3. Accommodation
Accommodation is the process by which the eye increases
optical power to maintain a clear image focus on an object
as it draws near the eye
This changes occur due to
change in Zonular tension induced
by ciliary muscles contraction
and change in the shape of lens
4. Mechanism of accommodation
Eye is at rest(unaccommodated)
Ciliary ring is large and keep zonules tense, zonuler tension
lens is kept compress
During accommodation ciliary muscles contract and shorten
the ring thus release zonuler tension on lens capsule,
This give way to elastic capsule to change the shape of lens-
become more convex
5. Mechanism of accommodation
Eye is at rest (unaccommodated)
Ciliary ring is large and keep
zonules tense, zonuler tension
lens is kept compress
During accommodation ciliary
muscles contract and shorten the
ring thus release zonuler tension
on lens capsule,
This give way to elastic capsule
to change the shape of lens-
become more convex
6. At rest radius of curvature of anterior surface of lens is
10 mm and posterior 6 mm
During accommodation curvature of anterior surface
become up to 6mm while posterior remains almost
same
Focal power can change
up to 14D
7. Near point and Far point
Near point or punctum proximal –
The nearest point at which small object can be seen
clearly
Far point or punctum proximal –
The distant (farthest) point at which object seen clearly
8. Far point and near point
of eye vary with state of
refraction
a. Emmetropia: the far point is
at infinity.
b. Myopia: the far point is a
finite distance in front of the
eye.
c. Hypermetropia: the far
point is a finite distance
behind the eye.
9. Range and amplitude of accommodation
Range of accommodation –
The distance b/n the near point and the far point
Amplitude of accommodation –
The difference b/n the dioptric power needed to focus at near
point (P) and far point (R) is called amplitude of
accommodation (A)
Thus A= P-R
12. Presbyopia
It is not an error of refraction
It a condition of physiological insufficiency of
accommodation – progressive fall in vision
The condition of failing near vision due to age –related
decrease in the amplitude of accommodation or increase in
punctum proximum is called Presbyopia
13. Pathophysiology
In emmetropic eye far point – infinity (∞)
Near point – varies with age at 10 years -7cm
40 years- 25 cm, 44 years – 33cm
At age of 10 years amplitude of accommodation (A) =
D power needed to see clearly at near point – D power needed
to see clearly at Far point
100/ 7 – 1/ ∞ = 14 D
At age of 40 years = 100/25 – 1/ ∞ = 4D
14. Causes –
1. Age- related changes in the lens-
Decrease elasticity of lens capsule
Progressive increase in size and hardness of lens ( sclerosis)
less easily moulded
2. Age related decline in
ciliary muscle power
15. Causes of premature Presbyopia -
Uncorrected hypermetropia
Premature sclerosis of crystalline lens
General debility causing presenile weakness of ciliary
muscle
Chronic simple glaucoma
16. Clinical features -
Difficulty in near vision
Asthenopic symptome
Intermittent diplopia – due to disturbed relationship b/c
accommodation and convergence
17. Treatment
Optical treatment – convex lens
rough guide
+ 1DS at 40-45 years
+ 1.5DS at 45-50 years
+ 2DS at 50-55 years
+ 2.5 DS at 55-60 years
Presbyopic spectacles-
Unifocal
Bifocal
Multifocal
19. Insufficiency of accommodation
When accommodative power is significantly less than the
normal physiological limits for the patient’s age
Causes –
Premature sclerosis of lens
Weakness of ciliary muscle due to systemic cause-
debilitating illness, anaemia, diabetes mellitus, pregnancy,
stress, malnutrition
Weakness of ciliary muscle associated with Primary open
angle glaucoma
20. Clinical feature-
Blurred vision, or difficulty in sustaining clear vision at
near;
Frontal headache and pain in the eye.
Often associated with convergence insufficiency
Treatment –
Treat the primary cause,
Convex lens correction
In some cases exercises such as accommodative facility
training are useful
22. Causes -
Drug induced cycloplegia - effect of atropine,
homatropne, cyclopentolate
Internal ophthalmoplegia -
(paralysis of ciliary muscle and sphincter pupillae)
may result from neuritis associated with diphtheria, syphilis,
diabetes, alcoholism, cerebral or meningeal diseases.
Paralysis of accommodation - (as a component of
complete third nerve paralysis)
may occur due to intracranial or orbital causes. The lesions
may be
traumatic, inflammatory or neoplastic in nature
23. Clinical features -
Blurred vision at near
Photophobia or a 'dazzling' effect
Enlarged pupil (mydriasis) due to complete paralysis of
sphincter pupillae
Treatment
1. Self-recovery -druginduced paralysis and in diphtheric
cases (once the systemic disease is treated).
2. Dark-glasses - reducing the glare.
3. Convex lenses for near vision - if the paralysis is
permanent
24. Spasm of accommodation
Exertion of abnormally excessive accommodation
Causes –
Drug induced – miotics ( echothiophate)
Spontaneous spasm – in children to compensation
refractive error
Excessive exertion of eye for near work in dim illumination
Stress
Anxiety
25. Clinical feature –
Defective vision due to induced myopia
Asthenopic symptoms
Diagnosis-
Retinoscopy under cycloplegic
Treatment –
Relaxation of ciliary muscle spam by strong cycloplegic
Prohibition of near work allow early recovery
Correction of refractive error
