From Looking to Seeing and understanding Accommodation
This talk on "accommodation" was presented by Dr. G. D.
Hoonka, at Hotel Krishna, 5 PM on July 25th 2004. This talk was presented
with OHP support, hand drawn diagrams and working models explaining
the process of accommodation in great details. The program was sponsored
by Himalaya Drugs.
In our school days our teachers often used to say - "Look at the
board and see what is written". But inspite of doing the same habitually,
difference between looking and seeing was not clear. Now we realize
that between the process of looking 'to seeing, some interaction does
occur between the eyes and brain. What is that and how it is accomplished?
is the theme of today's talk.
In context of senile cataract operation even with PC IOL, we know that
due to loss of accommodation the pleasure of reading or Computer work
is hampered. Therefore to explore the possibility to restore the accommodation
is the aim behind this talk, so as to share our experiences in this
regard and also to stimulate further research for creating physiologicaly
true replica of human lens with dynamic accommodation & also the
accommodating spectacles & medicine for presbyopes.
Working of the eye is comparable to a focussing camera which can be
set from infinity to a close up shot However far and near objects can't
be focussed simultaneously -It is always "One at a time".
For clear focussing of near objects eye has to increase its refractivity.
This process of changing focus is called accommodation. Theoretically
in animal Kingdom it can be achieved by any of the four ways:
.1. As seen in some snail's eyes - by elongating the eye to 'H'. In
camera it amounts to shifting the photographic plate backwards.
2. As seen in some fishes - by advancing the lens forward. But Tscherning's
calculation showed that in man 1 10 mm advance will be necessary whereas
anterior chamber is only 3mm deep. It is similar to shifting spectacles
towards tip of nose.
3. As seen in some birds like eagle - by increasing curvature of cornea.
4. By increasing curvature of lens in man raises its power from +18
to 23 D.
Before elaborating accommodation - mechanics, which is solely based
on elastic forces of lens and ciliary body against the action of ciliary
muscles, recalling their brief anatomy will be better for the proper
understanding of the subject :-
Anatomy of the structures responsible for the process of accommodation
The crystalline lens
Derived from embryonic ectoderm it is a bi-convex mass of pliable
(soft & elastic) tissue which is enclosed in a highly elastic capsule
and held in situ by the suspensory ligament - zonule of Zinn Its anterior
surface lies behind the iris and posterior surface resting in the retrolental
patellar fossa, is separated from the face of vitreous by a capillary
film of aqueous. Its equator lies only 0.5 mm from ciliary process (Sorsby
vol. 1 P-126). Its dimensions are shown in fig. 2.
As per B. Patnaik (Adler's Physiol) during accommodation in human lens
it is not the cortex but the nucleus which undergoes the deformation.
Thus as long as nucleus remains soft and elastic it can be moulded easily
by the thin but extremely elastic capsule, whereas hard & inelastic
presbyopic nucleus fails to mould.
The lens capsule
Thickness of capsule is not uniform. As per Fig. 3 it is thickest towards
equator and thinnest at posterior pole. Capsule is so elastic that:-
1. On capsulotomy, particularly in children, its cut edges gap and
role out.
2. After absorption of traumatic cataract undulating folds are seen
on the
floating capsule.
3. Long standing dislocated lens, owing to freedom of zonular pull becomes
almost spherical.
4. When capsule of water swollen lens is punctured by a fine needle
water jets out.
Thus it is the elastic recoil of the capsule that moulds the lens for
near vision.
The Suspensory Ligaments
Though derived from vitreous but look like fine hair but are strong
and visible to naked eyes. Their length is (stallard) between 2-7 mm
and resistance to rupture is equivalent to 100 g. in children and 60G
in elderly. Due to resemblance to hair it is also called "Zonula
Ciliaris". Further pull of the zonules to stretch the capsule is
comparable to pull of hair on skin which is easily demonstrable. Strands
of suspensory ligaments, passing forward and backwards cross each other,
are divided into three groups :-
1. Anterior group (AZ) :- They arise from pars plana of ciliary body
almost as for back as ora serrata (which is 8 mm behind limbus). After
lying in contact with ciliary body, curve towards equator of lens for
insertion into capsule 1.5 mm anterior to equator. They are longest
(7 mm).
2. Posterior group (PZ) :- They arise from the summits and sides of
ciliary processes and pass backwards for insertion into capsule 1 mm
posterior to equator. Their length is 4 mm.
3. Equatorial group (EZ) :- They arise from the summits of ciliary processes
and pass directly inwards to be inserted at the equator which is 0.5
m away. They are shortest (2 mm) and are pulled by radial fibers under
sympathetic control.
