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Annually, more than three million of United States citizens are diagnosed with Strabismus. Strabismus, commonly known by the term “crossed eye,” is a physiological disorder where the visual axes of the eyes are not parallel, causing a person’s eye to appear misaligned or pointing in different directions, usually due to poor eye muscle control. A patient with strabismus may report symptoms such as, having a misalignment of the eyes, blurred or double vision, and eye strains or headaches. Normally, there are two categorizations of this disorder; one of the two being tropias, which are misalignments of the eye that are always present, and the other being phorias, which are misalignments of the eye that are only present under specific conditions. Strabismus usually develops in infants and young children, most often by age 3. Yet  older children and adults can develop the condition as well. This dysfunction may get worse overtime but can usually be corrected with early treatment. There are various ways strabismus can be acquired. One of the forms of strabismus is congenital strabismus, which is present at birth. This form being the most common seen in adults, due to residual childhood strabismus. Some other causes of strabismus include: nerve injury, abnormalities of the muscles controlling the eye, direct or secondary trauma to the eye and surrounding structures, and neurological disorders or chromosomal aberrations, such as, Cerebral Palsy, Brain cancer, and Down syndrome. Children with disorders that may affect the brain are commonly subjected to strabismus. Previous records of Strabismus in the family line can increase prevalence in by as high as 25%. Having extreme uncorrected refractive errors, medical conditions that involve strokes and head injuries, and complications during pregnancy put you at risk of obtaining strabismus. There are two different types of strabismus, which are horizontal and vertical. Horizontal strabismus includes Esotropia and Exotropia. Esotropia is when the eye is pulled inward or towards the nose; it is most often identified in children between the ages of two and four, but it can occur at any age. Exotropia is when the eye is pulled outward or towards the temple, where the angle of degree may vary from small to large, meaning it can either be highly noticeable or slightly present. Vertical Strabismus categorizes both  hypotropia and hypertropia. Hypotropia is when the eye is pulled downward, meanwhile in hypertropia, the eye is pulled upward. Hypertropia is common in adults diagnosed with strabismus that has been in a latent stage.In strabismus, as a result of the eyes being at different points, the brain receives two contrasting images. Strabismus affects vision differently in children versus adults. In children the child’s brain trains itself to disregard the image the misaligned eye forms, only seeing the image the dominant eye creates. This leads to loss of depth perception and also causes double vision. In adults, since they are used to receiving pictures from both eyes, their brain can no longer ignore the picture from the turned eye. This causes them to often have double vision.Strabismus can be commonly corrected using prisms, which are made of two nonparallel refracting surfaces that intersect at the apex and deflect light rays passing through them, with the rays always bending toward the base of the prism. This structure allows them to alter the pathway of light, moving images onto the retina of the deviated eye or within range to allow the fusion of an image. Strabismic deviations are commonly expressed in prism diopters (?), which is the unit of measurement for the power of an ophthalmic prism. One diopter of prism is equal to deviation, in centimeters (cm), of a light ray passing through the prism, measured one meter, or 100 cm, away from the prism. As important as the amount of prism, is the direction of prism. The prism must displace viewed objects in the proper direction to achieve the desired visual correction. Prism direction can be specified using the prescriber’s method. The prescriber’s method specifies the direction if the prism in terms of the base, using base-up, base-down, base-in, referring to the direction of the nose, and base-out, referring to the direction of the temple. Prism lenses are necessary in aiding the eyes to work together. With strabismus, one or both eyes tend to pull up apart, due to muscle imbalances. The use of prism in the lenses tricks the brain into thinking the eyes are working together by shifting the image in the appropriate direction. They are often used for the relief of symptoms or phorias and long-standing strabismus of childhood. Prisms are most commonly put into glasses but a better alternative could be contacts.Contact lenses have been credited with providing better vision, more normal retinal image size, less distortion and a wider field of view than glasses. They can also help the patient’s visual development, visual functioning and binocularity. Contacts lenses without prisms have made fusion possible in a case study of an 11-year-old African