Using an Automated Hirschberg Test App to Evaluate Ocular Alignment

We present a protocol of using a smartphone app to perform Hirschberg test for measuring manifest and intermittent ocular misalignment (strabismus) under near and far fixation conditions.

The mobile app that we have developed can be used to measure strabismus for research or for clinical use. This method, facilitates an easy strabismus measurement, which currently requires extensive training and experience to be performed using only a standard phone camera and the app software. In the app interface, users can choose to perform a cover test or to measure a patient with both eyes fixated at a target simultaneously, at either a near or far fixation distance.

To measure tropia, with a single snapshot in with a near fixation, launch the app and set the mode to No Cover. Select Near and position the phone in a landscape orientation about 40 centimeters from the patient at eye level, with the rear camera facing the patient. Instruct the patient to fixate on the flashlight, which should be off at this point.

For tests requiring a precise accommodation, paste a fixation target on the back of the phone directly below or above the flashlight. When the patient confirms they are fixating, press the round button to take a snapshot to be analyzed by the app. When the analysis is complete, the app will show the detected limbus boundary, the center of the eye, and the location of the corneal reflection.

After verifying that these features have been detected without obvious errors, observe the measurements related to the eye alignment. Then, press the Save button to save the current test in the phone, or press the back arrow button to retest. To measure intermittent ocular misalignment for far fixation in the app, set the mode to Cover Test and select Far.

Hold the phone in a landscape orientation, about 40 centimeters from the patient at eye level with the flash camera positioned between the patient's eyes. Instruct the patient to look just above the phone, to a target about six meters away. Have the patient cover one eye and press the round button, the app will begin the detection.

While making sure that the patient is fixating properly, have them quickly remove the occlusion. The app will automatically take a picture as soon as the occlusion is removed. When the analysis is complete, the app will show the detected limbus boundary, the center of the eye, and the location of the corneal reflection.

After verifying that these features are detected without obvious errors, review the measurement outcomes for the eye alignment in prism diopters, before saving the data or repeating the test. For Angle Kappa measurement in the app, select measure Kappa angle. Position the phone in a landscape orientation about 40 centimeters from the patient at eye level, and instruct the patient to use the eye to be tested to fixate on the flashlight while covering the other eye.

While making sure the patient is fixating properly, tap the round button to take a snapshot, which will be analyzed by the app. When the analysis is complete, the app will show the detected limbus boundary, the center of the eye, and location of the corneal reflection. After verifying that these features are detected without obvious errors, view the measurement outcomes for Kappa angle in degrees.

In this representative analysis, the limbus boundaries of the two eyes and the corneal reflection of the flashlight, were detected correctly, suggesting that the ocular alignment measure is not subject to Image Analysis error. In this particular case, the patient had left exotropia as is obvious from the larger corneal reflection offset in the left eye. The app did not report which eye was deviated however, because in cases of a small strabismus angle and unknown Kappa angle, it would be unreliable for the app to determine the deviated eye.

For comparison, here an example without strabismus is shown. In this analysis, erroneous limbus detection can be observed. While the detection of the corneal reflection is correct, the green circle clearly does not match the limbus boundary, indicating that the test should be repeated.

In this analysis of 14 subjects, 10 subjects exhibited exotropia and four subjects presented with exotropia. Linear regression analysis, revealed that the eye measurements of the strabismus angles, were consistent with clinical cover test measurements. When using the app, there are two important things to keep in mind.

One, set the cover test and the fixation mode correctly for each specific test. Two, check if the eye feature detection is correct. The app only allows strabismus measurement and it doesn't give a diagnosis.

Clinicians will need to combine the ocular measurement with other vision measurements to make a diagnosis.