Clinical Measurements of Accommodation


 Clinical measurements of accommodation

Far point of accommodation is the point conjugate with the retina when the accommodation is fully relaxed. (Punctum remotum)

Near point of accommodation is the point conjugate with the retina when the accommodation is fully exerted. (Puntum Proximum)

The range of accommodation is the difference between the far point and the near point, that is the distance over which accommodation is fully exerted.

The amplitude of accommodation is the dioptric distance between the far point and the near point of accommodation

If the far point is located at optical infinity, that is zero diopters, then the amplitude of accommodation equals the dioptric equivalent of the near point of accommodation.

Techniques for determining the amplitude of accommodation:

Push up technique
Push down technique
Minus lens technique
Negative relative and positive relative accommodation
Methods of spheres
Dynamic retinoscopy and other optometers.

PUSH UP TECHNIQUE:
1. To identify the near point of accommodation, that is the target location for which the patient is exerting his maximum accommodative response.
2. That is to determine the closest distance at which the target remains absolutely sharp. (In practice, such an end point is difficult to identify.)
3. It is conventional to locate the position where the target exhibits the first, slight, sustained blur.(just begins to blur and remains blurred).
4. The patient should always be encouraged to try and clear the target when the patient reports the blur.
5. The reciprocal of that point is the near point of accommodation.
6. In this slight over estimation of the near point occurs.

PUSH DOWN TECHNIQUE:
1. The target is advanced a little beyond this point of the first sustained blur.
2. Target moved away from the patient until the target "just becomes absolutely clear".
3. This procedure attempts to obtain a more accurate determination of the near point. (How)
4. In this care must be taken to ensure that the target is advanced only slightly beyond the first blur position.
5. If the stimulus is moved several diopters closer than the near point, the patient may start to relax his accommodative response.
6. In this slight under estimation of the near point occurs.

The average value of the push up and push down techniques gives accurate assessment of the near point of accommodation.

MINUS LENS TECHNIQUE:
1. In this the target remains at a fixed position at 40cms and minus lenses are introduced in 0.25D steps to move the location of the optical image of the target.
2. The patient will report the first noticeable sustained blur that cannot be cleared by further conscious effort.
3. The total amplitude of accommodation is equal to the amount of minus lens power introduced, plus the 2.5D required to focus initially on the target.
4. It is not performed binocularly, when adopting the minus lens technique, the disparity-vergence stimulus remains fixed at approximately 2.5meter angles.
5. Thus, if the procedure were performed binocularly this would require increased accommodation while maintaining the vergence response at relatively constant level.

Differences between minus lens and push up techniques:

1. During testing with the minus lens the target remains fixed, accordingly, the psychological proximal stimulus to accommodation remains relatively constant.
2. In push up technique, in which the angular subtends of the target increases for higher dioptric stimuli, this is more natural than the minus lens technique.
3. Higher amplitude with the push up probably due to the additional proximally induced accommodation stimulated by the advancing the target.

Positive and negative relative accommodation:

Positive and negative relative accommodation represent changes in accommodation that can be elicited while the stimulus for vergence is held constant.
These are measured clinically by introducing increasing minus and plus lenses while the patient views a near target at 40cm.
The end point is taken when the patient reports the first, slight, sustained blur.
The amount of spherical power added to the original prescription represents the magnitude of relative accommodation.
Test should be performed with their distance prescription in place, and through the appropriate near prescription.

Changes of accommodation and vergence during assessment of PRA and NRA:


Positive relative accommodation:

1. Increased blur-driven accommodation
2. Increased accommodative convergence
3. Decreased disparity-vergence
4. Decreased convergent accommodation
5. Increased blur driven accommodation

Negative relative accommodation:

1. Decreased accommodative convergence
2. Decreased accommodative convergence
3. Increased disparity vergence
4. Increased convergent accommodation
5. Decreased blur driven accommodation

In fact during PRA testing, the compensatory decrease in disparity vergence is frequently less than the increase in accommodative convergence, resulting in an esophoric shift in fixation disparity.
Exophoric shift in fixation disparity is typically observed during NRA testing.

Methods of spheres:

Amplitude of accommodation may also be measured by having the eye fixate on a reading target (e.g. at 40cm). The accommodation is stimulated by placing minus spheres until the print blurs and accommodation is then relaxed by using the plus lenses until the onset of blurring noted. The sum of these two is the A.A.

Factors affecting measurements of the accommodative amplitude:

1. Monocular versus binocular:

Push up and push down techniques can be done both monocularly and binocularly also.
When testing monocularly proximal accommodation is induced and in binocularly in addition convergent accommodation also is considered.
Studies showed that binocular amplitude is higher than the monocular findings.
While doing binocularly care must be taken to measure the actual distance from the target to the spectacle plane along the visual axis.
If this is along the midline, the distance will be shorter so apparently elevated A.A.

2. Age:

Donders and Duane have reported a decline in amplitude at a rate of approximately 0.30D per year.
Hofstetter proposed three equations to represent the minimum mean and maximum expected amplitudes:
Minimum amplitude 15-(0.25) (age)
Mean amplitude 18.5-(0.3)(age)
Maximum amplitude 25-(0.4) (age)

3.Target size:
Variations in target size may produce significant changes in the subjective amplitude of accommodation.
Fonda proposed that the amplitude of accommodation should be measured using a target that subtends an angle of no more than 5 minutes of arc at the individual's near point.

Dynamic retinoscopy:

The principle of both static and dynamic retinoscopy is that a neutral reflex is observed when the point conjugate with the retina coincides with the retinoscope peephole.
This is the most common technique used to assess the objective amplitude.
Accommodative response is measured for a series of increasing stimulus levels until the maximum response is achieved.

Ex:
Target at 40cm          Neutral at 50cm

25cm                33cm
20cm                25cm
17cm                20cm
14cm                20cm
13.5cm             20cm

Therefore objective amplitude of accommodation is 5D.

In this a point is reached where further increases in the accommodative stimulus are not accompanied by an increase in the accommodative response. Thus the maximum response may be determined.

However, any optometer such as autorefractor, may be used to determine the objective amplitude provided that an adequate range of stimuli can be introduced.


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