DISEASES OF THE LENS

Anatomy of the Lens

The diseases of the lens make about 12% of eye diseases. The lens (lens crystallina) develops from ectoderm.

The lens is one of refractive environments of the eye, its mai function is refraction of light rays. The important feature of th lens is its accomodative capability. Without accomodation, whe looking at a distance, the refractive power of the lens is approxi mately 18-20 D, and at a short distance, at a forced accommoda tion, it can increase up to 35 D in children.

Iris and lens together form so called irido-lens diaphragm, which divides anterior and posterior parts of the eye, keeps the vitreous in the posterior part of the eye.

Anterior surface of the lens can be seen in the region of a pupil. Its centre is the anterior pole of the lens. The posterior surface is visible by biomicroscopy on slit-lamp, its centre is the posterior pole of the lens. A line of transition of anterior surface to posteri­or is named as the equator of the lens.

The lens is covered by the unstructured elastic capsule. The part of the capsule, which covers the anterior surface of the lens, is named the anterior capsule, and that one that covers the posterior surface — the posterior capsule. Suspensory Zinn's ligaments at­tach to the capsule on the equator, and go from the ciliary body. The lens is suspended by these ligaments. Under the anterior cap­sule there is a layer of cubical epithelium, transparent cells, which reproduce, displace to equator and transform in lens fibres. The old layers of fibres displace to the centre of the lens, condense and form a nucleus of the lens.

Size, form, consistence, colour of the lens change during the life.

In newborns the lens form is spherical, colourless, soft, with thickness of 4 mm and diameter of 6 mm. The lens of the child con­tains 30% of various proteins, 65% of water, 5% of mineral salts (K, Ca, P), vitamins (C, B2), glutathion, enzymes and lipids.

In adults thickness of the lens is about 3.6 mm, diameter — 9-10 mm. In the lens of the adult over 25-30 years old layers in the centre form a yellowish transparent firm nucleus, and periferal lay­ers form a soft transparent core. The chemical structure of the lens changes with the age: the amount of insoluble protein fractions in­creases, cholesterol, thirosin collects and the lens becomes yellow­ish. The amount of Ca increases, the amount of vitamins C and B decreases.

The bigger the nucleous, the firmer the lens and less accommo­dation ability and the farther from the eye the nearest point of clear vision. But these changes are physiological, the lens in adults, as well as in newborns, remains transparent. The lens has no vessels.

Nutrition to the lens is provided by the aqueous humour of anteri­or and posterior chambers of the eye. The metabolism occurs very slowly. The lens has no nerves, therefore pathological changes of the lens proceed painlessly.

Methods of the Lens Examination

With a wide pupil we may see almost the whole lens, but the edge of the lens is seen only in complete colobome of the iris or in aniridia. Lens examinations are carried out by using of the following meth­ods:

— lateral illumination;

— biomicroscopy on a slit lamp;

— investigation with passing light;

— ultrasound (in the cornea opacity, for measurement of the lens thickness);

— examination of the eye functions.

The main symptoms of the lens pathology are a loss of trans­parency, dislocalization, change of the form and size. The disea­ses of the lens are congenital and acquired.

Diseases of the Lens

The main signs of the lens pathology are loss of transparency, disorder of localisation, changes of its form and size. The diseases of the lens can be congenital and aquired.

Date: 2014-12-21; view: 962