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3.2.1.2 The Strength of Accommodation
Some results about the strength and amplitude of accommodation of myopes are:
Abbot et al. stated142: "A reduced accommodation response to negative lens-induced accommodative
demand was found in progressing myopes but not in stable myopes." In
addition, eyes with myopia were found to have lower amplitude of accommodation. Fong
stated143 "Eyes with lower amplitudes of accommodation must use more of their accommodative
reserve for near work. Myopia may be an adaptation that develops in eyes with
reduced accommodative amplitudes", and less accurate accommodation144. And a reduced
accommodation was speculated to create a blur signal, which might be responsible
for development of myopia80. Consequently, under-accommodation may precede the development
of myopia as clinical data suggest 145.
In contrast to these results in other experiments it was found by McBrien et al.146 that
"...late onset myopes having the largest amplitude of accommodation, followed by early
onset myopes, emmetropes, and hyperopes."
In this respect, it is interesting that wearers of glasses accommodate less and have lower
convergence demands than emmetropes or wearers of contact lenses due to optical reasons
147, 148, but that hard contact lenses were often found to stop progression of myopia
(not soft contact lenses, see section 3.19.1).
Some positive effects of pharmaceuticals like atropine can be at least partly attributed to
their attenuating effect on the accommodation (see section 3.17).
Chinese people have lower amplitudes of accommodation than Caucasians149, and Chinese
have a higher probability of becoming myopic.
Notes:
From an evolutionary point of view there might be a linkage between both facts:
- Without any accommodation the only one way to focus is to adjust the length of the eye as
happens in emmetropization (see section 3.3.5), and for dominating near work this means
to become myopic.
- Question: is there a rule like "the lower the accommodation before the onset of myopia, the
higher the probability and the degree of the myopia that will develop later"?
For information about the related issue tonic accommodation (resting state accommodation) see
section 1.4.1, the interaction between accommodation and convergence is discussed in sections
3.4.2 and 3.4.3.
3.2.1.3 Timing- and Hysteresis- Effects of Accommodation
Some results about the time it takes to accommodate and to recover are:
Accommodation causes a substantial hysteresis of the ocular shape: With normal people,
after accommodation, as Walker et al. stated150, "... ocular shape had become more prolate
[i.e. stretched]. This shape remained unchanged after 1 hour of sustained accommodation
and then returned to baseline dimensions after 2 h of accommodation ... Ocular shape returned
to baseline dimensions after 45 min of accommodative relaxation." (see section
3.6.4).
Myopes showed reasonable aftereffect of accommodation17, i.e. it took an average of 35
seconds for early-onset myopes and 63 seconds for late-onset myopes until the accommodation
of the ciliary muscle was released. Non-myopes, however, showed no myopic
aftereffect. It was speculated that this transient pseudomyopia is either the cause or a precursor
of permanent myopia 151.
Additionally, it was shown that during the progression of myopia there is a significant near
work after-effect (sometimes called nearwork-induced transient myopia - NITM17), causing
transient myopia that still existed 10 seconds and 30 seconds after the near task 152, 153.
Correspondingly, there is a reduced facility, i.e. a reduced maximum frequency of accommodation:
"Mean distance facility was significantly lower (9.7 cycles per minute
(cpm)) in the myopic group compared with the mean distance facility in the emmetropic
group (15.6 cycles per minute). There was no significant difference in the near facilities of
the two groups (11.5 cpm in myopes versa 12.9 cpm in emmetropes)."154
More recent experiments confirmed the fact that myopic children are showing a significantly
larger nearwork-induced transient myopia, but demonstrated additionally that this
nearwork-induced transient myopia was still evident after 3 minutes, which is significantly
longer than what was previously reported for adults155.
There is a time lag of accommodation for myopes 130, 156. However, Rosenfield et al.
stated132 that "...stable myopes also exhibited the largest lag of accommodation."
For children it is generally recommended (Note: but very often not done) to do the refraction
after applying a cycloplegic agent. The residual accommodation, which exists if no
cycloplegic agent is used results in overcorrection of myopiaFehler! Textmarke nicht definiert..
Notes:
- Maybe the hysteresis of the reshaping of the lens before accommodation and after accommodation
contributes to this effect as well (see section 1.3.3).
- This aftereffect and hysteresis of accommodation can be responsible for a steady progression of
myopia in school kids: If the kid has to change in class permanently and fast between near work
and reading from the distant blackboard, the mentioned delay can give the impression that the
current glasses are too weak; new, stronger glasses, however, can easily induce additional myopia
as described in section 3.3, and these cycles can repeat again and again. The long duration of the
aftereffect (which was still evident after 3 minutes) strongly supports this model. Bifocal glasses
might help in these cases, but care has to be taken when fitting these bifocals (see section 3.2.2).
- Maybe the anomalies of myopic accommodation can be explained by a lack of nitric oxide (NO) in
the smooth ciliary muscle (see sections 3.12.4 and 4.2.3.4).
