definition of Accomodation by Medical dictionary

Adjustment of the dioptric power of the eye. It is generally involuntary and made to see objects clearly at any distance. In man (and primates), this adjustment is brought about by a change in the shape of the crystalline lens. In some animals this adjustment occurs either as a result of an anteroposterior movement of the crystalline lens, or of an alteration in the curvature of the cornea. See aniso-accommodation; ciliary muscle; accommodative reflex; Fincham’s theory; Helmholtz’s of accommodation theory.
amplitude of accommodation The maximum amount of accommodation A that the eye can exert. It is expressed in dioptres, as the difference between the far point and the near point measured with respect either to the spectacle plane or the corneal apex or some other reference point. Thus,

A = K − B

where B is the near point vergence and K is the far point vergence. A is always positive. In the emmetropic eye, A = −B, because the far point is at infinity and K = 0. So, if the near point of an emmetrope is at 25 cm from the spectacle plane, the amplitude of accommodation is equal to − [−1/(25 ✕ 10−2)] = 4 D. The amplitude of accommodation declines from about 14 D at age 10 to about 0.5 D at age 60 (although the measured value is usually higher due to the depth of focus of the eye). See minus lens method; push-up method.
astigmatic accommodation Postulated unequal accommodation along different meridians of the eye attributed to a differential action of the ciliary muscle which would lead to a difference in the curvature of the surfaces of the crystalline lens along different meridians. Syn. meridional accommodation.
closed-loop accommodation Accommodation response to visual stimuli in normal viewing conditions. See open-loop accommodation.
components of accommodation The process of accommodation is assumed to involve four components: reflex, vergence (convergence), proximal and tonic accommodation (also called resting state of accommodation). See convergence accommodation; proximal accommodation; reflex accommodation; resting state of accommodation.
consensual accommodation Accommodation occurring in one eye when the other eye has received the dioptric stimulus.
convergence accommodation 1. Accommodation induced directly by a change in convergence. 2. That component of accommodation induced by the binocular disparity of the retinal images. Syn. vergence accommodation.
correction induced accommodation Ocular accommodation induced when changing from spectacles to contact lenses in near vision. Spectacles induce less accommodation in myopes and more accommodation in hyperopes than that exerted by an emmetrope fixating at a given distance. Contact lenses do not induce any different accommodation than that required for a given distance. Consequently, myopes require more accommodation and hyperopes less accommodation when they transfer from spectacles to contact lenses. However, this change in accommodative demand is accompanied by a similar change in convergence, so that a myope transferring to contact lenses accommodates and converges more than with spectacles and the reverse applies for a hyperope. See correction induced convergence.
far point of accommodation A point in space conjugate with the retina (more specifically the foveola) when the accommodation is relaxed. In emmetropia, the far point is at infinity; in myopia, it is at a finite distance in front of the eye; in hyperopia, it is a virtual point behind the eye (Fig. A4). Syn. far point of the eye; punctum remotum. See far point sphere.
ill-sustained accommodation See accommodative insufficiency.
inert accommodation See accommodative infacility.
insufficiency of accommodation See accommodative insufficiency.
lag of accommodation 1. The amount by which the accommodative response of the eye is less than the dioptric stimulus to accommodation, as usually occurs when fixating an object at near. It could be due to uncorrected hyperopia or indicate accommodative insufficiency. Absence of an accommodative lag may indicate latent hyperopia. Syn. lazy lag of accommodation. 2. The condition occurring in dynamic retinoscopy in which the neutral point is situated further from the eyes than is the retinoscopic target. See hyperopic defocus.
lead of accommodation The amount by which the accommodative response of the eye is greater than the dioptric stimulus to accommodation, as occurs when fixating at distance, in a few individuals. It can also occur as a result of a spasm of accommodation. See myopic defocus.
mechanism of accommodation Process by which the eye focuses onto an object. It does so by contracting the ciliary muscle which releases the tension on the zonular fibres, allowing the elastic lens capsule to increase its curvature, especially that of the front surface. Along with these changes are an increase in the thickness of the lens, a decrease in its equatorial diameter and a reduction in pupil size. The ciliary muscle is controlled by the parasympathetic system, which is triggered by an out of focus retinal image. See convergence accommodation; proximal accommodation; accommodative response; ciliary muscle; near reflex.
meridional accommodation See astigmatic accommodation.
microfluctuations of accommodation Involuntary variations in the contraction of the intraocular muscles responsible for accommodation and resulting in changes of about 0.1-0.5 D with a frequency of 0.5-2.5 Hz.
