Optical Issues in Measuring Strabismus

Abstract

Potential errors and complications during examination and treatment of strabismic patients can be reduced by recognition of certain optical issues. This articles reviews basic as well as guiding principles of prism optics and optics of the eye to equip the reader with the necessary know-how to avoid pitfalls that are commonly encountered when using prisms to measure ocular deviations (e.g., during cover testing), and when observing the corneal light reflex to estimate ocular deviations (e.g., during Hirschberg or Krimsky testing in patients who do not allow for cover testing using prisms).

Keywords: Angle kappa; Prentice position; Prentice's rule; angle lambda; frontal plane position; minimum deviation position; prism diopters.

Figure 1

Light path through a 15^Δ prism. The amount of deflection (in cm) of a light ray that is caused by a prism, measured 1 m (100 cm) away from the prism, is equal to the power of the prism in prism diopters (^Δ). Thus, a 15^Δ prism, as in this illustration, deviates a light ray 15 cm toward the base of the prism, when measured 1 m away from it

Figure 2

Relationship between prism diopters and degrees. For angles smaller than 45° (or 100^Δ), the number of prism diopters per degree is about 2. For angles larger than 45° (or 100^Δ), this approximation becomes invalid, and as one approaches 90°, the number of prism diopters per degree goes up to infinity

Figure 3

Positioning of prisms. (a) Prentice position. Glass prisms are calibrated for use in this position, so the line of sight makes a right angle with one of the surfaces. (b) Minimum deviation position. Plastic prisms, including plastic prism bars, are calibrated in this position, so the line of sight makes an equal angle with each prism surface. (c) Frontal plane position. Holding plastic prisms in this position, with the back surface flat to the face of the patient, closely approximates the minimum deviation position, which would otherwise be difficult to estimate in clinical practice

Figure 4

Interface between two stacked prisms. While the first glass prism is in the Prentice position, with the light ray being perpendicular to the first surface of the prism, the second glass prism is nowhere near the Prentice position, with the light ray going in at an angle far from perpendicular (adapted from reference 4)

Figure 5

Detection of prism with lensmeter. Displacement of the cross-line target away from the reticle center indicates presence of prism, with the decentration in the direction of the prism base. In this illustration, the cross-line target is displaced 4 rings to the right, indicating the presence of 2^Δ base-in or-out prism if dealing with a spectacle lens for the right eye or left eye respectively

Figure 6

Schematic representation of angle lambda and corneal light reflex positioning. (a) Normal angle lambda (λ) between the patient's line of sight (directed at the examiner's handlight) and pupillary axis is associated with slight nasalward positioning of the corneal light reflex (the reflection of the handlight from the anterior surface of the cornea) with respect to the pupillary center. (b) Eye with normal angle lambda (λ) is not fixating on the handlight, with strabismic deviation (δ), showing nasalward displacement of the corneal light reflex from the pupillary center. Note the imaginary string that connects the corneal center of curvature (CR) with the handlight. (c) Large angle lambda (λ+) caused by a temporally dragged, but fixating fovea, showing the same amount of nasalward displacement of the corneal light reflex from the pupillary center as in (b) with the deviated eye with normal angle lambda (λ) not fixating on the handlight