What is a Cover Test? A Comprehensive Guide

A comprehensive guide on what a cover test is & how it's used for detecting & treating strabismus & amblyopia (lazy eye). Learn about its types & risk factors.

What is a Cover Test? A Comprehensive Guide

A coverage test or cover-up test discovered is an objective determination of the presence and degree of ocular deviation. It is usually performed by orthoptists, ophthalmologists, and optometrists during eye exams. There are two main types of coverage tests: the cover test and the discovery test. The cover test is used to determine if a heterotropia (i.e., misalignment) is a tropia or a phoria, while the discovery test is used to differentiate between the two.

Additionally, the prismatic coverage test is used to measure the deflection angle of a small-angle tropia. Early detection and treatment of strabismus are essential to reduce the risk of functional disability and mitigate the amblyogenic burden. This requires that all medical professionals, especially those who work with children, be competent to recognize a possible strabismus, so that it can be referred as soon as possible to an eye doctor. Treatment options may include observation, optical correction, supernegative therapy, prisms, occlusion therapy, visual therapy, and surgery.

Strabismus (a.k.a. strabismus) is an ophthalmic condition in which the eyes don't align correctly with each other when focusing to look at an object (i.e., misalignment can occur occasionally or constantly). Strabismus can cause permanent functional disability, psychosocial distress, and significantly interfere with social and psychological development, especially if it is persistent during childhood. This functional disability can manifest as amblyopia (strabic) or lazy eye, in which the brain cannot process inputs from one eye and, over time, favors the other eye, causing structural changes in the visual pathway and the cortex.

This ultimately results in a decrease or total loss of vision in an otherwise normal eye. Pseudo-strabismus (i.e., false strabismus) may be due to structural causes that usually resolve over time as the person gets older. These causes include a wide, flat nose; hypothelorism or ocular hypertelorism (i.e., abnormal decrease or abnormal increase in the distance between the eyes) or strange skin that covers the medial corner. Triggers for intermittent strabismus can include fatigue and stress.

Risk factors for primary strabismus include family history of strabism, low birth weight, preterm delivery, and maternal smoking. The physical examination of possible strabismus requires a flashlight or other light source and an occluder. The basic physical exam includes light reflection, as well as coverage and discovery tests. Other tests such as the Brückner screening test, prismatic coverage test, alternative prismatic coverage tests, Parks—Bielschowski three-step test can be found in other publications.

Factors that affect test performance such as abnormal head posture should be taken into account when performing these tests. The Brückner test is useful for identifying the presence of small-angle strabismus. At a distance of approximately 50 cm from the patient, the examiner uses a direct ophthalmoscope (OD) with the largest diameter of light to see the patient's two red reflections simultaneously while fixing the patient on an objective adjacent to the OD. DO lenses are adjusted until the skin around the eyes is in focus.

The asymmetry of red reflections in terms of size, shape, color and brightness may suggest an eye disorder such as strabismus and anisometropia (i.e., imbalance (refractive error) or opacity of the medium that obstructs the visual axis). The cover-up and discovery test is used to differentiate whether a misalignment is a tropia or a phoria; that is if the previous coverage test shows that there is no tropia it is used to determine if a phoria (i.e., large ones may be associated with asthenopia (i.e., eye strain)). Keep in mind that small subtly noticeable phores are common and not pathological. The examiner occludes one eye for approximately 1-2 seconds and then quickly removes the occluder to restore binocular vision; if a phoria occurs this eye will become orthotropic again (i.e., looking directly) to re-establish sensory fusion with the other eye; speed and smoothness of refixation indicate strength of fusion.

The prismatic coverage test is used to measure deflection angle of small-angle tropia; magnitude of tropia estimated from cover and cover-up test; prism selected (alone or on prism bar) underestimates magnitude; simultaneously this prism should occlude fixing eye; direction of change in fixation determines type of tropia; appropriate interpretation outlined in table below; keep in mind that amblyopia known sequel to long-established strabismus but can itself be cause of strabismus due to inferior visual experience (sensation cognition processing perception) in one eye causing it to deviate from correct alignment.

Kelli Roswick
Kelli Roswick

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