An Unreliable Visual Field with a Suspicious Superior Defect
Mr. Bianchi, 70, has moderate nuclear sclerotic cataracts and a family history of glaucoma. His IOP is 21 mmHg OU, and the optic discs are suspicious. The physician orders a Humphrey 24-2 SITA-Fast visual field. The technician positions him, gives the standard instructions, and the test takes about 3.5 minutes per eye. The printout shows fixation losses 6/12 OD, false-positive errors 18%, false-negative errors 25%, and a foveal threshold of 24 dB OD with acuity of 20/40. There is a dense superior defect that respects the horizontal meridian. The technician noticed that Mr. Bianchi removed his glasses because they fogged, and that he repeatedly lifted his head to look at the fixation light. The physician's schedule is full, and the technician considers whether to repeat the test now or send the patient home.
1. How reliable is this field, and which findings may be artifact rather than true loss?
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The field is unreliable as printed. Fixation losses of 6/12 (50%) exceed the usual 20% threshold and suggest the patient was not consistently looking at the fixation target; his head-lifting confirms this. False-positive errors of 18% are modestly above the typical 15% threshold, and false-negative errors of 25% are within the commonly accepted range of under about 33%.
The dense superior defect that respects the horizontal meridian is suspicious for a lens-rim artifact, especially because he removed his glasses and may have been tested with an ill-fitting trial lens or no correction; a true nerve-fiber-bundle defect would not ordinarily stop sharply at the horizontal midline. Fogging and head movement can also create generalized depression. The foveal threshold of 24 dB with 20/40 acuity is plausible, but the reliability indices mean the field should be repeated before it is used to manage glaucoma.
2. What should the technician do before repeating the test, and what test conditions should be used?
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Before repeating, confirm the correct near correction for the test distance, usually a trial lens adjusted for vertex distance rather than his distance glasses. Clean the lens and use an anti-fog agent, or position it slightly away from the eye if fogging persists. Occlude the non-tested eye completely, set the chin rest and forehead strap, and check that the eye is centered. Give a brief reinstruct: look at the center light, respond only when a light is seen, blink normally, and keep the head still. Offer a rest break and monitor fixation during the test. Because of the cataract and the unreliable first attempt, SITA-Standard may give more interpretable data than SITA-Fast, even though it takes longer. Repeat one eye at a time and compare with the first test.
3. What should be documented and communicated, and what should the technician avoid saying?
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Document the reliability indices, the artifacts observed (fogging, head lifting, removed glasses), the correction used, test strategy, and the patient's behavior. Note that the test was repeated and why, and attach both printouts. Tell the physician directly that the first field was unreliable and that a superior defect may be lens-rim artifact.
Tell the patient that the test needs to be redone because the machine did not get a clear reading, not that he has glaucoma or nerve damage. Do not interpret the field or tell him the defect is real or false. If he is tired, reschedule rather than push through, and record the reason.