Diplopia in the context of Extraocular muscles

Eye strain, also medically termed as asthenopia (from astheno- 'loss of strength' and -opia 'relating to the eyes'), is a common eye condition characterized by non-specific symptoms such as fatigue, pain in or around the eyes, blurred vision, headache, and occasional double vision.

These symptoms tend to arise after long-term use of computers, staring at phone screens, digital devices, reading, or other activities that involve extended visual tasks. Various causes contribute to eye strain, including uncorrected vision problems, digital device usage, environmental factors, and underlying health conditions. When concentrating on a visually intense task, such as continuously focusing on a book or computer monitor, the ciliary muscles and the extra-ocular muscles are strained, also contributing to the symptoms. These symptoms are broadly classified into external (related to the ocular surface) and internal symptom factors (related to eye muscles).

Strabismus is an eye 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 ("lazy eyes") and loss of depth perception. If onset is during adulthood, it is more likely to result in double vision.

Strabismus can occur out of muscle dysfunction (e.g., myasthenia gravis), 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). Another condition that produces similar symptoms is a cranial nerve disease. 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 test.

Mucormycosis, also known as black fungus, is a severe fungal infection that may cause fulminant fungal sinusitis, usually in people who are immunocompromised. It is most curable when diagnosed early. Symptoms depend on where in the body the infection occurs. It most commonly infects the nose, sinuses, eyes and brain resulting in a runny nose, one-sided facial swelling and pain, headache, fever, blurred vision, bulging or displacement of the eye (proptosis), and tissue death. Other sites of infection may include the lungs, stomach and intestines, and skin. The fatality rate is about 54%.

It is spread by spores of molds of the order Mucorales, most often through inhalation, contaminated food, or contamination of open wounds. These fungi are common in soils, decomposing organic matter (such as rotting fruit and vegetables), and animal manure, but usually do not affect people. It is not transmitted between people. Risk factors include diabetes with persistently high blood sugar levels or diabetic ketoacidosis, low white blood cells, cancer, organ transplant, iron overload, kidney problems, long-term steroids or use of immunosuppressants, and to a lesser extent in HIV/AIDS.

Binocular disparity is the difference between the images from the left and right eyes. This difference can be caused by one or more of the following:

• horizontal disparity: a difference caused by the left and right eyes looking from slightly different directions, which can cause depth differences between objects to be perceived;
• vertical disparity, which is associated with vertical misalignments and tilting the head; and
• cyclodisparity which is associated with unequal rotations of the two eyes around their visual axes;
• aniseikonia: a significant difference in the size of the retinal images in the two eyes.

Vertical disparity, cyclodisparity and aniseikonia can impair depth perception and may cause double vision (diplopia).

Esotropia (aka ET) (from Greek eso 'inward' and trope 'a turning') is a form of strabismus in which one or both eyes turn 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, orthoptic exercises, or eye muscle surgery.

Exotropia is a form of strabismus where one or both eyes are deviated outward. It is the opposite of esotropia and usually involves more severe axis deviation than exophoria. People with exotropia often experience crossed diplopia. Intermittent exotropia is a fairly common condition. "Sensory exotropia" occurs in the presence of poor vision in one eye. Infantile exotropia (sometimes called "congenital exotropia") is seen during the first year of life, and is less common than "essential exotropia" which usually becomes apparent several years later.

The brain's ability to see three-dimensional objects depends on proper alignment of the eyes. When both eyes are properly aligned and aimed at the same target, the visual portion of the brain fuses the two forms from the two eyes into a single image. When one eye turns inward, outward, upward, or downward, two different pictures are sent to the brain. Thus, the brain can no longer fuse the two images coming from the two eyes. This causes loss of depth perception and binocular vision. The term is from Greek exo meaning "outward" and trope meaning "a turning".