near point of accommodation The nearest point in space that is conjugate with the foveola when exerting the maximum accommodative effort. Syn. punctum proximum. See push-up method; near point rule; near point sphere; Scheiner’s test.
negative accommodation 1. A relaxation of accommodation below the apparent zero level or when shifting from near to distance vision. 2. See relative amplitude of accommodation.
objective accommodation 
Accommodation measured without the subject’s judgment. This is accomplished by dynamic retinoscopy, by autorefractors or by visually evoked cortical potentials. The term is sometimes used incorrectly to refer to the amplitude of accommodation without the influence of the depth of focus (e.g. as measured by stigmatoscopy). See subjective accommodation; optometer; evoked cortical potential; dynamic retinoscopy; stigmatoscopy.
ocular accommodation The amplitude of accommodation referred to the front surface of the cornea. Symbol: A. See spectacle accommodation.
open-loop accommodation Accommodative response occurring without the usual stimulus to accommodation, such as a blurred retinal image. In these conditions, the accommodative system of the eye tends to return to its position of rest (or tonic accommodation). Examples: looking at an empty field; looking through a very small artificial pupil (0.5 mm or less). See closed-loop accommodation.
paralysis of accommodation Total or partial loss of accommodation due to paralysis of the ciliary muscle.
positive accommodation Normal accommodation that occurs when looking from a distant to a near object.
proximal accommodation That component of accommodation initiated by the awareness of a near object. Syn. psychic accommodation. See components of accommodation; resting state of accommodation.
psychic accommodation See proximal accommodation.
range of accommodation The linear distance between the far point and the near point. Part of the range of accommodation is virtual in the case of the hypermetrope.
reflex accommodation An adjustment of the refractive state of the eye stimulated by blur and aimed at reducing blur. It may be initiated when the eye changes fixation from far to near, or it may be induced by convergence. The amount of reflex accommodation rarely exceeds 2 D. See components of accommodation; near reflex.
relative amplitude of accommodation The total amount of accommodation which the eye can exert while the convergence of the eyes is fixed. It can be positive (using concave lenses until the image blurs). This is called positive relative accommodation (PRA). It can be negative (using convex lenses until the image blurs). This is negative relative accommodation (NRA). See binocular vision single zone of clear.
reserve accommodation See near addition.
resting state of accommodation The passive state of accommodation of the eye in the absence of a stimulus, i.e. when the eye is either in complete darkness, or looking at a bright empty field. In this condition, the pre-presbyopic eye is usually focused at an intermediate point (about 80 cm on average, although there are large variations), that is, the emmetropic eye becomes myopic. This is presumably due to a balance between a parasympathetic innervation to the circular fibres of the ciliary muscle and a sympathetic innervation to the longitudinal fibres of the ciliary muscle. Thus, the resting state of accommodation would correspond to a position of equilibrium between the two systems. Accommodation from this state to the near point of accommodation would be the response to parasympathetic stimulation; and accommodation from this state to the far point of accommodation would be the response to sympathetic stimulation. Syn. dark accommodation; dark focus (these terms are not strictly synonymous but as they have been found to correlate well, they have been adopted as synonyms); tonic accommodation (TA). See accommodative hysteresis; night myopia; space myopia; tonus; tonic vergence.
spasm of accommodation Involuntary contraction of the ciliary muscle producing excess accommodation. It may be constant, intermittent, unilateral or bilateral. Patients typically complain of blurred distance vision and sometimes changes in perceived size of objects, and discomfort. If the patient is a low hyperope or emmetrope, it will give rise to pseudomyopia (false myopia, hypertonic myopia, spurious myopia). Diagnosis is facilitated by cycloplegic refraction to rule out latent hyperopia. Although spasm of accommodation can be a separate entity, it is often associated with excessive convergence (esotropia) and miosis; this is referred to as spasm of the near reflex. Management includes removal of the primary cause, if possible (e.g. uveitis, or patient taking parasympathomimetic drugs), correction of the underlying refraction, if any, changes in the visual working conditions, positive lenses, accommodative facility exercises and, only rarely, cycloplegics. See accommodative facility; metamor-phopsia.
spectacle accommodation The amplitude of accommodation referred to the spectacle plane. Symbol: As. See ocular accommodation.
subjective accommodation Measurement of the accommodation based on the subject’s judgements, such as the push-up or push-out method or the minus lens method. See objective accommodation; minus lens method; push-up method.
tonic accommodation See resting state of accommodation.
vergence accommodation See convergence accommodation.

