Most eye disability is about how much a person sees — acuity, field, how much light arrives. Double vision is a different kind of injury: the patient sees two of everything. And here the rating does not ask how sharp the vision is, but where the doubling appears in the field of gaze. It has a clause of its own — clause 57 — and in both schedules it is built around the same principle: the closer the doubling to straight-ahead gaze, the higher the percentage.
What double vision and strabismus are
Double vision (diplopia) in the eyes is almost always caused by the two eyes not aiming together at the same point. The causes vary: a palsy of one of the ocular motor nerves (the third, fourth or sixth), thyroid eye disease restricting the eye muscles, a fracture of the orbital floor entrapping a muscle, a heterophoria that has lost its compensation, or myasthenia gravis in which the doubling varies with fatigue.
The functional loss is direct and immediate: one cannot drive safely, descending stairs becomes dangerous, and sustained reading is exhausting. Some patients compensate by tilting the head, others close one eye instinctively. This is why the schedule rates by direction of gaze: it measures not the squint itself but the extent to which the doubling intrudes into the fields of gaze actually used.
The logic: the direction of gaze sets the percentage
In both schedules, the doubling is rated by the direction in which it appears, and the percentage rises as the direction approaches straight-ahead gaze. Up-gaze — the least-used direction — at the bottom; down-gaze and the sides in the middle; straight-ahead gaze, the position for walking and driving, at the top. And where the doubling exists in several directions, the rule is explicit: rated by the direction yielding the highest percentage, and only one rung is selected.
A structural point worth knowing: clause 57 gives a single percentage, with no split between one eye and both. This makes sense — double vision is by its nature a phenomenon of the two eyes together; there is no “double vision in one eye”.
At the National Insurance Institute: clause 57
| Finding | Percentage |
|---|---|
| Heterophoria or convergence weakness with functional disturbance | 10% |
| A squint of ≥15 prism dioptres, without diplopia | 10% |
| Squint with diplopia — on up-gaze | 10% |
| Squint with diplopia — on down-gaze or to the sides | 15% |
| Squint with diplopia — on forward gaze | 20% |
Note the second row: it is the one place in the clause where the schedule quantifies the size of the deviation in a number — 15 prism dioptres or more — and awards it even where there is no doubling at all. A large squint, even if the brain has learned to suppress the second image, counts as an impairment in its own right.
At the Ministry of Defence: clause 57
| Finding | Percentage |
|---|---|
| Heterophoria or convergence weakness with functional disturbance | 10% |
| Squint without doubling, or with doubling on up-gaze | 10% |
| Squint with doubling on down-gaze and/or to the sides | 15% |
| Squint with doubling on straight-ahead gaze | 20% |
| Squint with doubling requiring occlusion of one eye | 25% |
The two schedules meet in the middle — 15% on down-gaze and to the sides, 20% on forward gaze — but part at the extremes. The Ministry of Defence does not rate by dioptres; instead it groups “a squint without doubling” and “doubling on up-gaze” together at 10%. And more importantly, it adds a fifth rung the National Insurance Institute lacks: doubling so severe that it requires occlusion of one eye — 25%.
That rung deserves attention. A patient forced to cover an eye to be rid of the doubling becomes effectively one-eyed; he trades one problem for another. This is why it sits at the top of the scale, functionally close to the loss of an eye — a subject discussed in one-eye blindness and disability.
What should be documented before the committee
- A mapping of the doubling by direction of gaze. Because the percentage is set by the most severe direction, documentation showing clearly in which directions there is doubling — and above all whether it exists on straight-ahead gaze — is what fixes the rung. A generic description of “double vision” loses percentages.
- Measurement of the squint angle in prism dioptres, especially relevant to the National Insurance track and the 15-dioptre row.
- A cause workup — nerve palsy, thyroid disease, orbital fracture, myasthenia. In myasthenia, where the doubling varies with fatigue, it is worth documenting it at the end of the day and not only in the morning.
- Documentation of any need to occlude an eye, where present — it is the criterion for the top rung at the Ministry of Defence.
- A stable state, preferably after rehabilitation or surgery has ended, because fresh doubling after an injury may improve with time.
The injuries that lead to double vision often overlap with others — an orbital fracture in a road accident or a work accident, for example — and the general framework for setting the rating is on eye disability assessment.
Bottom line
Double vision is the clause where how sharply one sees is beside the point; what matters is where the gaze breaks. In both schedules the percentage rises as the doubling approaches straight-ahead gaze — 10% up, 15% down and to the sides, 20% forward — and the Ministry of Defence adds 25% where the state requires covering an eye. Whoever reaches a committee with a precise mapping of the directions of doubling, and not just the diagnosis, knows the rung will be set by what the eyes actually do in each direction.
Frequently asked questions
How many disability percentages does double vision carry?
The answer depends mainly on the direction of gaze in which the doubling appears. At the National Insurance Institute, clause 57 rates diplopia on up-gaze at 10%, on down-gaze or to the sides at 15%, and on straight-ahead gaze at 20%. At the Ministry of Defence the scale is similar — 10%, 15%, 20% — but with a fifth rung the National Insurance Institute lacks: diplopia requiring occlusion of one eye gives 25%. In both schedules this is a single percentage, not a split between one eye and both, because double vision is by its nature a binocular phenomenon.
Why is doubling on straight-ahead gaze worth more than on up-gaze?
Because it strikes what you use all the time. Straight-ahead gaze is the position for walking, driving and recognising faces; down-gaze serves reading and descending stairs. Doubling there disrupts basic daily function. Extreme up-gaze, by contrast, is rarely used, so doubling appearing only there is rated lower. The schedule is built around this functional logic: the closer the region of doubling to the useful centre, the higher the percentage.
I have a squint without double vision. Is that worth anything?
It may be. At the National Insurance Institute a strabismus of 15 prism dioptres or more, even without diplopia, gives 10% — the one rung where the schedule quantifies the size of the deviation in a number. Convergence weakness or a heterophoria with functional disturbance also gives 10%. At the Ministry of Defence there is no rung by dioptres, but a squint without doubling, or doubling only on up-gaze, is likewise rated 10%.
What happens when the doubling is so severe that an eye must be covered?
That is the most severe rung, and it exists only at the Ministry of Defence: doubling requiring occlusion of one eye gives 25%. The functional meaning is that the patient becomes effectively one-eyed — giving up one eye to be rid of the doubling. This is conceptually close to the functional loss of an eye, whose consequences are set out in [one-eye blindness and disability](/en/one-eye-blindness-disability/).
