Titmus Fly Component of Stereopsis Testing
What It Is and How to Perform It
The Titmus Fly is a gross stereopsis test that measures depth perception at 3000 seconds of arc using polarized glasses and a large fly image where the wings appear to lift off the page when stereopsis is present. 1
Test Administration
Standard Protocol
- Position the test booklet at 40 cm from the patient while they wear polarized glasses 2
- Ask the patient to pinch or point to the wings of the fly, which should appear elevated above the page 1
- The fly's wings provide a disparity of 3000 arcseconds, representing coarse stereopsis 1
- A "pass" is recorded when the patient accurately identifies and reaches for the elevated wings 1
Modified Protocol for Improved Accuracy
Use a two-step verification process to eliminate false positives, particularly in patients with strabismus or those repeatedly exposed to the test 3, 1:
- Step 1: Administer the standard Titmus Fly test as described above
- Step 2: If the patient passes, re-test immediately with both polarizers oriented in the same direction (eliminating stereoscopic cues) 1
- If the patient still "pinches" above the plate without disparity cues present, this represents a false positive response 1
- This modified protocol increases sensitivity from 81% to 90% 1
Interpretation
Quantitative Assessment
The amount of perceived depth correlates directly with stereoscopic threshold and can be used to estimate finer stereoacuity 4:
- Measure how high above the page the patient perceives the wings to be elevated
- This perceived depth measurement correlates accurately with measured stereoscopic threshold 4
- A regression equation exists to convert perceived depth into estimated stereoacuity 4
Clinical Pitfalls
The Titmus Fly has a notoriously high false positive rate, with specificity of only 26% in some populations 3:
- Patients with childhood-onset strabismus who have repeated exposure are particularly vulnerable to learned responses 3
- Monocular cues (such as the visible offset of the printed images) can be detected without true stereopsis 3, 1
- Always use the modified presentation method in patients with small-angle strabismus to improve accuracy 3
When to Use This Component
The Titmus Fly is most appropriate for young children who cannot complete more detailed stereoacuity testing or patients with manifest strabismus where only gross stereopsis assessment is possible 3, 1:
- It serves as a screening tool for the presence versus absence of coarse stereopsis 1
- For patients who pass the fly test, proceed to finer stereoacuity measures (Titmus circles or random dot tests) 1
- The test can be performed without polarized glasses using newer panagraphic versions, which is advantageous for young children who refuse to wear glasses 2