The Cilliary Muscles
This is a circum lenticular muscle and its chief mass is composed of
involuntary smooth muscle fibers that are divided into three parts with
a common origin in the ciliary tendon which blends with scleral spur.
Here it is noteworthy that during the muscular contraction it is the
inserted part which is pulled towards the fixed origin.
1. Meridional Fibers (MF) :- (Brucke's Muscle) The greater part of
ciliary muscle is composed of meridional fibers running anterio-posterior
on the inner aspect of the sclera to find diffuse insertion into the
suprachoroid (about 8 mm behind the limbus) where muscle stars are produced.
Under the parasympathetic control through short ciliary nerves its contraction
pulls the suprachoroid and ciliary body forwards and inwards by 0.5
mm (Davson). As a result periphery of the vitreous is also compressed
so that patellar fossa along with lens move forwards (High lights).
2. Annular or circular fibers (Mullar's Muscle) (AF) :- Most of the
remaining fibers run so obliquely in interdigiting V shaped bundles
that it appears running in a circle around the ciliary body concentrically
with the root of iris. Their sphincter like action constricts the ciliary
ring around the lens & thereby relaxing the anterior and posterior
zonules. These are parasympathetically controlled through short ciliary
nerves and are synergistic to meridional fiberes.
3. Radial Fibers (RF) :- The third portion of the ciliary muscle which
is composed of few thin fibers, arising most anteriorly fans out to
find insertion in the root of the iris in close relation to the dilator
pupillae. Under sympathetic
control through long ciliary nerves, they focus distant objects,by pulling
the equator of the lens further.
Enervation
The accommodation is regulated by the autonomic nervous system. Due
to antagonistic control by parasympathetic and sympathetic it has come
in line with the pupillary activity. Whereas as per Sherington's principle
constrictor or dilator activity can be "one at a time", otherwise
if simultaneous, antagonist activity will be nullifying and ineffective.
Thus Retardation or acceleration, Bradycardia or Tachycardia, sympathetic
or parasympathetic activity can be only one at a time.
Participation of sympathetic elements in human accommodation has been
debatable since long. By now sufficient evidence has accumulated that
active accommodation for distant vision does exist. It is based on the
presence of
sympathetically enervated genuine radial fibers, which on stimulation
by subconj. inj. adrenaline (Briggs, Alpren and Bennett) leads to hypermetropia
due to extra flattening of lens. Whereas sympathetic palsy, seen in
Horner's syndrome or in experimental superior cervical ganglionectomy
induces myopia and meiosis. This effect cannot be attributed to merely
vasoconstriction which decreases the volume of ciliary body.
Stimulus for Accommodation
The reply to question that "What triggers the accommodation ?
is contained in the ocular estimation of distance of the object looked
at. This is estimated by the combination of the following clues :-
1. Blurring of image: - By itself this clue is not useful since eye
cannot determine whether blurring is due to nearness, remoteness or
due to fog. Similarly blurring due to chromatic and spherical aberration
is also of limited use.
2. Vengeance of light rays: - Rays from near object are oblique or
divergent so that closer the object greater the vergence and lesser
would be brightness of the image due to Stiles - Crawford's effect which
is appreciated easily in night by watching the car lights The oblique
light will appear dull and small.
3. Physiological Diplopia :- Whenever object nearer than the object
of fixation is looked at it produces blurred images on the temporal
sides of both fovea and also the diplopia which is normally suppressed
mentally& the degree of suppression varies with the distance. When
such blurred images are projected to visiopsychic area (19) the related
object is oriented as nearer. After determining the distance it is upto
the area 4 & 8 to send signal to Edinger-Westphal Nucleus (EWN)
to see it clearly. Thus ground is prepared by the blurred images but
it is the desire to see clearly that triggers the accommodation Therefore
our presumption that accommodation is initiated as a voluntary reaction
but
completed by the reflex mechanism is also supported clinically.
Whenever corticonuclear tract (CNT) between motor area 4 & 8 and
EWN is disturbed by brainstem tumour inverse Argyl Robertson Pupil is
produced Le. only accommodation is lost where as light reaction is retained.
I wish some MRI study to confirm the exact afferent of this psycho-optical
reflex.
Nervous Pathways for Accommodation
Book diagrams based on clinical findings showing accommodation pathways
are not anatomical but just schematic only. That is how Gray's Anatomy
writes it as presumptive and Parson - Diseases of the Eye also mention
it as conjectural.
1) Thus information of blurred images via area (19) reaches to EWN.
2) Normal Suppression of blurred image continues unless signal from
area 4,8 reaches EWN to clear the image.
