Table 1 Positive diagnostic tests for functional visual loss and explanatory/therapeutic opportunities they provide.

Observation of the patient’s ability to navigate obstacles in the waiting room and clinic room

Consider video recording the patient’s gait to help persuade carers that vision may be better when navigating than at other times. This is good because it shows the potential for improvement

Observation of patient’s ability to use a mobile phone in waiting room and clinic including assessment of font size

Discuss how the font size they can see on their phone is better than their near vision achieved on formal testing. This shows that their brain can see better when they are relaxed and looking at interesting content compared to doing a stressful eye test

Observing eye contact. Often starts off with avoidance of eye contact initially which improves when more distracted or discussing things unrelated to visual loss.

Discuss with the patient and carer that you noticed this, and that good eye gaze depends on good vision. Discuss when eye gaze became better and how recreating this feeling may help in therapy.

Optokinetic drum eliciting horizontal jerk nystagmus (for those who have vision between no perception and hand movement) = acuity of at 6/120

Show carers how an optokinetic drum (or even an iPhone video of one) can induce nystagmus and thus provides evidence of a pathway between the eye and brain

Mirror Gazing. Ability to follow a reflection in a mirror that’s being rotated also denotes vision better than light perception

This may be more useful as a therapeutic manoeuvre similar to the optokinetic drum than a diagnostic one.

Modification of visual acuity checking – for example start with smallest type and work upwards, or use single letter optotype, removing cue of other letters about what is ‘hard’

Consider doing the test in the standard way afterwards and share with the patient how they found it easier to see when not confronted by a task of increasing difficulty.

Lack of improvement in acuity with moving the chart nearer.

Explain how vision should improve in this situation but in functional visual loss, the brain “expects” to see badly regardless of how the image is present.

Use of technology that promotes saccadic eye gaze – eg ‘peekaboo vision’ app designed for young children that produces saccades towards a striated target at higher acuities than they are aware of, even if they can’t tap the correct target.

This can be explained similarly to nystagmus induced by an optokinetic drum (see above). Your brain IS seeing the target, enough to move your eyes towards it, but not enough to give you a signal to make a tap.

Fogging test in which vision in the good eye is worsened to show that vision must be coming from the bad eye by:

a) progressively fogging the good eye

b) blocking vision with an over-powered refractive correction (eg ± 5 dioptre), whilst the “bad” eye has either a minimal power lens (eg ± 0.5 dioptre lens) or the patient’s known refractive correction or c) blocked using paired cylinders of opposite signs over the good eye (eg +3 and −3) placed initially with their axes aligned to cancel each other out – then with the cylinder axis rotated by 15–20 degrees to create blurring.

However fogging is done, this is an opportunity to explain back to the patient or carer afterwards that vision in the bad eye can be temporarily improved by confusing the brain with different eye tests. “The brain thinks it is seeing out of the good eye, which is why vision improves, even though its actually seeing out of the bad eye”. It helps move the conversation away from the eye to the brain where the problem is.

Bagolini glasses – Where the patient perceives one light with two long striations arranged in a saltire’ cross, binocular single vision is present which should not be possible if the patient has a structural cause for visual loss.

Explanation of the mechanics of the test^a will be understood by very few patients – but you can nonetheless explain how you need two eyes to be working to see the cross, so like fogging its evidence that the brain’s visual pathway works better than the patient perceives it to.

Prism dissociation testing – through various manipulations^b prisms can induce diplopia in individuals with normal binocular vision. Even if the patient doesn’t experience diplopia, then the patient can be seen looking between one image and another

The test can be discussed with patients and potentially saccadic movements shown to carers as evidence that there is double vision even though the person doesn’t experience it, again pointing to a brain rather than an eye problem

Stereopsis testing – as a substitute for acuity testing. For example, ability to see a 55 degree 3-D image on the Frisby Stereotest is equivalent to 6/12 vision in both eyes

Not so helpful as a demonstration of normal vision, but discussion is another way to emphasise what, at a minimum, you might expect visual acuity to be if functional visual loss could improve.

Tunnel vision – eg same visual field at 1 m as at 2 m – should be conical and not a cylinder.

Explain that the brain ‘expects’ to see a tunnel which is why the patient has that response. The laws of physics mean that a field must get wider the further away it gets so the explanation is that the brain is suppressing the additional visual information to fit with its expectation. Use of a diagram may help (Figure 2).

Spiralling and other visual fields – including spiralling, crossing of isopters, clover leaf pattern and stacking. All common in functional visual loss but can also occur due to poor technique or retinopathy, as attention dependent.

Explain that spiralling is a feature that doctors look for when diagnosing functional visual loss. Spiralling happens because during the test the person gets more and more tunnel vision as the test goes on. Ask if they felt “spaced out” during the test and explain how the brain tends to make tunnel vision when it’s spaced out or under threat.