (the MLF having crossed from the opposite side), then the "one and a half syndrome" occurs which, simply put, involves paralysis of all conjugate horizontal eye movements other than abduction of the eye on the opposite side to the lesion.

Quantitative comparisons between different eyes and conditions are usually made using RMS (root mean square). To measure RMS for each type of aberration involves squaring the difference between the aberration and mean value and averaging it across the pupil area. Different kinds of aberrations may have equal RMS across the pupil but have different effects on vision, therefore, RMS error is unrelated to visual performance. The majority of eyes have total RMS values less than 0.3 µm.

The most common method of classifying the shapes of aberration maps is to consider each map as the sum of fundamental shapes or basis functions. One popular set of basis functions are the Zernike polynomials. Each aberration may be positive or negative in value and induces predictable alterations in the image quality.

Because there is no limit to the number of terms that may be used by Zernike polynomials, vision scientists use the first 15 polynomials, based on the fact that they are enough to obtain a highly accurate description of the most common aberrations found in human eye. Among these the most important Zernike coefficients affecting visual quality are coma, spherical aberration, and trefoil.

Zernike polynomials are usually expressed in terms of polar coordinates (ρ,θ), where ρ is radial coordinate and θ is the angle. The advantage of expressing the aberrations in terms of these polynomials includes the fact that the polynomials are independent of one another. For each polynomial the mean value of the aberration across the pupil is zero and the value of the coefficient gives the RMS error for that particular aberration (i.e. the coefficients show the relative contribution of each Zernike mode to the total wavefront error in the eye). However these polynomials have the disadvantage that their coefficients are only valid for the particular pupil diameter they are determined for.

In each Zernike polynomial formula_1, the subscript n is the order of aberration, all the Zernike polynomials in which n=3 are called third-order aberrations and all the polynomials with n=4, fourth order aberrations and so on. formula_2 and formula_3 are usually called secondary Astigmatism and should not cause confusion. The superscript m is called the angular frequency and denotes the number of times the Wavefront pattern repeats itself.

List of Zernike modes and their common names: