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.

Many people with amblyopia, especially those who only have a mild form, are not aware they have the condition until tested at older ages, since the vision in their stronger eye is normal. People typically have poor stereo vision, however, since it requires both eyes. Those with amblyopia further may have, on the affected eye, poor pattern recognition, poor visual acuity, and low sensitivity to contrast and motion.

Amblyopia is characterized by several functional abnormalities in spatial vision, including reductions in visual acuity, contrast sensitivity function, and vernier acuity, as well as spatial distortion, abnormal spatial interactions, and impaired contour detection. In addition, individuals with amblyopia suffer from binocular abnormalities such as impaired stereoacuity (stereoscopic acuity) and abnormal binocular summation. Also, a crowding phenomenon is present.

These deficits are usually specific to the amblyopic eye. However, subclinical deficits of the "better" eye have also been demonstrated.

People with amblyopia also have problems of binocular vision such as limited stereoscopic depth perception and usually have difficulty seeing the three-dimensional images in hidden stereoscopic displays such as autostereograms. Perception of depth, however, from monocular cues such as size, perspective, and motion parallax remains normal.

Amblyopia has three main causes:

- Strabismic: by strabismus (misaligned eyes)

- Refractive: by anisometropia (difference of a certain degree of nearsightedness, farsightedness, or astigmatism), or by significant amount of equal refractive error in both eyes

- Deprivational: by deprivation of vision early in life by vision-obstructing disorders such as congenital cataract

Convergence insufficiency or convergence disorder is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn towards each other, or sustain convergence.

Concomitant esotropia – that is, an inward squint that does not vary with the direction of gaze – mostly sets in before 12 months of age (this constitutes 40% of all strabismus cases) or at the age of three or four. Most patients with "early-onset" concomitant esotropia are emmetropic, whereas most of the "later-onset" patients are hyperopic. It is the most frequent type of natural strabismus not only in humans, but also in monkeys.

Concomitant esotropia can itself be subdivided into esotropias that are ether "constant," or "intermittent."

- Constant esotropia

- Intermittent esotropia

A patient can have a constant esotropia for reading, but an intermittent esotropia for distance (but rarely vice versa).

When observing a person with strabismus, the misalignment of the eyes may be quite apparent. A patient 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/or 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.

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.

Although astigmatism may be asymptomatic, higher degrees of astigmatism may cause symptoms such as blurry vision, squinting, eye strain, fatigue, or headaches. Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches.

Concomitant esotropias can arise as an initial problem, in which case they are designated as 'Primary,' as a consequence of loss or impairment of vision, in which case they are designated as 'Secondary,' or following overcorrection of an initial Exotropia in which case they are described as being 'Consecutive'. The vast majority of esotropias are primary.

A spasm of accommodation (also known as a ciliary spasm, an accommodation, or accommodative spasm) is a condition in which the ciliary muscle of the eye remains in a constant state of contraction. Normal accommodation allows the eye to "accommodate" for near-vision. However in a state of perpetual contraction, the ciliary muscle cannot relax when viewing distant objects. This causes vision to blur when attempting to view objects from a distance. This may cause pseudomyopia or latent hyperopia.

Although antimuscarinic drops (homoatropine 5%) can be applied topically to relax the muscle, this leaves the individual without any accommodation and, depending on refractive error, unable to see well at near distances. Also, excessive pupil dilation may occur as an unwanted side effect. This dilation may pose a problem since a larger pupil is less efficient at focusing light (see pupil, aperture, and optical aberration for more.)

Patients who have accommodative spasm may benefit from being given glasses or contacts that account for the problem or by using vision therapy techniques to regain control of the accommodative system..

Possible clinical findings include:

Normal Amplitude of accommodation and Near point of convergence

Reduced Negative relative accommodation

Difficulty clearing plus on facility testing

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 patient age at which the damage occurs: patients whose vision is lost or impaired at birth are more likely to develop esotropia, whereas patients 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 afflicted 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%.

In with-the-rule astigmatism, the eye has too much "plus" cylinder in the horizontal axis relative to the vertical axis (i.e., the eye is too "steep" along the vertical meridian relative to the horizontal meridian). This causes vertical beams of light to focus anterior to (in front of) horizontal beams of light, within the eye. This problem may be corrected using spectacles which have a "minus" cylinder placed on this horizontal axis. The effect of this will be that when a vertical beam of light in the distance travels towards the eye, the "minus" cylinder (which is placed with its axis lying horizontally – in line with the patient's excessively steep horizonal axis/vertical meridian) will cause this vertical beam of light to slightly "diverge", or "spread out vertically", before it reaches the eye. This compensates for the fact that the patient's eye converges light more powerfully in the vertical meridian than the horizontal meridian. Hopefully, after this, the eye will focus all light on the same location at the retina, and the patient's vision will be less blurred.

In against-the-rule astigmatism, a plus cylinder is added in the horizontal axis (or a minus cylinder in the vertical axis).

Axis is always recorded as an angle in degrees, between 0 and 180 degrees in a counter-clockwise direction. Both 0 and 180 degrees lie on a horizontal line at the level of the center of the pupil, and as seen by an observer, 0 lies on the right of both the eyes.

Irregular astigmatism, which is often associated with prior ocular surgery or trauma, is also a common naturally occurring condition. The two steep hemimeridians of the cornea, 180° apart in regular astigmatism, may be separated by less than 180° in irregular astigmatism (called "nonorthogonal" irregular astigmatism); and/or the two steep hemimeridians may be asymmetrically steep—that is, one may be significantly steeper than the other (called "asymmetric" irregular astigmatism). Irregular astigmatism is quantified by a vector calculation called topographic disparity.

The first symptoms most people notice are difficulty reading fine print, particularly in low light conditions, eyestrain when reading for long periods, blurring of near objects or temporarily blurred vision when changing the viewing distance. Many extreme presbyopes complain that their arms have become "too short" to hold reading material at a comfortable distance.

Presbyopia, like other focal imperfections, becomes less noticeable in bright sunlight when the pupil becomes smaller. As with any lens, increasing the focal ratio of the lens increases depth of field by reducing the level of blur of out-of-focus objects (compare the effect of aperture on depth of field in photography).

The onset of correction for presbyopia varies among those with certain professions and those with miotic pupils. In particular, farmers and homemakers seek correction later, whereas service workers and construction workers seek eyesight correction earlier. Scuba divers with interest in underwater photography may notice presbyopic changes while diving before they recognize the symptoms in their normal routines due to the near focus in low light conditions.

An eye that has no refractive error when viewing distant objects is said to have "emmetropia" or be "emmetropic" meaning the eye is in a state in which it can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object in this case is defined as an object located beyond 6 meters, or 20 feet, from the eye, since the light from those objects arrives as essentially parallel rays when considering the limitations of human perception.

An eye that has refractive error when viewing distant objects is said to have "ametropia" or be "ametropic". This eye cannot focus parallel rays of light (light from distant objects) on the retina, or needs accommodation to do so.

The word "ametropia" can be used interchangeably with "refractive error". Types of ametropia include myopia, hyperopia and astigmatism. They are frequently categorized as spherical errors and cylindrical errors:

- Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refractive error frequently have blurry vision.

- Nearsightedness: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness. This can arise from a cornea or crystalline lens with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia). Myopia can be corrected with a concave lens which causes the divergence of light rays before they reach the cornea.

- Farsightedness: When the optics are too weak for the length of the eyeball, one has hyperopia or farsightedness. This can arise from a cornea or crystalline lens with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia). This can be corrected with convex lenses which cause light rays to converge prior to hitting the cornea.

- Presbyopia: When the flexibility of the lens declines, typically due to age. The individual would experience difficulty in near vision, often relieved by reading glasses, bifocal, or progressive lenses.

- Cylindrical errors cause astigmatism, when the optical power of the eye is too powerful or too weak across one meridian, such as if the corneal curvature tends towards a cylindrical shape. The angle between that meridian and the horizontal is known as the axis of the cylinder.

- Astigmatism: A person with astigmatic refractive error sees lines of a particular orientation less clearly than lines at right angles to them. This defect can be corrected by refracting light more in one meridian than the other. Cylindrical lenses serve this purpose.

Accommodative infacility is the inability to change the accommodation of the eye with enough speed and accuracy to achieve normal function. This can result in visual fatigue, headaches, and difficulty reading. The delay in accurate accommodation also makes vision blurry for a moment when switching between distant and near objects. The duration and extent of this blurriness depends on the extent of the deficit.

