Wayne EngineeringDevelopers of Visual and Motor Skills
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The Franzblau Strabismomeler is a high tech, accurate and patient friendly instrument based on the Hess Lancaster Test. It has many functions and should be used as an integral pan of an amblyopic, strabismic or neurological evaluation.
The equipment consists of an illuminated screen with a control panel, a laser flashlight and a pad of recording charts. The screen has a horizontal and vertical grid printed in black on a green background. There are nine green lights which can be illuminated one at a time in each of the cardinal meridians.
The hand-held laser flashlight produces a small, discreet red light which allows accurate measurements to within one prism diopter.
When anaglyph glasses are worn, the eye behind the green filter can only sec the illuminated green light and the eye looking through the red filter can only sec the red light.
The recording chart is a representation of the patient's projection of the light when he/she perceives it being coincident with each of the nine targets. When completed, it will provide a permanent record for the patient's file and copies can be readily sent to other professionals involved in the management of the patient.
The patient wears their habitual Rx or proposed Rx in a trial frame. He/she is positioned one meter from the screen with the central green light aligned horizontally and vertically with the patient's eyes. In other words, the patient should be at eye level and centered on the midline of the body. Before placing the anaglyph glasses on the patient, demonstrate how the procedure is done, by placing the red laser beam on the central target. Be sure to caution a child not to look directly at the light or shine it in anyone's eyes. Explain that you want him to place the red spotlight on the green light. Then using the central panel, move the position of the green LED to a few locations to show the patient where the target may be.
When you are sure the patient understands the instructions, place the anagiyph glasses on the patient For conformity, it is preferable lo have the red lens over the right eye. However there are exceptions.
The room illumination should be subdued, but not dark.
Note on the recording sheet whether the red lens is on the right or the left eye. Use a red felt tip pen to record where the patient placed the red spot of light relative lo the green target. Draw an arrow from the target to the point where the patient placed the red spotlight (see figure 1).
If the red lens is in front of the right eye, then placement of the red light to the right of the target indicates exophoria or exotropia. If the red light is placed to the left of the green target (with the red lens over the right eye), a measurement of esophoria or esotropia is indicated.
Because this is direct foveal projection, the patient will project light in the same direction as the eye deviation. Therefore, if the right eye is behind the red lens and the red light is projected to the right of the green light (temporal), the patient has an exo deviation. Conversely, if the red lens was before the left eye and the red spot was placed to the right of the illuminated target (crossed or nasal projection), an eso deviation is illustrated. In addition, placement of the red light above the green target with the red lens in front of the right eye would indicate a measurement of a right hyperphoria or hypertropia.
Begin the test by asking the patient to place the red spot on the green light. Start with the central target to measure the subjective angle in primary gaze. Follow this same procedure for the other 8 positions of gaze and record the results on the recording sheet.
The patient's head should remain in the primary position and eye movements should be directed to the peripheral targets, rather than a head turn towards the target.
Most of the black lines on the screen are 8 cm apart. By definition, the deviation of light one centimeter from a distance of one meter is equal to one prism diopter (pd.). Therefore, you can easily calculate the amount of the deviation in prism diopters by how many centimeters the projected light is from the target. These values remain constant up to the 24 prism diopter coordinate. As the target approaches the periphery, compensation can be made for the increased distance from the patient to the screen. However, it is generally not necessary because it only induces a one to two prism diopter error for most muscle field testing. The fixations would have to be greater than 35 pd. before becoming a significant factor.
As mentioned previously, one of the primary functions of The Franzblau Strabismometer is to determine whether or not the deviation is concomitant. Non-concomitancy is indicated when there is more than five prism diopters difference between measurements in the cardinal meridians. A non-concomitant deviation can be due to paralysis or paresis of one or more of the extraocular muscles. There are many excellent tests on how to determine the exact muscle(s) involved based on Strabismometry. The reader is urged to review this material for further information. A recent onset of a non-concomitant deviation suggests a neurological or vascular etiology, such as cerebral vascular accident (stroke) or other acute onset diseases. Adult patients with slow progressive onset of diplopia may indicate a chronic disease process, i.e. multiple sclerosis, myasthenia gravis, neoplasm, etc. A consultation with a neurologist or neuro-ophthalmologist is strongly indicated in those cases where the etiology of the gradual onset diplopia is unknown.
A significant difference in the subjective angle of deviation as determined by the Strabismometer and the objective angle as measured by the cover test or Hirshberg method strongly confirms a diagnosis of anomalous correspondence. I personally have seen cases of "surgically corrected" strabismics who showed an obvious post-surgical exotropia on the cover test but demonstrated an eso deviation on the Strabismometer.
