Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009
.pdf
Accommodation and its Anomalies 111
The dioptric power of the eye in this accommodative state is called it’s dynamic refraction. The dioptric power of the resting eye is called it’s static refraction. Anatomical changes associated with accommodation are (Fig. 8-1):
•Decreased radius of curvature of anterior surface of the lens from 10 mm to 6 mm.
•Increased thickness of the lens by 0.50 mm.
•Decreased equatorial diameter of the lens from 10 mm to 9.6 mm.
•Anterior axial displacement of the anterior pole of the lens by 0.30 mm and consequent decreased depth of anterior chamber also by 0.30 mm.
•During accommodation, all the cardinal points of the eye move toward the anterior surface of the lens (Fig. 8-1).
MECHANISM OF ACCOMMODATION
Understanding of mechanism of accommodation is based on Von Helmholtz theory (1909). The shape of the lens is basically a balance between the elastic forces of the lens capsule and viscoelasticity of the lens mass.
112 Textbook of Visual Science and Clinical Optometry
1.Far point (Punctum Remotum): It is the furthest point at which objects can be focussed on the retina, when the ciliary muscle is relaxed, i.e. accommodation is at rest. The far point which is conjugate to the retina (Fig. 8-2) varies according to it’s static refraction (emmetropia, myopia or hypermetropia).
2.Near point (Punctum Proximum): It is the closest point at which small objects can be focussed on the retina clearly after maximum accommodation and the near point is conjugate to the retina (Fig. 8-2).
3.Range of accommodation: It is the distance between the far and near points, e.g. from infinity to 20 cm.
4.Amplitude of accommodation: It is the difference in refractive power of the eye between near point and far point. Amplitude of accommodation gradually declines to become zero at the 6th decade.
Fig.8-2: Far point and near point in an emmetropic eye.
F = Far point, N = Near point and R = Range of accommodation
Following two other phenomena occur along with accommodation:
•Pupillary miosis–It increases depth of focus and reduces aberrations.
•Convergence, i.e. inward rotation of the eyes.
A stimulus to either accommodation or convergence can cause
both the change since they are controlled by the same neurological pathway.
Q.Calculate the amplitude of accommodation and range of accommodation of an eye with far point at 1 metre and a near point of 10 cm.
A.Refractive power of the eye at far point is 1/1 = 1 diopter
Accommodation and its Anomalies 113
Refractive power of the eye at near point is 1/0.10 = 10 diopter
So, the amplitude of accommodation is = 10D–1D = 9 diopter. The range of accommodation is = 1 m–10 cm = 100 cm–10 cm = 90 cm
INSUFFICIENCY OF ACCOMMODATION
It is a situation where the accommodative power of a person is persistently lower than appropriate level expected for the person’s age. It is the most common form of accommodative disorder.
AETIOLOGY
•Early onset of presbyopia.
•Fatigue of ciliary muscle due to general weakness, influenza, poor health, anaemia, debilitating illness, etc.
•Impaired effectiveness of ciliary muscle due to increased IOP in primary open angle glaucoma (POAG).
•Working in dim/poor light for long hours.
COMPLAINTS
•Blurred vision
•Difficulty in maintaining good vision at near
•Frontal headache.
TREATMENT
•Treatment of the causal factor or illness.
•Near correction—Weakest convex lens which allows near vision at normal reading distance. Full correction is avoided to encourage exercise and stimulation of the available accommodation.
•Encourage near work at good illumination.
PARALYSIS OF ACCOMMODATION
Bilateral paralysis of accommodation is less common than paresis (partial paralysis/weakness).
114 Textbook of Visual Science and Clinical Optometry
AETIOLOGY
Aetiology of paralysis of accommodation can be classified as follows (Table 8-1).