Fig. A4 A hypermetropic eye H and a myopic eye M, fixating an object at the far point M Renlarge picture

Fig. A4 A hypermetropic eye H and a myopic eye M, fixating an object at the far point MR

Table A2 Mean amplitude of accommodation as a function of age, in Caucasians (the plane of reference is the spectacle plane)
age (years) Duane
(N 5 2000 subjects, push-up method)
Turner
(Nh 5 500
subjects, push-out method)
10 13.5 13.0
15 12.5 10.6
20 11.5 9.5
25 10.5 7.9
30 8.9 6.6
35 7.3 5.75
40 5.9 4.4
45 3.7 2.5
50 2.0 1.6
55 1.3 1.1
60 1.2 0.7
65 1.1 0.6
70 1.0 0.6

Table A3 Relationship between viewing distance and spectacle and ocular accommodation of a contact lens wearer and of four corrected hyperopes (with thin spectacle lenses).The vertex distance was 14 mm and ocular accommodation was calculated using the formula A = K − B. The ocular accommodation exerted by a contact lens wearer is the same for all refractive errors and equal to that of an emmetrope
distance from
spectacle lens (cm)
spectacle accom. (D) ocular accom. (D) of contact lens wearer ocular accom. (D) of
hyperopes
+2 +4 +6 +8
67 1.49 1.46 1.55 1.64 1.74 1.86
50 2.00 1.95 2.06 2.18 2.31 2.46
40 2.50 2.42 2.55 2.71 2.87 3.05
33 3.03 2.91 3.07 3.25 3.45 3.67
25 4.00 3.79 4.00 4.24 4.49 4.77
20 5.00 4.67 4.94 5.22 5.54 5.88
16 6.25 5.75 6.07 6.42 6.80 7.22
14 7.14 6.49 6.86 7.25 7.68 8.14
12 8.33 7.46 7.88 8.32 8.81 9.34
10 10.00 8.77 9.25 9.77 10.34 10.95

Table A4 Relationship between viewing distance and spectacle and ocular accommodation of a contact lens wearer and of five corrected myopes (with thin spectacle lenses). The vertex distance was 14 mm and ocular accommodation was calculated using the formula A = K − B. The ocular accommodation exerted by a contact lens wearer is the same for all refractive errors and equal to that of an emmetrope
distance from spectacle
lens (cm)
spectacle accom.
(D)
ocular accom. (D) of contact lens wearer ocular accom. (D) of myopes
4 6 8 10 12
100 1.00 0.99 0.89 0.84 0.80 0.76 0.72
67 1.49 1.46 1.31 1.25 1.18 1.13 1.07
50 2.00 1.95 1.75 1.66 1.58 1.50 1.43
40 2.50 2.42 2.17 2.06 1.96 1.87 1.78
33 3.03 2.91 2.61 2.48 2.36 2.25 2.14
25 4.00 3.79 3.41 3.24 3.08 2.93 2.80
20 5.00 4.67 4.21 4.00 3.80 3.62 3.46
16 6.25 5.75 5.18 4.92 4.69 4.47 4.26
14 7.14 6.49 5.85 5.57 5.30 5.05 4.82
12 8.33 7.46 6.73 6.40 6.10 5.82 5.55
10 10.00 8.77 7.92 7.54 7.18 6.85 6.55

Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann

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