====== How to Measure Strabismus ====== - Ensure proper spectacle/contact lens prescription - Use an accommodative target: an object that requires a fixed accommodative effort to resolve - Acuity Chart with optotypes - Scott Fixation Device {{ ::scott_fixation_devices.png?150 |}} - Video with formed images - Finger puppets - Wiggle sticks - Fusion may need to be suspended with 45-minute occlusion test first - only for intermittent or largely variable deviations ==== Detection of Strabismus ==== * Single Cover Test and Cover/Uncover Test * Detects presence of a tropia * Fusion is not suspended * Alternate Cover Test * Detects basic deviation= Phoria + Tropia ==== Quantify Strabismus ==== * Observation is helpful before any cover testing to assess level of control of intermittent deviations * watch how their eyes look with distance and near targets * you should to be able to estimate the amount of manifest deviation with practice * Corneal Refection tests * Accommodation not controlled and fusion not suspended * **Hirschberg test** * Compare light reflexes and estimate ocular alignment without prisms {{ ::hirschberg_test.png?600 |}} * **Modified Krimsky Test** * Most useful when there is poor fixation in one eye * The Prism is held in front of the preferred eye (which isn't deviated) with the appropriate power to ensure both corneal light reflexes are symmetrically centered in the pupil. * The original Krimsky test required holding the prism over the deviating eye. This test requires the prism over the aligned eye. {{ ::modified_krimsky.png?600 |}} * Scleral Comparison * compare the amount of white visible between the eyes * **Prism Cover Tests** * Single Prism Cover Test * Prism is placed over the deviated or paretic eye * Fellow eye covered * End point is when any movement stops * Measures Tropia * Simultaneous Prism Cover Test * Same as Single Prism Cover test except the prism is placed in front of the deviated eye at the same time the fellow eye is occluded. * Alternate Prism and Cover Test * Prism is placed over the deviating or paretic eye and the cover is rapidly shifted from one eye to the other * End point is the prism that causes no additional movement. * This measures the manifest (tropia) and latent (phobia) components to the deviation * This measurement is most useful for pre-operative planning * This measurement may overestimate the control of deviation in the setting intermittent deviation * Prolonged cover testing can break down any tenuous sensory fusion and worsen the control of the strabismus * Measurements should be recorded for the following eye positions * Distance Fixation (6 meters) * Primary position * Right, left, up and downgaze * Head tilts if there is a vertical deviation * Near fixation (1/3 meter) * Primary position * Reading position if there are reading concerns ==== Positioning Prisms ==== * The deviation that a prism produces or neutralizes is dependent on the position of the prism as it is held before the patient. * When prisms are used to measure a strabismus deviation, the prism displaces the image so that no movement of either eye is needed to fixate on the target. === Prentice position === * The line of sight of the deviated eye is perpendicular to the posterior face of the prism. * This is the proper way to hold a **glass prism**. * Fresnel press-on prisms are calibrated in the Prentice position. {{ ::prentice_position.png?300 |}} === Minimum Deviation Position === * The visual axis makes an equal angle with each prism surface. This is the position in which **plastic loose prisms** are calibrated. {{ ::minimum_deviation_position.png?300 |}} === Frontal Plane Position === * **Horizontal and vertical prism bars** are calibrated for use in the frontal plane position. {{ ::frontal_plane_position.png?300 |}} === Prism Stacking === * Stacking two prisms in the same direction will induce measurement error * [[prism_stacking|See Table with values of stacked prisms]] * Prisms held in the same direction over either eye will induce some error, albeit smaller. * This error has been incorporated into surgical tables as the sum of the two prism values were used when the tables were developed rather than the actual measured deviation. * [[actual_value_of_two_prisms_held_over_either_eye|See Table with actual value of prisms held in the same direction over either eye]] ==== Spectacle Induced Errors ==== * High powered spectacles (generally ≥5 D spherical equivalent) create a built in prismatic effect in strabismic patients that must be taken into account when performing cover testing. * High minus glasses will induce a base in prism effect for esotropes and a base out prism effect in exotropes. * In both cases this will have the effect of making the deviation by the PCT appear larger than the true deviation by 2.5 D %. * i.e., a patient wearing -10.00 D glasses with a 40Δ esotropia by PCT will have a true deviation of 30Δ. * See [[true_ocular_alignment_with_hyperopic_spectacles|True Ocular Deviation with Hyperopic Spectacles Table]] and * High plus glasses will induce a base out prism effect for esotropes and base in effect for exotropes. * In both cases this will make the measured deviation smaller than the true deviation by 2.5 D % (i.e., a patient wearing +10.00 D glasses with a 40Δ esotropia by cover testing will have a true deviation of 50Δ ). * See [[true_deviation_with_myopic_spectacle_correction|Ture Ocular Deviation with Myopic Spectacles Table]] * Anisometropic spectacles * Prism is induced in the presence of an anisometropic spectacle correction when the visual axis is not aligned with the optical axis of the lenses. * This can cause diplopia and account for differences in measurements obtained in secondary and tertiary gaze positions. ===== References ===== - {{ ::comp_of_acc_and_non-acc1.pdf |Comparison of Accommodative and Nonaccommodative Targets For the Assessment of Ocular Deviations. Scott, W.E. et al 1979}} - [[https://vimeo.com/126071279|How to Perform a Basic Cover Test by Christopher Kirkpatric and Tony Klauer]] - {{ ::ophthalmic_prisms_measurement_errors_and_how_to_minimize_them.pdf |Ophthalmic Prims: Measurement Errors and How to Minimize Them. Thompson & Guyton 1982}} - {{ :artifacts_introduced_by_spectacle_lenses_in_the_measurement_of_strabismic_deviations.pdf |Artifacts Introduced by Spectacle Lenses in the Measurement of Strabismic Deviations. Scattergood, Brown and Guyton. 1983}} {{tag>strabismus tutorial}}