Dictionary of terms relating to
eyesight and vision

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The eye's ability to direct and coordinate movement as it quickly and voluntarily shift from one target to another.

Saccades Dysfunction

A condition in which the individual's ability to scan along a printed page and move his eyes from point to point is inadequate. Symptoms include frequent loss of place while reading, skip or transpose words, and have difficulty comprehending because of an inaccurate eye movement. Vision therapy is an effective treatment option. (See "Ocular Motor Dysfunction")

Saccadic Test

Measures the eyes ability to move quickly and precisely from point to point.


Sarcoidosis is an inflammatory disease, an immune system disorder, characterized by small lumps or granulomas in lymph nodes and other organs. More on Sarcoidosis


The white protective covering of the eye. (See the diagram of the eye)

Slit Lamp (Biomicroscope)

This instrument can examine ocular tissue from the front of the cornea to the back of the lens. A narrow "slit" beam of very bright light produced by a lamp. This beam is focused on to the eye which is then viewed under magnification with a microscope. A joystick control is employed to enable instrument to be moved left-right and up-down. A chin rest, head rest and fixation target is also required. Some slit lamps have a tilting mechanism to enable the lamp to be directed from different angles.

Snellen eye chart

A Snellen chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Hermann Snellen who developed the chart in 1862.

Spatial Relation

The ability to judge the relative position of one object to another and the internal awareness of the two sides of the body. These skills allow the individual to develop the concepts of right, left, front, back, up, and down. This ability is needed in reading and math. (See "Directionality/Laterality")


An ophthalmic lens with no cylindrical power or addition. It has the same power in all parts of the lens.


To be unable to direct both eyes simultaneously toward a point. Also known as strabismus (turned eye). For more information, see "Strabismus".


The ability to perceive a three dimensional depth which requires adequate fusion (union) of the images from each eye.

Stereopsis Test

Measures depth perception that is dependent on the accuracy of eye teaming.

Strabismus (clinical condition)

Turned eye (s), the eyes are misaligned. It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. In strabismus, the eyes send conflicting images to the brain, and the brain cannot combine these images as it would in normal vision. The brain compensates by ignoring one image in favor of the other, causing a loss of depth perception. Strabismus in more common in children, and affects four percent of all children (although it may also appear later in life).

It is characterized by using the following categories:
Strabismus is also known as squint. It may also be referred to as cross-eyes (convergent- turning inward) or wall eyes (divergent- turning outward). Treatment options may include one or more of the following: optical lenses, bi-focal lenses, prisms, surgery, vision therapy, or Botulinum Toxin Type A (Oculinum, Botox®) injections. For more information about Strabismus, please click here.  (See "Esotropia", "Exotropia", "Hypertropia", and "Hypotropia")

Streff Syndrome

Named after the optometrist who originally described it, Dr. John Streff. This functional vision loss is also known as Non-Malingering Syndrome. Signs include reduced visual acuity in both eyes at distance and near. The visual acuity at near is more reduced than the distance acuity. Frequently patients will have reduced stereopsis, large accommodative lag on dynamic retinoscopy, and a reduced visual field (tubular or spiral field). The syndrome is associated with a visual or emotional stress occurring in the child's life. It is more prominent in girls (ages 7-13) than boys. Treatment includes a low plus lens and/or vision therapy. This condition is sometimes incorrectly diagnosed by doctors as hysterical amblyopia.

Subconjunctival Hemorrhage

A blood spot on the eye. It occurs when a small blood vessel under the conjunctiva (the transparent coating that covers the inner eyelid and the white of the eye) breaks and bleeds. A common condition caused spontaneously from coughing, heavy lifting, or vomiting. In some cases, it may develop following eye surgery or trauma. It tends to be more common among those with diabetes, hypertension, and taking blood thinners (including aspirin). A subconjunctival hemorrhage is essentially harmless. The blood naturally absorbs within one to three weeks and no treatment is required. If a mild irritation is present, artificial tear drops can be used. You can speed up the healing process by applying cool compresses for the first two days and then warm compresses in the following days.

Subjective Refraction

The procedure in which the patient is asked to report on which lens combination provides the clearest vision. While this is the method of choice for determining prescription in those able to understand the task and respond to the examiner, it is less reliable in children.

Subluxation of the lens

Subluxation implies displacement of the crystalline lens, whereas luxation refers to a lens that is totally dislocated. The term ectopia lentis has been used interchangeably with subluxation, although this term should be reserved for bilateral cases. Lens subluxation can be either acquired or due to congenital systemic causes. Infants are rarely born with displaced lenses; rather, they develop due to a predisposing systemic condition.

Acquired subluxation is slightly more common than lens displacement associated with underlying systemic disorders. Trauma accounts for a large percentage of all acquired lens subluxations, with mechanical stretching of the zonules. This occurs as the eye is compressed in an anterior-posterior direction, such as with impact by a fist or ball. The subsequent distention of the globe in the medial-lateral plane ruptures the zonular fibers. Other acquired causes include chronic cyclitis, syphilis, buphthalmos, ciliary body tumor and severe or pathological myopia.

The pathophysiologic mechanism of lens subluxation from congenital causes varies depending on the condition. The direction of displacement in each case is characteristic, although not completely diagnostic. In Marfan's syndrome, the lenses tend to displace supero-temporally due to abnormal collagen vascular tissue and faulty lens zonules. It is typically present at birth and is non-progressive. Because the zonules are still attached to the lens, some accommodation remains.

Homocystinuria, a defect in amino acid metabolism, results in brittle zonules that rupture. This allows the lens to displace inferonasally or even into the anterior chamber. There is no accommodation, and the condition may progress.

Two other conditions worth mentioning are simple ectopia lentis and ectopia lentis et pupillae. Simple ectopia lentis is an autosomal-dominant condition where the lenses are dislocated superotemporally, but there are no other associated systemic abnormalities. Ectopia lentis et pupillae is likewise an isolated inherited condition, albeit autosomal-recessive, where the lenses displace temporally in opposite directions.

The main concern with lens subluxation is the development of secondary angle closure glaucoma. Anytime the crystalline lens displaces, there is always the possibility that the lens can come into apposition with the back surface of the iris (or the front surface of the iris during complete lens dislocation into the anterior chamber). This will lead to pupillary block, iris bombé and secondary angle closure. Also, if the lens completely dislocates into the anterior chamber, the lens may touch the cornea, irreversibly damaging the endothelial cells with subsequent corneal edema and decompensation.

Suppression of Binocular Vision

When the brain ignores the image that is seen by one eye. It is the result of weak eye teaming skills (binocularity).

Suppression Test

Determines if there is any tendency for the visual processing center of the brain to ignore or suppress visual data from one eye.


Sympathomimetics are eyedrops that decrease the formation of aqueous humor in the eye and increase the steady flow of the aqueous humor out of the eye.