Presbyopia is a condition associated with aging of the eye that results in progressively worsening ability to focus clearly on close objects. Symptoms include difficulty reading small print, having to hold reading material farther away, headaches, and eyestrain. Different people will have different degrees of problems. Other types of refractive errors may exist at the same time as presbyopia.

Presbyopia is a natural part of the aging process. It is due to hardening of the lens of the eye causing the eye to focus light behind rather than on the retina when looking at close objects. It is a type of refractive error along with nearsightedness, farsightedness, and astigmatism. Diagnosis is by an eye examination.

Treatment is typically with eye glasses. The eyeglasses used have higher focusing power in the lower portion of the lens. Off the shelf reading glasses may be sufficient for some.

People over 35 are at risk for developing presbyopia and all people become affected to some degree. The condition was mentioned as early as the writings of Aristotle in the 4th century BC. Glass lenses first came into use for the problem in the late 13th century.

Aphakia is the absence of the lens of the eye, due to surgical removal, a perforating wound or ulcer, or congenital anomaly. It causes a loss of accommodation, far sightedness (hyperopia), and a deep anterior chamber. Complications include detachment of the vitreous or retina, and glaucoma.

Babies are rarely born with aphakia. Occurrence most often results from surgery to remove congenital cataract (clouding of the eye's lens, which can block light from entering the eye and focusing clearly). Congenital cataracts usually develop as a result of infection of the fetus or genetic reasons. It is often difficult to identify the exact cause of these cataracts, especially if only one eye is affected.

People with aphakia have relatively small pupils and their pupils dilate to a lesser degree.

Streff syndrome is a vision condition primarily exhibited by children under periods of visual or emotional stress.

Frequently patients will have reduced stereopsis, large accommodative lag on dynamic retinoscopy, and a reduced visual field (tubular or spiral field). Streff Syndrome was first described in 1962 by an optometrist, Dr. John Streff as Non-malingering syndrome. In 1962, Dr. Streff and Dr. Richard Apell expanded the concept to add early adaptive syndrome as a precursor to Streff syndrome. Dr. Streff believed the visual changes were induced by stress from reading. There is dispute on the taxonomy of functional vision defects. Some research indicates that Streff syndrome may be caused by a dysfunction in the magnocellular pathway of the retinal ganglion cells. These cells are only 10% of the retinal nerve cells and register motion detection.

Early Adaptive Syndrome

Most optometrists agree that Streff syndrome is a generalized reduction in visual performance that is not caused by structural damage. It is a disease involving vision distress primarily of the accommodation system. Hans Selye described stress, distress and eustress. It is most common in girls ages 8 to 14. Hand held reading material is often positioned excessively close. Reading aloud shows signs of elevated pitch and stumbling over common words. History of homework avoidance and falling class performance are often present. If the patient is directed to read aloud and +.50 lenses are then used, there is usually a dramatic improvement as observed by patient and parent. Abnormal results on color vision or visual field testing is not uncommon. Visual field often presents as constricted 'tubular' at multiple test distances. The poor visual performance is understood as distress, and treatments are usually to provide the patient with low powered reading glasses. The "relaxing" nature of reading glasses is believed to reduce the near vision stress and allow normal function. The emotional effects of chronic near vision stress are also reduced.

The "non-Malingering" name is a refutation that the patient is malingering.

There are many causes of blurred vision:

- Use of atropine or other anticholinergics

- Presbyopia—Difficulty focusing on objects that are close. Common in the elderly. (Accommodation tends to decrease with age.)

- Cataracts—Cloudiness over the eye's lens, causing poor night-time vision, halos around lights, and sensitivity to glare. Daytime vision is eventually affected. Common in the elderly.

- Glaucoma—Increased pressure in the eye, causing poor night vision, blind spots, and loss of vision to either side. A major cause of blindness. Glaucoma can happen gradually or suddenly—if sudden, it is a medical emergency.

- Diabetes—Poorly controlled blood sugar can lead to temporary swelling of the lens of the eye, resulting in blurred vision. While it resolves if blood sugar control is reestablished, it is believed repeated occurrences promote the formation of cataracts (which are not temporary).

- Diabetic retinopathy—This complication of diabetes can lead to bleeding into the retina. Another common cause of blindness.

- Hypervitaminosis A—Excess consumption of vitamin A can cause blurred vision.

- Macular degeneration—Loss of central vision, blurred vision (especially while reading), distorted vision (like seeing wavy lines), and colors appearing faded. The most common cause of blindness in people over age 60.

