Abstract

Introduction

Vergence disorders can be a source of eyestrain and uncomfortable vision. Measurement of heterophoria is an important component for the clinical examination for vergence disorders. Measurement of heterophoria requires a method for the elimination of binocular fusion and a method for determining the angle between the lines of sight of the eyes and the position the lines of sight would assume if they intersected at the object of regard.

Two common clinical methods of heterophoria measurement are the von Graefe prism dissociation method (VG) and the tangent scale method that is commonly known as the modified Thorington test (MT). The VG test uses prism dissociation to prevent binocular fusion. Alignment of the diplopic images by a rotary prism or a prism bar provides measurement of the heterophoria. The MT test employs a Maddox rod to prevent binocular fusion. A penlight is pointed toward the patient through a hole in the center of the test card. A line is seen by the eye covered by the Maddox rod and a tangent scale is seen by the other eye. The position of the line on the tangent scale is reported by the patient.

One of the factors that can be considered in the evaluation of a clinical test is the repeatability of the results obtained with it. A metric often used in the evaluation of repeatability of clinical tests is found by multiplying the standard deviation of the differences between pairs of measurements on a series of subjects by 1.96 to get a 95% limits of agreement between the repeat measurements. This value is sometimes referred to as a coefficient of repeatability.

Previous studies have reported better repeatability for MT testing than for VG testing. Morris [1] tested adult subjects, ages 22 to 31 years, on separate days. Near point VG and MT tests were performed on separate days by one examiner with subjects viewing through a phoropter at a test target. Coefficients of repeatability for the VG test were 3.3 prism diopters ( ) for 20 trained observers and 2.9 for 20 untrained observers. On the MT, coefficients of repeatability were 2.0 for the trained observers and 1.6 for the untrained observers. Slightly lower coefficients of repeatability were obtained when the subjects had kinesthetic input from holding some part of the test target or the target support.

Rainey et al. [2] reported on repeatability of test results between two examiners for 72 subjects between the ages of 22 and 40 years. Test distance for both the VG test and the MT test was 40 cm. VG testing was done using phoropter rotary prisms, while the MT testing was performed without a phoropter. Coefficients of repeatability for the VG test were 6.7 using a flash presentation of the target in which subjects viewed the target intermittently between

degrees of downgaze, test cards were moved down from their position for straightforward gaze by about 13.7 cm and they were tilted back about 20 degrees so that the subjects’ lines of sight would be approximately perpendicular to the cards. Four recording forms, each with a different test sequence, were used to counter-balance test order. The recording form used was changed with each consecutive subject number. All tests were done by two examiners.

For the VG test, dissociation of the eyes was induced with an 8 base-down loose prism held over the subject’s right eye. Subjects viewed a vertical column of letters on a test card at 40 cm from them. They were instructed to keep the letters clear to control accommodation. A rotary prism was placed over the subject’s left eye in a clamp mounted on a table. Subjects were instructed to report when the columns of letters were aligned so that the upper column of letters was directly above the lower one, and the reading on the rotary prism was noted when the subjects reported alignment. Two values, one with the prism starting from the base-in side and one starting from the base-out side, were averaged, and the result was recorded. Exo findings (Base-in for alignment) were recorded as negative values, and eso findings (base-out for alignment) were treated as positive numbers. For downgaze measurements, subjects were instructed to turn their eyes down rather than their heads, and the prisms were tilted forward by approximately 20 degrees.

A Bernell Muscle Imbalance Measure (MIM) near test card was used for the MT test. This is calibrated for a 40 cm test distance. For lateral phoria testing, there is a horizontal row of numbered dots separated by 1 when the card is at 40 cm and a hole in the center of the row. Light from a penlight was directed through the hole in the center of the card toward the subject. A red Maddox rod was placed over the subject’s right eye oriented so that the subject saw a vertical line. Subjects were then instructed to close their eyes. As soon as they opened their eyes, they reported the number through which the vertical red line passed and whether the line was to the left or right of the white light. The number indicated the phoria magnitude. The position of the red line relative to the white light indicated the direction of the phoria, to the left for exo (recorded as a negative number) and to the right for eso (recorded as a positive number).

Results

The mean findings on both tests were a small amount of exophoria (Table 1). A coefficient of repeatability was determined by finding the mean and standard deviation of the differences between the two examiners and then multiplying the