AC / A quotient

The AC / A quotient is a ratio that is used in ophthalmology and is calculated using a special formula. It is needed to determine possible causes for particular strabismus diseases .

description

The AC / A quotient is used in strabismus medicine ( strabology ). It is a measure of the ratio of the accommodative convergence movement to the accommodation expended and indicates how many prism dioptres of accommodative convergence are achieved per diopter of accommodation. The AC / A quotient is thus an indicator of a pathological state of binocular vision . It can be measured in two different ways and using an optimal eyeglass correction.

Heterophoria method

In the heterophoria method , the deviation of one eye is measured while switching off the two-eyed vision ( fusion-free setting ) when looking into the distance (5 m) and near (0.33 m). The AC / A quotient is calculated using the following formula:

${\ displaystyle AC / A = PD + (FN-FF) / A}$

where: PD = pupillary distance in cm, FN = near deviation in prism dioptres, FF = far deviation in prism dioptres, A = accommodation (as the reciprocal of the test distance in the vicinity).

Gradient method

With the gradient method, the deviation of an eye is measured at a given distance while switching off binocular vision. Now in the same distance minus lenses are attached, (Akkommodationsanforderung) until the fixed optotype ( the optotypes ) are blurred. The deviation is now measured again and the thickness of the minus lenses is noted. The AC / A quotient is then calculated using the formula:

${\ displaystyle AC / A = (FL-F) / A}$

where: FL = misalignment in prism dioptres with minus lenses, F = misalignment in prism dioptres without minus lenses, A = thickness of the minus lenses held in front of them in dioptres.

rating

The heterophore method and the gradient method will seldom provide an identical AC / A quotient, the gradient method leading to more reproducible but less clinically relevant data. The normal value is around 2–3 ° (4–6 Pdptr.) Of accommodative convergence per diopter of accommodation performed. Anomalies in this relationship can trigger latent or manifest deviations from the squint of the convergence or divergence excess type, as well as of the convergence and divergence insufficiency type. An increased AC / A quotient can be associated with normal or reduced accommodation ( hypo- accommodation ).

literature

Herbert Kaufmann (Ed.) : Strabismus . 4th fundamentally revised and expanded edition, with Heimo Steffen, Georg Thieme Verlag, Stuttgart, New York 2012, ISBN 3-13-129724-7 .

Individual evidence

^ ^A ^b Albert J. Augstein: Ophthalmology. With the assistance of JF Collins. 2nd edition, Springer Verlag 2013, page 111, ISBN 978-3-662-05919-7 .

[Augstein-1] A ^b Albert J. Augstein: Ophthalmology. With the assistance of JF Collins. 2nd edition, Springer Verlag 2013, page 111, ISBN 978-3-662-05919-7 .