- Eye infection, inflammation, or injury.

- Sjögren's syndrome, a chronic autoimmune inflammatory disease that destroys moisture producing glands, including lacrimal (tear)

- Floaters—Tiny particles drifting across the eye. Although often brief and harmless, they may be a sign of retinal detachment.

- Retinal detachment—Symptoms include floaters, flashes of light across your visual field, or a sensation of a shade or curtain hanging on one side of your visual field.

- Optic neuritis—Inflammation of the optic nerve from infection or multiple sclerosis. You may have pain when you move your eye or touch it through the eyelid.

- Stroke or transient ischemic attack

- Brain tumor

- Toxocara—A parasitic roundworm that can cause blurred vision

- Bleeding into the eye

- Temporal arteritis—Inflammation of an artery in the brain that supplies blood to the optic nerve.

- Migraine headaches—Spots of light, halos, or zigzag patterns are common symptoms prior to the start of the headache. A retinal migraine is when you have only visual symptoms without a headache.

- Myopia—Blurred vision may be a systemic sign of local anaesthetic toxicity

- Reduced blinking—Lid closure that occurs too infrequently often leads to irregularities of the tear film due to prolonged evaporation, thus resulting in disruptions in visual perception.

- Carbon monoxide poisoning—Reduced oxygen delivery can effect many areas of the body including vision. Other symptoms caused by CO include vertigo, hallucination and sensitivity to light.

Refractive error, also known as refraction error, is a problem with focusing light accurately onto the retina due to the shape of the eye. The most common types of refractive error are near-sightedness, far-sightedness, astigmatism, and presbyopia. Near-sightedness results in far away objects being blurry, far-sightedness and presbyopia result in close objects being blurry, astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision, headaches, and eye strain.

Near-sightedness is due to the length of the eyeball being too long, far-sightedness the eyeball too short, astigmatism the cornea being the wrong shape, and presbyopia aging of the lens of the eye such that it cannot change shape sufficiently. Some refractive errors occur more often among those whose parents are affected. Diagnosis is by eye examination.

Refractive errors are corrected with eyeglasses, contact lenses, or surgery. Eyeglasses are the easiest and safest method of correction. Contact lenses can provide a wider field of vision; however they are associated with a risk of infection. Refractive surgery permanently changes the shape of the cornea.

The number of people globally with refractive errors has been estimated at one to two billion. Rates vary between regions of the world with about 25% of Europeans and 80% of Asians affected. Near-sightedness is the most common disorder. Rates among adults are between 15-49% while rates among children are between 1.2-42%. Far-sightedness more commonly affects young children and the elderly. Presbyopia affects most people over the age of 35. The number of people with refractive errors that have not been corrected was estimated at 660 million (10 per 100 people) in 2013. Of these 9.5 million were blind due to the refractive error. It is one of the most common causes of vision loss along with cataracts, macular degeneration, and vitamin A deficiency.

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.

Cycloplegia is paralysis of the ciliary muscle of the eye, resulting in a loss of accommodation. Because of the paralysis of the ciliary muscle, the curvature of the lens can no longer be adjusted to focus on nearby objects. This results in similar problems as those caused by presbyopia, in which the lens has lost elasticity and can also no longer focus on close-by objects. Cycloplegia with accompanying mydriasis (dilation of pupil) is usually due to topical application of muscarinic antagonists such as atropine and cyclopentolate.

Accommodative insufficiency (AI) involves the inability of the eye to focus properly on an object. AI is generally considered separate from presbyopia, but mechanically both conditions represent a difficulty engaging the near vision system (accommodation) to see near objects clearly. However, AI is the term used for a patient where normal near vision is expected, whereas presbyopia is specifically the loss of accommodation due to age. Approximately 80 percent of children diagnosed with convergence excess also demonstrate AI, a relationship attributed to the accommodative convergence.

Eye floaters and loss of accommodation are among the earliest symptoms. The disease may progress to severe uveitis with pain and photophobia. Commonly the eye remains relatively painless while the inflammatory disease spreads through the uvea, where characteristic focal infiltrates in the choroid named Dalén-Fuchs nodules can be seen. The retina, however, usually remains uninvolved, although perivascular cuffing of the retinal vessels with inflammatory cells may occur. Papilledema, secondary glaucoma, vitiligo, and poliosis of the eyelashes may accompany SO.