Table 8-1: Aetiology of paralysis of accommodation
Unilateral cause |
Bilateral cause |
Cycloplegic induced |
Diabetes mellitus |
Blunt trauma |
Alcoholism |
Paralysis of IIIrd cranial nerve |
Encephalitis |
Tear in iris sphincter |
Diphtheria |
Horner’s syndrome |
Botulism |
Adie’s syndrome |
Syphilis |
Herpes zoster |
Multiple sclerosis |
|
Lead poisoning |
|
Arsenic poisoning |
|
Typhoid |
|
|
The prognosis is good in cases due to diphtheria or drugs. However, in traumatic cases the condition is often permanent. Full mydriasis is usual in total paralysis of accommodation.
TREATMENT
i.Treatment of causative factor or illness
ii.Appropriate convex lens for adequate near work is advised.
SPASM OF ACCOMMODATION
It is defined as a situation where accommodation is found to be always higher than expected. The excess accommodation is caused by involuntary contraction of the ciliary muscle. Myopia develops due to the excess of accommodation. Pseudomyopia develops in this accommodative disorder.
AETIOLOGY
i.It is typically seen in young myopic patient.
ii.It is also found in young patients involved in too much of near work in insufficient illumination and bad posture.
iii.It is also observed in young patients suffering from mental anxiety.
Accommodation and its Anomalies 115
iv.It may be induced artificially by instillation of miotics, i.e. pilocarpine eyedrops in treatment of primary open angle glaucoma (POAG) in young patients.
v.It is also observed in patients suffering from uveitis.
COMPLAINTS
i.Eye strain or Asthenopia
ii.Blurring of vision for distance due to induced myopia.
DIAGNOSIS
Atropine paralyses the tone of the ciliary muscle which is +1.00D. Retinoscopy under atropinisation (cycloplegic refraction) reveals that the value is higher in such cases.
TREATMENT
i.Removal of environmental, working condition and aetiological factors.
ii.Atropinisation, i.e. use of cycloplegic drugs for several weeks and reassurance.
iii.Psychotherapy, if indicated.
iv.Correction of refractive error.
C H A P T E R
Errors of
9 Refraction
INTRODUCTION
The refractive status of an eye during minimal accommodation may be of the following types: (Fig. 9-1):
Fig. 9-1: Image formation in various states of refraction
a.Emmetropia: It is a condition in which parallel rays of incident light are brought to a focus upon the light sensitive layer of the retina during minimal accommodation. It indicates absence of refractive error.
b.Ammetropia: It indicates presence of refractive error, i.e. absence of emmetropia wherein, parallel rays of incident light are not focused on the light sensitive layer of the retina during minimal accommodation. Ammetropia may be of the following types:
i.Hypermetropia (or Hyperopia): Incident parallel rays of light are brought to a focus behind the retina.
ii.Myopia: Incident parallel rays of light are brought to a focus in front of the retina.
120Textbook of Visual Science and Clinical Optometry
iii.Astigmatism: Incident parallel rays of light are brought to a line of focus instead of a single point focus due to inequality in curvature of different meridians.
Most definitions of refractive status of the eye theoretically refer to the term “when the accommodation is at rest”. However, in practice it is not possible to obtain zero accommodation even in the absence of any optical stimulus to accommodation. Due to this fact, the term “minimal accommodation” is used in the definitions.
HYPERMETROPIA
It is also called long sightedness.
TYPES
Based on Anatomical Features a. Axial Hypermetropia
•It is due to relatively short axial length.
•1 mm axial length shortening will cause +3.00D of hypermetropia.
•Physiologically majority of all infants are axial hypermetropic due to small size of the globe at birth.
•Pathologically, axial hypermetropia will develop when the retina is pushed forward in ocular tumour, central serous retinopathy, etc.
b. Curvature Hypermetropia
•It is due to the increased radius of curvature of the refractive surfaces, i.e. cornea and lens.
•1 mm increase in radius of curvature, i.e. flattening will cause +6.00D of hypermetropia.
•It is seen in cornea plana.
c. Index Hypermetropia
•It is due to increase in refractive index of the lens cortex relative to the nucleus, which is often seen in elderly.
d. Absence of Refractive Element
• It is due to removal of the lens, i.e. aphakia.