![]() ![]() ![]() ![]() When the left eye is covered, neither the right nor left eye moves. Since we can see this eye deviation without breaking fusion with the cover paddle, we are looking at a tropia. To start, we can see that the left eye of the patient deviates outward. The misalignment is only visible once the patient is dissociated (once the cover paddle is placed over the left eye). This is a simple example of the cover-uncover test for a patient with exophoria. ![]() When the occluder is removed, the patient is able to regain fusion and the eye moves back inwards and appears aligned. The left eye moves slightly outwards - in an exo direction. In the second image, the cover paddle is brought over the left eye. When looking at the patient without occluding an eye, the patient's eyes appear aligned. For the below examples, the occluder is translucent so you can see how the eye moves under the cover paddle. The occluder is often opaque so that the patient can't see through it. For the images, the gray circle is the occluder used to cover one eye. This gets confusing, so let's look at two examples of a cover-uncover test. However, strabismus due to nerve damage or muscle tissue scarring is often different in magnitude in various positions of gaze (termed incomtant or non-comitant). These types of strabismus are called comitant or concomitant deviations. Most forms of strabismus, for example, are the same magnitude in all positions of gaze. ComittancyĬomittancy is essential how consistent the deviation is in all positions of gaze. The magnitude of a phoria or tropia is usually evaluated with an alternating cover test. The magnitude of deviation may also be expressed in degrees of rotation. A prism diopter is of one represents a prism that deflects a beam of light a distance of one centimeter on a surface that is one meter away from the prism. The measurements are recorded using the optics unit of prism diopters (Δ). The magnitude of a phoria or tropia (strabismus) is often graded with prism lenses to estimate how much prism is needed to stop the eye from moving during the cover test. Think of magnitude as the "how much" question when evaluating an eye deviation. Excyclo - the eye(s) tends to rotate outwards.Incyclo - the eye(s) tends to rotate inwards. Hypo - the eye(s) tends to turn downwards relative to the fellow eye. Hyper - the eye(s) tends to turn upwards relative to the fellow eye. Ortho - the eye(s) shows no misalignment. Intermittent - the eye does not deviate constantly but may deviate under certain conditions or stressors (such as reading or near work).Duration of MisalignmentĬonstant - the eye is deviated at all times and all distances. If only manifest under dissociative conditions, the misalignment is termed a heterophoria. Ocular misalignment that is visible without dissociating the patient with a cover paddle or prisms is termed a heterotropia. The results of a cover test are reported in type, duration, direction of misalignment, magnitude, and comitancy. Abnormal eye positioning could be a cause for numerous symptoms, such as double vision (diplopia), eye strain (asthenopia), reading difficulties, headaches, and more. The cover test is a critical component to understanding how a patient's eyes move and team. The cover test is commonly performed at distance (6m) and at near (40cm), though in some cases the cover test is performed in multiple positions of gaze (up-left, up, up-right, right, down-right, down, down-left, left, and central gaze). In each step of the test, the clinician may also use a prism lens to measure the amount of misalignment of either the strabismus (tropia) or phoria. The alternative cover test described here is again looking for very small phoric misalignment. While moving the paddle back-and-forth, the clinician is looking for subtle movement of each eye. Here the clinician covers an eye and moves the paddle to the next eye. The second step of the test is called the alternating cover test. When watching the previously covered eye, the clinician is looking for a phoria (the "natural" eye position of a patient when fusion is disrupted). When covering an eye and watching the non-covered eye, the clinician is looking for strabismus (also referred to as squint or tropia). This step is called the cover-uncover test. The cover paddle is then removed, and the clinician looks for movement of the previously covered eye. First, the clinician covers an eye with a hand or paddle and looks for movement of the fellow eye. The cover test is a measurement of eye posture (or eye alignment). Good eye teaming allows sustained, single, and comfortable vision, and is the basis for depth perception. Eye teaming, or binocular vision, is a visual efficiency skill where both eyes work together in a precise and coordinated way. A key component of a comprehensive eye examination is the assessment of eye teaming, or binocular vision. ![]()
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