Hess screen

screen, Hess

A black tangent screen for measuring and classifying strabismus. It consists of a chart divided by red lines into small sections of 5º separations. As the screen is flat, the lines are curved in a pincushion pattern (i.e. the middle of the lines is closer to the centre of the screen than the ends of the lines). At the respective positions on the lines in the eight major meridians are small red dots indicating positions 15º and 30º from the fixation point. Two green threads extend from the upper corners of the screen and meet a third green thread, which originates on the end of a pointer. The three threads form a figure Y. The patient, wearing a red lens in front of one eye and a green lens in front of the other, moves the pointer until the common origin of the green threads is superimposed on each of the red dots. The discrepancy between the two on the screen indicates the extent of the deviation in various directions of gaze. It is a particularly useful test to indicate an incomitant strabismus as the field plots obtained with each eye are compared and an estimate of which muscle/s is affected can be made. Moreover, if the inner (15º) and outer (30º) plots are affected differently the incomitancy is of mechanical origin. Note: there exists a computerized version of the Hess screen.