4.4 Common Traps in Visual Field Testing
Key Takeaways
- A lens-rim artifact (dense peripheral ring scotoma) comes from a trial lens held too far from the eye or decentered, not from disease.
- Droopy lids or prominent brows create superior field depression; tape the lid up before assuming a true superior defect.
- High false positives inflate the field (white supranormal points); do not report a 'better' field without checking reliability first.
- A cloverleaf pattern means the patient quit responding mid-test, not that they have four-quadrant disease.
4.4 Common Traps in Visual Field Testing
Most wrong answers in this domain - and most clinical errors in real life - confuse an artifact for real disease. An artifact is a defect on the printout caused by the testing conditions rather than by anything wrong with the eye or pathway. The COA's job is to recognize the artifact, fix its cause, and repeat the test, not to let a flawed result reach the chart as pathology. The skill is pattern recognition: each artifact has a signature shape and a one-step fix.
Artifact versus disease at a glance
Before the specific traps, hold one principle: a defect that follows an anatomic boundary and repeats is probably real, while one that has an odd shape, tracks a setup flaw, and vanishes when you fix that flaw is an artifact. Every trap below is a variation on that theme - a mechanical or attentional problem that paints a false picture on the map.
Trap 1: The lens-rim artifact
If the trial lens sits too far from the eye or is decentered, its edge blocks peripheral stimuli and produces a dense ring-shaped scotoma at the field's outer edge. It looks alarming but is purely mechanical. Fix it by moving the lens close to the eye and centering it on the pupil, then repeat. A true peripheral constriction will persist; a lens-rim artifact disappears.
Trap 2: Lid and brow depression
Ptosis (droopy upper lid), dermatochalasis, or a heavy brow shadows the superior field, creating an arc of depression along the top. Before calling it a superior defect, tape the lid up and re-test. A persistent superior arcuate defect after taping may be real glaucomatous loss.
Trap 3: Reliability indices that mislead
| Finding | False conclusion to avoid | Correct response |
|---|---|---|
| High false positives | "The field improved" | Reinstruct, repeat; values are inflated |
| High false negatives | "Disease progressed" | Likely fatigue; offer break, repeat |
| High fixation losses | "New scotoma" | Reposition, re-explain fixation, repeat |
| Cloverleaf pattern | "Four-quadrant loss" | Patient quit mid-test; repeat |
Trap 4: Uncorrected refraction
Forgetting the near add (covered in 4.2) produces a uniform generalized depression that mimics early diffuse glaucoma or media opacity. Always confirm the trial lens before interpreting low overall sensitivity.
Trap 5: Small pupil and media
A pupil under about 2 mm, a dense cataract, or vitreous opacity all dim the whole field by reducing the light reaching the retina. The result mimics generalized depression. Note pupil size in the record and avoid testing through a constricted pupil when possible; the physician weighs media clarity when reading the map. A cataract that worsens between visits can make a glaucoma field look like it is progressing when the nerve is actually stable - another reason to document media status.
Trap 6: The learning effect
Many patients perform poorly on their very first field simply because the task is unfamiliar, then improve on subsequent visits. This learning effect can make an early field look falsely abnormal and a later field look falsely improved. The fix is patient education and a practice run, plus interpreting the first one or two fields with caution rather than as a true baseline.
Worked trap
A patient's printout shows a near-perfect, even slightly supranormal field with scattered white points, and the header reads FP 30%. The tempting answer is to celebrate improvement. The correct call: the high false-positive rate means the patient pressed for non-existent stimuli, inflating sensitivity. Reinstruct - "press only when you truly see the light" - and repeat before any comparison to prior tests.
Why repeating beats reporting
The single highest-value habit in field testing is the willingness to stop, fix, and repeat rather than report a flawed test. A field that reaches the physician with an uncorrected artifact can trigger the wrong clinical decision: an artifactual superior depression may be read as glaucoma progression and prompt an unnecessary medication change, while a false-positive-inflated field may hide real worsening. Because perimetry guides long-term glaucoma management, a single bad test that is accepted can distort the trend line for years. Catching the artifact at the chair costs a few minutes; missing it can cost a correct diagnosis.
Distinguishing artifact from disease
The practical test is simple: a true defect is reproducible and respects anatomic boundaries (the horizontal raphe for glaucoma, the vertical midline for neurologic loss), while an artifact usually has a non-anatomic shape, correlates with a setup problem you can name, and disappears once you fix that problem. If you tape the lid and the superior depression vanishes, it was the lid. If you center the lens and the ring scotoma vanishes, it was the rim. If the defect persists across two well-performed, reliable tests and follows an anatomic pattern, treat it as real and make sure the physician sees it.
Trap-avoidance checklist
- Is the lens close and centered? (lens rim)
- Is the upper lid taped if droopy? (superior depression)
- Are false positives, false negatives, and fixation losses within limits? (reliability)
- Is the correct near add in place? (generalized depression)
- Is pupil size documented and adequate? (media/small-pupil artifact)
- Is this the patient's first field, where a learning effect may apply?
A Humphrey field shows a dense ring-shaped scotoma at the extreme periphery, with a normal central field. What is the most likely cause?
A patient with prominent ptosis shows depression of the superior visual field. Before reporting a superior defect, what should the COA do?