- Open question: What is responsible for the time delay and the aftereffect of the accommodation, a
muscular problem, or a hysteresis problem of the flexibility of the lens? It appears to be rather
likely that a muscular problem is a key issue for myopia, i.e. not a consequence, but a reason.,
3.2.1.4 Aniso-Accommodation
For the case, that the object, which is looked at has a different distance from each eye Charman
stated157: "It is shown that the asymmetric convergence caused by the changes in fixation required
to read a line of text results in unequal accommodation demands to the two eyes. Since experimental
evidence suggests that the required aniso-accommodation response cannot be achieved,
and that accommodation in both eyes tends to match the response to the lower of the two demands,
in general the retinal image in at least one eye must be out-of-focus."
This effect can result in a risk particularly for children, which are reading in unfavorable positions,
i.e. lying on the side when reading (common when reading in bed).
Charman stated the conclusion157: "...maintain a symmetric posture and ... keep working distance
as long as practicable."
3.2.1.5 Is there a Connection between Blur Sensitivity and
Accommodation Deficits?
As the detected blur of an image can trigger the accommodation function there is the possibility
that the reduced blur sensitivity of myopes is the reason for deviations of the accommodation performance.
The results are, however, contradictory:
Rosenfield et al. stated130: "These results demonstrate that myopes are less sensitive to the
presence of blur, and may at least partially explain why previous reports have demonstrated
a larger lag of accommodation in this refractive group."
Schmid et al. stated158: "There was no correlation between blur thresholds and refractive
error magnitude, refractive error progression over the past year, or contrast sensitivity."
Note:
The reason for the contradictory results must be seen in differences in the setup of the experiments.
3.2.1.6 Accommodation and the Nervous System
The ciliary muscle, which performs the accommodation, is triggered by actions of the sympathetic
and the parasympathetic nervous system. The parasympathetic system is responsible for the contraction
of the ciliary muscle, and the sympathetic system is responsible for the relaxation, as Holland
stated: 159 "In effect, the sympathetic system facilitates a rapid shifting of accommodation
from near out to far as one would expect in a "fight or flight" situation."
In general, as Holland summarized other studies159 "... mental activity is accompanied by a shifting
in accommodation towards far."
Correspondingly, Davies reported160: "Increasing the cognitive demand led to a significant reduction
in the accommodative response in all subjects."
Note:
Obviously, near work, which requires mental concentration creates a conflict: The mental activity
triggers accommodation for far, the near work requires accommodation for near. Plus-glasses for
near work should be able to solve or at least reduce this conflict.
There are two different hypotheses about the nervous system existing, both supported by numerous
papers17:
Myopia is caused by "a deficit in the sympathetic innervation."
Myopia is caused by "a deficit in the dual [i.e. both sympathetic and parasympathetic] innervation."
The nitric oxide (NO) metabolism plays a role in the functioning of the sympathetic and the parasympathetic
nervous system161. Results with respect to the connection between NO metabolism
and myopia are discussed in section 3.12.6.
3.2.1.7 Accommodation and Biochemistry
Rada summarized238: " ...the chick choroid [the layer between retina and sclera] undergoes a rapid
and dramatic increase in thickness in response to myopic defocus. This thickness change may be
the result of changes in choroidal blood flow...or by the contraction and relaxation of nonvascular
smooth muscle cells present within the choroidal stroma. It is hypothesized that choroidal
thickening is a rapid mechanism for reducing the refractive error, by pushing the retina to
the focal point. Concomitant with choroidal thickening, the rate of vitreous chamber elongation
slows dramatically, as does proteoglycan synthesis in the chick sclera."
In other words, near focus initiates multiple biochemical processes, and it is likely that contractions
of smooth muscles are involved. The ciliary muscle, which is responsible for accommodation,
consists of smooth muscle fibers.
See also section3.21.2 about the interworking between mechanics and biochemistry.
3.2.1.8 Summary of Results about Accommodation
Quite a number of people react with myopia when exposed to extensive near work and accommodation,
and myopes have an accommodation problem. The open question is still162, whether myopia
is caused by:
Too much accommodation, i.e. near work, potentially at a too short working distance.
Too weak accommodation (under-accommodation), by which the focus is put at a distance
behind the retina at least temporarily, or accommodation hysteresis, i.e. a delayed
relaxation of accommodation after near work.
The impact of the accommodation-caused temporary stretching of the sclera on the biochemistry
of the sclera (see section 3.6.1)
An unbalance of the vergence system (vergence is the adjustment of the axes of the two
eyes to each other according to the distance of the object, see section 3.4)
Too weak connective tissue to cope with the extra stress on this connective tissue during
accommodation.
An overreaction on the imaging effect, which accompanies (potentially inadequate) accommodation
(see section 3.3).
Abnormal physical or physiological properties of components of the eye.
Apparently, however, near work creates myopia only in connection with other, so far in detail still
unknown processes.
Moreover, the reason for myopia can be rather specific, i.e. different for different people, and different
for different grades of myopia.
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