Red Cap Testing: - Red Desaturation Testing (i.e., Mydriatic red caps ) color comparison testing is done monocularly. Depressions of central field function, including color perception, is common in both optic nerve and chiasmal disease. Color desaturation may occur disproportionately to retention of acuity and form perception. No bright light tests are performed before red desaturation testing.

Alternate Comparison:
In optic nerve disease, central depression may be easily elicited by asking the patient to describe changes in their perception of the color of the red cap. Occlude the suspect eye. Using your white clinic coat as the background have the patient compare the color of the red cap with their good eye then with their suspect eye. Record the responses as equal, slightly dimmer, much dimmer, etc. Positive response requires further neurologic testing, but optic nerve disease is suspect.

Simultaneous Testing:
Have the patient fixate the central red cap (Fig. 1) while placing a second red cap to either side. The patent is asked to look at the central cap and the patient is asked if either the central or peripheral cap is equal to or brighter than the other. In a normal eye the central cap should be equal to or brighter than the peripheral cap. If the patient reports the central cap as dimmer or duller red, optic nerve disease is suspected.

Fig. 1 Central Fixation Red Cap

Again using your white clinic coat as the background and a button as a fixation point (Fig. 2) have the patient compare the color of the red cap with their good eye then with their suspect eye as the red cap is moved away from or toward the central point of fixation against the suspect eye.


Fig.2 One Red Cap

Alternately, two caps may be used, (Fig. 3) one placed centrally and the other eccentrically, and the patient asked to describe any differences in color intensity. The patient may report one cap as being faded . The same procedure should have already been performed on the non-suspect eye so the patient will have something to compare.


Fig.3 Two Red Caps

In suspected chiasmal syndromes (disease), one must compare color perception to either side of the central fixation point. Simultaneously presenting two caps, each on either side of fixation. This provides the patient with a maximal visual stimulus about which they may make a subjective and sensitive judgment in perception (Fig. 4). To further substantiate an apparent temporal field defect, the cap should demonstrably "brighten" or take on color as it is passed through the horizontal meridian into the nasal field. The vertical fields can also be examined in a similar manner.


Fig.4 Two Red Caps Both Sides Of Fixation

Photostress, brightness comparison, red desaturation, and amsler grid testing are all done monocularly and before the patient is dilated or Goldmann tonometry have been run and with the patient wearing their best correction for the testing distance. These patients usually have reduced acuity in one eye that cannot be explained and vision that cannot be improved with the aid of a pinhole. If vision is improved with a pinhole this indicates that your refraction is off and needs to be re-evaluated. The reason for running these tests is to differentiate between an optic nerve or retinal problem. Lesions of the retina which can reduce acuity do not always cause a proportionate reduction in color perception. Defects within the optic nerve create faulty conduction of the nerve impulses, leading to a reduction in color perception. Read pages 15 & 16 in Dr. Alexander's book.

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