The clinical test that I am undergoing is referred to as the Howell Card Dissociated Phoria Testing. The Howell Card test, essentially investigates how well the eyes work together (binocular vision), or the ‘dissociation’, allowing the determination of the type of the phoria that the subject has. Phoria is the optical term that refers to the degree of deviation Root, 2010 of one eye that occurs when the eyes are not synchronised.
There are different types of phoria that the Howell Card test can determine. Esophoria is termed to patients who experience an inward deviation of one eye towards the nose, which is usually caused by distance vision. Contrastingly, exophoria insinuates an outward shift of one eye mostly due to working at near distance. Macdonald, 1931
Essentially, the Howell Card is held a distance of thirty-three centimetres away from the right eye, while a six diopter base up prism is placed in front of the eye Goss, Reynolds, Todd, 2010. On the card, there is a row of numbers and an arrow pointing down from the middle at zero. To the left of the zero, there are even numbers against a blue strip, and on the right are odd numbers on a yellow strip. Looking through the 6D prism results in double vision by breaking binocular vision in a vertical manner, creating a top and bottom row. Opto Tube Channel, 2015
The optometrist asked me to look at the arrow on the top row and state the number the arrow pointed down on the bottom row. If the arrow corresponded to an even number on the blue side, then that was indicative of exophoria. Conversely, an odd number to the right of zero on the yellow side revealed esophoria Bernell Corporation 2018. The graduation of the scale is designed with the thirty-three centimetres working distance and the 6D base up prism. So when the subject reports the corresponding number, the number would effectively equal to the degree of phoria in diopters Goss, Reynolds, Todd, 2010.
According to this test, I have exophoria. However, it is normal to be slightly exophoric according to Evan BJW (2007). Data reveals 68% of population ranges from 2D esophoria and 8D exophoria on the scale, averaging at 3 D exophoria.
As this method requires the judgement of the patient, this is a subjective method. One issue is that when looking at the card, the arrow tends to shift side to side creating difficulty in finding the corresponding number. However, adequate time was allowed so that the movement stopped and the arrow would settle on just one number. This is a significant factor of this test, as it is once again subjective. Hence, in order to gain more reliable and accurate results, it is important to let the arrow fix on just one number rather than pointing out the first number it points to.