Binocular Vision Lecture 5 - Strabismus Correction

In a tropia, the eyes are misaligned, causing images to fall on non-corresponding retinal points, resulting in double vision unless neural adaption occurs.

Light is bent towards the base of a prism, making the image appear shifted towards the apex.

A prism can redirect light to ensure the image falls on corresponding points on both retinas, correcting double vision in tropias.

When correcting strabismus, the prism's apex is placed in the direction the eye deviates. The image is pointed at the eye, but in optometry, the prism's base direction, not apex, defines the direction.

A phoria is a latent eye misalignment that is not obvious when both eyes are working together. Prisms can be used to measure the difference between the associated (active) and dissociated (passive) positions of the eyes.

Prisms are added to the eyes until the associated eye position (when both eyes are working together) matches the dissociated eye position (when the eyes are looking independently).

When correcting a phoria, the prism is placed so that its base is directed in the direction of the eye movement that needs to be corrected. For example, an eye that tends to drift outward (exo deviation) would be corrected with a base in.

A prism's base is positioned in the direction opposite to the eye's deviation. For example, if the eye deviates outward (exo), the base of the prism is directed inward.

For an eye that deviates inward (eso), the prism's base is placed outward.

For an eye that deviates outward (exo), the prism's base is placed inward.

Prism bar cover test, Maddox rod and Maddox wing are all used to measure the size of a deviation in prism dioptres. These tests are dissociated tests, meaning they prevent fusion of the eyes.

The Maddox rod distorts an image beyond recognition, preventing fusion. This is achieved by using small planoconvex cylinders on a coloured glass disc, usually red. A spot light seen through the disc is distorted into a streak of light perpendicular to the axis of the grooves.

When using the Maddox rod to measure horizontal phorias, the patient is asked to describe the position of the red line relative to the spot of light. If the red line is to the right of the spot, this indicates an esophoria, as the right eye has drifted inwards.

When using the Maddox rod to measure horizontal phorias, the patient is asked to describe the position of the red line relative to the spot of light. If the red line is to the left of the spot, this indicates an exophoria, as the right eye has drifted outwards.

A base out prism is used to correct an esophoria. The prism is placed in front of the right eye with the apex pointing to the left, moving the red line towards the spot of light.

A base in prism is used to correct an exophoria. The prism is placed in front of the right eye with the apex pointing to the right, moving the red line towards the spot of light.

To measure vertical phorias with the Maddox rod, the grooves are placed vertically. The patient is asked if the red line is above, below or going straight through the spot of light.

A base up prism is used to correct a right hyperphoria. The prism is placed in front of the right eye with the apex pointing down, moving the red line towards the spot of light.

A base down prism is used to correct a left hyperphoria. The prism is placed in front of the right eye with the apex pointing up, moving the red line towards the spot of light.

The Maddox wing is a test that uses different objects for both eyes, preventing fusion. The Maddox wing is designed to work at 30cm. The patient views through two eye slits and sees arrows in one eye and a tangent scale in the other.

To measure horizontal phorias with the Maddox wing, the patient is asked which number on the tangent scale the arrow points to. A value of zero indicates orthophoria, an even number indicates exophoria, and an odd number indicates esophoria.

To measure vertical phorias with the Maddox wing, the patient is asked which number on the tangent scale the red arrow points to. A value of zero indicates orthophoria, an even number indicates hyperphoria, and an odd number indicates hypophoria.

To measure cyclophoria with the Maddox wing, the patient is asked if the red bar appears horizontal. If the red bar appears tilted, the angle of tilt indicates the degree of cyclophoria.

The Maddox wing is limited because the numbers and arrows are poor stimuli for accommodation, potentially overestimating exophoria and underestimating esophoria. The septa seen by both eyes also means dissociation is not complete.

The findings of subjective vision assessments using the Maddox rod and Maddox wing are recorded in a similar way as cover test findings. Orthophoria is noted, and if a phoria is present, the prismatic power and type of phoria are recorded.