Strabismus or heterotropia is a manifest deviation of one eye from its normal position which occurs despite both eyes being open and uncovered.
A majority of squint sufferers who have excellent cosmetic results from surgery and good visual acuity in each eye may still lack normal stereopsis (binocular depth perception). They develop distance judgement by monocular cues and these are usually superior to those available to applicants who have lost an eye. However, their fine distance judgement for near distances is inferior to those with normal binocular vision.
A latent squint is likely to become manifest under the influence of such factors as illness, fatigue, stress, drugs or alcohol. A well conducted cover test on each eye should unmask latent squint. The tests described below are designed to detect those who lack binocular vision.
Test at near (30cm) and at six metres. Use an accommodation fixation target at both distances. (N5 size print at near and 6/12 letter at distance). Ask the subject to look at the fixation target, cover one eye and observe the other eye for a refixation movement. Repeat for the other eye. Any refixation movement indicates heterophoria, possibly squint.
Test at near (30cm). Hold card still and ask subject to name any pictures seen. Pass is three pictures i.e. cat, star and car. There are two Lang stereo tests. Lang 1 tests to 550 degrees of arc disparity. Lang 2 tests to 200 seconds of arc disparity. Normal stereoscopic thresholds are around 10 seconds of arc, thus this test is quite crude.
The subject wears red/green goggles. A pass is identifying four lights, one red, two green and one white. Test at six metres only. Those who fail can undergo further tests, for example six-metre Vectograph or Bagolini lens test to confirm if they truly lack binocular vision.
The test is also useful to determine if an applicant has alternating suppression. This is when the input from one eye is suppressed alternatively with the other eye. The suppression allows an individual to use both eye alternatively so that diplopia does not occur. Such cases are seen more favourably for certification.
A Class 2 applicant with strabismus may be assessed as having a condition that is not of aeromedical significance if:
If in any doubt the ME should consult the CAA Medical Unit