3) I therefore can presume that When desire from 4,8 to "See"
clearly stops the su pression & opens the window of EWN, the calculated
information is released from here to accomplish the accommodation
Theories of Accommodation
Existing theories of accommodation as they stand cannot explain all
the phenomenon of the accommodation However while awaiting the precise
mechanism of accommodation, now every one has agreed that the essential
feature of accommodation is lenticonus formation
(1) Helmholtz' Theory :- In the act of accommodation all fibers of Ciliary
muscle contract to relax all zonules so that elastic lens becomes spherical
in shape by reducing its radius of curvature of anterior surface from
1 10 mm to 6 mm
i.e. equal to posterior surface. Against this theory it is argued that
in aged people due to increased diameter of the lens relaxation of zonules
and lenticonus shall be persistent. But even if it is so the hard and
inelastic lens cannot bulge any more.
(2) Tscherning's Theory :- By this theory it was rightly contested
that during the act of accommodation anterior surface of lens assumed
not a spherical but a hyperbolic lentinconus. But to explain this it
was suggested that all fiberes of ciliarly muscle contract to tighten
all zonules and thereby to flatten the equator by which anterior surface
of lens bulged as nipple. But the so called tightening of the zonules
is contradicted by the trembling of the lens seen during full accommodation
However formation of lenticonus is confirmed by the work of Finchem
but, later on Nordenson found that periphery of un-accommodated lens
is also relatively flat (fig.6) and therefore peripheral flattening
cannot be part of accommodation of as suggested by Tscherning.
3. Schachar's Theory :- In 1992 Schachar proposed a new theory As per
this in the act of accommodation while meridional & annular fibers
of ciliary muscle contract to relax anterior and posterior group of
zonules, at the same time contractions of radial fibers tightens the
equatorial zonules for flattening the lens periphery and thereby to
produce the lenticonus. Since parasympathetically (MF & AF) induced
lenticonus and sympathetically (RF) induced flattening cannot be simultaneous
as proposed, this theory which is mixture of Helmoltz's and Tscherening
becomes improbable and unacceptable. This is against the Sherington's
principle of reciprocal enervation.
4. Mv Unified Theory :- In line with reciprocal pupilary activity I
have to propose this theory that can explain all phenomenon of accommodation
(a) For Near Vision-Contraction of MF & AF --Relaxing AZ &
PZ--Lenticonus In other words C. Ring Shrinks -- Cap. Shrinks -- Nucleus
squeezed out.
During the accommodation sympathetically controlled equatorial zonules
do not participate. It is supported by the fact that length of EZ is
not altered on account of equal shrinkage in the ciliary ring and lens
diameter by 1 mm.
(b) For Active Distant Vision - Sympathetic contraction of RF-- stretching
of EZ pull & flatten the equator so that lens diameter approximately
increases to 9.2 mm & radius of curvature of anterior surface increases
to 9.9 mm.
In other words Zonule stretches -- Cap stretches -- Nucleus Sucked in.
This theory can explain all the lenticular changes enumerated below:-
1. Lens moves forward by 0.5 mm due to vitreous compression by MF.
2. Diameter of Lens reduced by 1.3 mm
3. Thickness of the lens increases by 1.25 mm
4. Anterior pole bulges forward by 1.75 mm (total of 1+3)
5. Forward shift of iris making AC shallow by 1.5 mm (AC = 2.5-3.6 mm)
6. Wrinkles from equator disappear due to loose zonules.
7. During the maximum accommodation smaller & lose lens of young
ones trembles.
Mechanism of Accommodation
Now the details of Near Vision accommodation can be elaborated, stepwise
as following:-
1. Parasympethitically initiated contraction of MF of CM compresses
the vitreous from ora serrata onward. So that retrolental patellar fossa
vaults anteriorly (demonstrate) pushing the the lens by 0.5 mm in the
directions of least resistance towards pupil. At the same time by
the contractions of AF ciliary ring is not only constricted but it is
also pulled forward and inward so that zonular tension of AZ. and PZ
is removed. Synergetic Contraction of MF & AF maximize zonular relaxation.
Forward shift of lens by 0.5 mm accounts for about 1.50 accommodation
which is utilized by the dynamically accommodating IOL(Highlights).
2. Due to relief from zonular stretching in attempt to reduce the surface
area lens changes its shape. If lens capsule was uniformly thick, then
it must have assumed spherical shape (Demonstrate). But on account of
thinner anterior capsule nucleus is moulded as a lenticonus. As soon
as thinnest post. polar capsule, which is under maximum stretch, gets
relieved from zonular stretch, then similar to elastic recoil of catapult
(Demonstrate) the thinner opposite anterior pole is made to herniate
towards the least resistance i.e. pupilary area inform of lenticonus.