American female at the Illinois Eye Institute diagnosed with compound myopic astigmatism, constant alternating exotropia, and myopic degeneration OU.With continued contact lens wear and compliance to a vision therapy program, it is possible to maintain ocular alignment and increase binocular function. Prism incorporated into spectacles facilitates fusion, but can sometimes cause symptoms of eye strain, resulting in patients who are unable to wear such a correction. Patients with vertical heterophorias often present with symptoms of diplopia, or double vision, due to difficulties in fusing images. In a study, a base down prism was used in a gas permeable contact lens to correct for a vertical heterophoria which is an uncommon practice, but can significantly improve functional vision and quality of life for patients unable to adapt to spectacle prism. A 55-year-old female with a history of diplopia secondary to a decompensating vertical heterophoria presented for a contact lens evaluation to eliminate the use of spectacles. A gas permeable contact lens with 4.0 prism diopters of base down prism was fit over the right eye, and the diplopia was eliminated. Some studies have supported the claim that adding prism into contact lens can significantly improve the quality of life and vision of a person who has strabismus. Specialeyes has taken an approach by using a base down prism in their custom soft toric contact lens, each coming with a standard with 1.0 diopter of base down prism. The design of the lens enables the company to vary or customize the amount of prism in their lenses up to 4.0 diopters. The increase in amount of base down prism is generally used to stabilize the the toric or multifocal toric contact lens on the eye. The higher amount of irregular rotation, the greater amount of additional prism will be needed to correct the unstable lens. They also stated that adding additional prism to soft toric contact lenses may prove beneficial for treating a binocular dysfunction such as vertical heterophoria which is a vertical misalignment of the eyes causing vertigo, which is ringing in the ears, diplopia, headaches, fatigue, etc. In an additional study by Specialeyes a fully customized soft multifocal contact lens design with customizable base down prism was tested on a 41-year-old Caucasian male. This case was used to reduce asthenopia and diplopia for a patient wearing multifocal toric contact lenses. The multifocal lenses did improve visual performance for the patient however the patient experienced binocular complications. A vertical deviation was then determined and the customization of vertical prism was changed from 1.0 diopters to 1.75 diopters in order to address the vertical deviation. With this binocularity and further visual performance improved and the patient experienced improved near and intermediate vision. Another study tested a small amount of vertical base down prism, which can be up to 4.0 diopters, incorporated into a rigid gas permeable contact lens to align the diplopic images of a 49-year-old female patient. Due to a post retinal detachment surgery the patient experienced significant binocular vertical diplopia at both distance and near, causing her to abandon contact lens wear. Later on, while performing a vertical alignment test, a left hyperphoria of four prism diopters at both distance and near revealed. These issues was then eliminated by the incorporation of the appropriate amount of vertical prism into a bitoric rigid gas permeable contact lens.Strabismus affects approximately 2-4% of the population in the U.S, which is  approximately 1 in every 25 to 50 individuals. Strabismus is a disorder causing a person’s eye to appear misaligned or pointing in different directions, usually due to poor eye muscle control. Patients with strabismus have a misalignment of the eyes, and can experience blurred or double vision, and eye strains or headaches. Strabismus can be acquired either at birth or secondary to trauma and neurological complications. Strabismus can be commonly corrected using prisms, which alter the pathway of light, moving images onto the retina of the deviated eye or within range to allow the fusion of an image. Prisms are most commonly put into glasses but a better alternative could be contacts. Contact lenses have been credited with providing better vision, more normal retinal image size, less distortion and a wider field of view than glasses. They can also help the patient’s visual development, visual functioning and binocularity. Prism incorporated into spectacles facilitates fusion, but can sometimes cause symptoms of eye strain, resulting in patients who are unable to wear such a correction. Specialeyes has taken an approach on this topic by using a fully customized soft multifocal contact lens design with customizable base down prism was tested on a 41-year-old Caucasian male with a vertical deviation. With this binocularity and further visual performance improved. Thus, although it is a uncommon practice, base down prism can be placed in contact lenses in order to treat strabismus.

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