Table of Contents
- 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
- 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
- Single Cover Test and Cover/Uncover Test
- Detects presence of a tropia
- Fusion is not suspended
- Alternate Cover Test
- Detects basic deviation= Phoria + Tropia
- 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
- 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.
- 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
- 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.
- 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.
- The visual axis makes an equal angle with each prism surface. This is the position in which plastic loose prisms are calibrated.
- Horizontal and vertical prism bars are calibrated for use in the frontal plane position.
- Stacking two prisms in the same direction will induce measurement error
- 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.
- 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.
- 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 ).
- 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.