contact lens use Indications & Contraindications

Indications of contact lens use

1. Optical indications include anisometropia, unilateral aphakia, high myopia, keratoconus and irregular astigmatism. Optically they can be used by
every patient having refractive error for cosmetic purposes.

Advantages of contact lenses over spectacles:
(i) Irregular corneal astigmatism which is not possible to correct with glasses can be corrected with contact lenses.

(ii) Contact lenses provide normal field of vision.

(iii) Aberrations associated with spectacles (such as peripheral aberrations and prismatic distortions) are eliminated.

(iv) Binocular vision can be retained in high anisometropia (e.g., unilateral aphakia) owing to less magnification of the retinal image.
 

 (v) Rain and fog do not condense upon contact lenses as

CONTACT LENSES

what is contact lens

Contact lens is an artificial device whose front surface
substitutes the anterior surface of the cornea.
Therefore, in addition to correction of refractive error,
the irregularities of the front surface of cornea can
also be corrected by the contact lenses.
Parts, curves, and nomenclature for contact lens
To understand the contact lens specifications
following standard nomenclature has been recommended
(Fig. 1).

Fig. 1. A contact lens.

1. Diameters of the lens are as follows :
i. Overall diameter (OD) of the lens is the linear measurement of the greatest distance across the physical boundaries of 

SPASM OF ACCOMMODATION

(continue  ACCOMMODATION ANOMALIES)

Spasm of accommodation refers to exertion of abnormally excessive accommodation.

Causes
1. Drug induced spasm of accommodation is known to occur after use of strong miotics such as echothiophate and DFP.

2. Spontaneous spasm of accommodation is occasionally found in

PARALYSIS OF ACCOMMODATION

(continue  ACCOMMODATION ANOMALIES)
Paralysis of accommodation also known as
cycloplegia refers to complete absence of accommodation.

Causes
1. Drug induced cycloplegia results due to the effect of atropine, homatropine or other parasympatholytic drugs.

2. Internal ophthalmoplegia (paralysis of ciliary muscle and sphincter pupillae) may result from neuritis associated with diphtheria, syphilis,
diabetes, alcoholism, cerebral or meningeal diseases.
 
3. Paralysis of accommodation as a 

INSUFFICIENCY OF ACCOMMODATION

(continue  ACCOMMODATION ANOMALIES) Insufficiency of accommodation

The term insufficiency of accommodation is used when the accommodative power is significantly less than the normal physiological limits for the patient's
age. Therefore, it should not be confused with presbyopia in which the physiological insufficiency of accommodation is normal for the patient's age.

Causes
1. Premature sclerosis of lens.
2. Weakness of ciliary muscle due to

ANOMALIES OF ACCOMMODATION

Anomalies of accommodation are not uncommon.
These include: (1) Presbyopia, (2) Insufficiency of accommodation, (3) Paralysis of accommodation, and (4) Spasm of accommodation.(last three anomalies discussed in other posts)

PRESBYOPIA
Pathophysiology and causes:

Presbyopia (eye sight of old age) is not an error of refraction but a condition of physiological
insufficiency of accommodation leading to a progressive fall in near vision.

Pathophysiology. To understand the pathophysiology of presbyopia a working knowledge about
accommodation (as described in this post) is mandatory. As we know, in an emmetropic eye far point is infinity and near point varies with

Accommodation eye

Definition. As we know that in an emmetropic eye, parallel rays of light coming from infinity are brought to focus on the retina, with accommodation being at rest. However, our eyes have been provided with a unique mechanism by which we can even focus the
diverging rays coming from a near object on the retina in a bid to see clearly (Fig. 1).
This mechanism is called accommodation. In it there occurs increase in

 FIG 1 Effect of accommodation on divergent rays
entering the eye

Fig. 2. Changes in the crystalline lens during
accommodation.

the power of crystalline lens due to increase in the
curvature of its surfaces (Fig. 2).
At rest the