By lenticonus, depending upon the age, full amplitude of accommodation
upto 15D can be achieved upto the age of 10 years.
3. Proportionate to parasympathetic discharge amount of accommodation
is regulated. When stimulation for near vision stops, ciliary ring relaxes
and recoils back to distant vision state, the tightened anterior zonules
& posterior zonules restore the site, size and shape of the lens
to the un-accommodated state. However equatorial zonules remains unaltered.
4. Since concentrically positioned ciliary muscle contract equally all
around the lens in both the eyes, any amount of astigmatism or anisometropia
cannot be corrected by the efforts of accommodation. Whereas in presbyopia
although hardened lens fails to bulge despite the contraction of ciliary
muscle, even then anterior shift of lens or PC IOL will provide atleast
10 accommodation so that people beyond 60 yrs. also can focus objects
upto one meter but not within a meter. Working of accommodating IOL
is based on the anterior shifting of Capsular bag.
Active accommodation for distance vision
Our daily life is dominated by the effortless distant vision because
rays from objects 6 meter (20') away are regarded parallel and being
parallel they come to focus on fovea of emetropic eye i.e. without any
accommodation Whereas fact remains that although our eyes are not telescopic
eagle's eyes still on account of distant vision accommodation fruits
and birds on tree top of +20' height are clearly seen & successfully
hit if desired. At a distance of one meter 10 accommodation is necessary
and at 6 M & 10M it will be +0.170 & 0.100 respectively. Even
this small amount of accommodation is of paramount importance for shooting
fruits, birds and playing outdoor games like badminton and Cricket with
the pitch of 20M long, so as to accurately hit and catch the ball. This
infact can be achieved by fine adjustment within the amplitude of 10
and indeed it is perfectible with the practice. However change in size
of retinal image of moving ball or shuttle cork also helps in estimation
of correct distance and correct focussing.
Mechanism of active distance vision accommodation
During sports when active DV is required Sympathetic stimulation by
contracting the Radial fibers pull the equatorial zonules thereby lens
is further flattened. It is noteworthy that degree of contraction of
Radial fibers and flattening of lens is finely adjusted by the sympathetic
discharge from the hypothalemic center which is also influenced by pscho-sensory
stimuli.
In short :- NV = Cilary Ring Shrinks --+ Cap Shrinks --+ Squeezed Nucleus
DV = Ciliary Ring stretches --+ Cap. Stretches --+ Sucked in Nucleus
(Demonstrate sucked in drop by a dropper)
Recent Trends
Can we restore the accommodation lost by lens extraction or presbyopia
? At least that seemed improbable until now. But accumulating evidence
strongly suggest that it is indeed possible with the ultramodern techniques
some of which are worth mentioning.
For Presbyopia
Though multifocal glasses are of great value still with the modern
trend to discard spectacles, in USA conductive keratoplasty using radiowaves
is approved by FDA. Apart from this at New Mexico University an artificial
Bionic Muscle is being developed and its transplantation around the
ora Serrata will either elongate or shorten the AP length of the eye
similar to snail's eye is under trial.
For senile cataract
PC Bifocal/ multifocal and dynamically accommodating plate or two pieces
IOl are already implanted in metrocities Similar to normal near vision
contracting forces of ciliary muscles are transformed into ante nor
shift of accommodating IOL Optic by 0.50 to 0.63 mm. Due to the increased
distance between the lens and retina, psudophakic accommodation between
+13 to 1.70 enables the eye to enjoy the boon of near vision without
spectacles in majority of the cases.
For infantile cataract
IOL with increasing power is under trial. This acrylic lens has 3-4
water tight vacuum chambers stacked on top of one another. Power of
lens can be increased stepwise by pricking a hole by LASER in one of
the chamber in which rush of aqueous raises its refractive index &
refractivity. So repeated change of IOL is not required.
Conclusion
In conclusion I would like to recall the story of Panda vas brothers
Four of them were simply looking about the bird but only Arjun who was
seeing the bird's eye attentively could shoot it down. The most significant
lesson derived from this story is that looking is different from seeing
because seeing is the intentional mental process which triggers the
accommodation making the looked at things visible. Ultimately vision
remains the power and art of seeing things because eyes see what brain
shows in the range of looking, seeing, watching, observing, gazing or
staring Thus our vision is not only clearly focussed by the mind but
also modified by emotions and sometime even deranged to produce illusions,
delusions & hallucinations. How perfectly it has been described
in the literature as- "Jaki rahi bhavana jaisi, prabhu moorat dekhi
tin taisi "
Therefore while ending the talk I would like to reiterate that eyes
look and mind sees.