050 Les méthodes de mesure de l’accommodation

Accommodation is the mechanism whereby the converging power of the eye is increased, so that a distinct image is still retained for near objects. This is mainly due to an increased vergence of the crystalline lens. Accommodation is driven by several stimuli (defocus, horizontal vergences, near vision), but in different way. Therefore, when accommodation in measured, its value greatly depends upon the stimulus with which is induced.There are other mechanism allowing the eye to have some degree of multifocality. This capability is named apparent accommodation or pseudo-accommodation and it is present even if the ciliary muscle has been paralysed (cycloplegia) or the crystalline lens is absent (aphakia). The more important factors allowing some degree of multifocality are corneal spherical aberration and pupillary diameter. In a lesser degree, multifocality is also present when some types of IOL are implanted after cataract surgery (pseudophakia).Severing true from apparent accommodation is not easy, especially with the instruments and techniques usually employed in clinical practice. The most usual subjective techniques are:-Push up;-Push down;-Minus to blur;-Near cross cylinder;-Near duo chrome test;-Less used are Duane test, focometer induced accommodation, Helmoltz test.Objective tests in clinical practice are limited to the streak retinoscopy (so called “Dynamic Retinoscopy) and to aberrometers (Tschernig, Shack-Hartmann). Other expensive or experimental way to measure objectively the accommodation can be infrared dynamic photorefractors, UBM, Hartinger coincidence refractometer, pattern VEP.