VISUAL ACUITY , LENS AND FUNDUS OCULI

THE REDUCED OR SCHEMATIC EYE

• In the eye, light is actually reflected at the anterior surface of the cornea and at the ant. and post. surfaces of lens.

• However, in order to represent the process of reflection in a more simplified way an imaginary concept the reduced or schematic eye has been evolved.

• In this concept, the lens is not considered to be present; the ant. surface of the cornea is assumed to possess the whole of the refractive power of the eye with a power of +60 diopters.

THE REDUCED OR SCHEMATIC EYE

• The whole eye is assumed to have the refractive index of 1.333. the total axial length of the eyeball is about 24mm.

• The center of curvature for the imaginary corneal surface having a power of +60 D is 5.5 mm behind the principal plane.

• Light will therefore pass undeviated through the center of curvature which is therefore the optical center or the nodal point of the reduced eye.

THE VISUAL ACUITY (ACUTNESS OF VISION)• Visual acuity is defined as the shortest distance by which two lines can be separated and still be visualized as two lines.

• If two black objects on a white background are separated by a space subtending an angle of less than 1 minute then the two objects are not seen separated but are seen fused with each other.

• When the visual angle is increased to 1 minute, the objects are seen separately. Thus 1 minute is the minimum angle for the normal eye.

THE VISUAL ACUITY (ACUTNESS OF VISION)• Clinically visual acuity for distant vision is

measured with SNELLEN’S test types. These are a series of letters of varying sizes so constructed that the top letter is visible to the normal eye from 60 meters and the subsequent lines from 36,24,18,9,6 and 5 meters respectively.

• The height of each letter, when seen from its corresponding distance, subtends a visual angle of 5 minutes.

THE VISUAL ACUITY (ACUTNESS OF VISION)

• Each line in the letter subtends 1 minute of arc.• Visual acuity for near vision is measured with the help of words printed in small type.

• In doing the test, the subject is made to sit at distance of 6 meters from the chart. The eyes are tested one by one.

THE VISUAL ACUITY (ACUTNESS OF VISION)

• If he can only see the top line, he has a VA of 6/60.This means that the subject can read that letter from 6 meters which a normal eye should be able to read from 60 meters.

• If he can read even the smallest line, then his VA is 6/5; this means that the subjects VA is better than the normal average eye which should read the lowest line from 5 meters.

THE VISUAL ACUITY (ACUTNESS OF VISION)

• Thus the numerator corresponds to the distance in meters b/w the subject and the chart. This is kept as 6 meters (20ft) as mentioned above.

• The denominator corresponds to the distance in meters at which the smallest row of letters read by patient should be read by normal eye.

• Thus VA decreases in the order, 6/5,6/6,6/9,6/12,6/18,6/36 and 6/60.

LENS• The lens of eyeball is crystalline in nature. It is

situated behind the pupil.• It is biconvex, transparent and possesses the

elastic property.• The lens is an avascular structure and it receives its

nutrition from the aqueous humor.• Lens refracts light rays and helps focus the image

of the objects on retina.• The focal length of human lens is 44mm and its

refractory power is 23 D.• Lens is supported by suspensory ligaments (zonular

fibers) which are attached with ciliary bodies.

STRUCTURE OF THE LENS

• The lens is formed of three components.

1) The capsule

It is highly elastic membrane covering the lens.

2) The anterior epithelium• It is a single layer of cuboidal epithelial cells situated beneath the capsule.

• At the margins, the epithelial cells are elongated.

• The epithelial cells give rise to the lens fibers present in the lens substance.

3) The lens substance• Lens is formed by long lens fibers derived from the ant. epithelium.

• The lens fibers are prismatic in nature and are arranged in concentric layers.

CHANGES IN THE LENS DURING OLD AGE

• After 40 t0 45 years, the lens looses its elastic property. So, the amplitude of accommodation is decreased.

• The person cannot see the near objects clearly. This condition is known as presbyopia.

• In old age the lens becomes opaque and this condition is called cataract.

CATARACT

• It is opacity or cloudiness in the natural lens of the eye. It is the major cause of blindness worldwide.

• When the lens becomes cloudy, light rays cannot pass through it easily, and vision is blurred.

• Cataract develops in old age after 55 to 60 years.

CATARACT

• The lens is situated within the sealed capsule. The old cells die and accumulate within the capsule.

• Over the years, the accumulation of cells is associated with accumulation of fluid and denaturation of the proteins in the lens fibers causing cloudiness of lens and blurred image.

CAUSES OF CATARACT

1. Age

2. Eye injuries

3. Previous eye surgery

4. Diseases like diabetes, Wilson's disease and hypocalcaemia.

5. Long term use of drugs such as steroids diuretics and tranquilizers

6. Long term unprotected exposure to sun light

7. Alcoholism

8. Family history

9. Diet containing large quantity of salt

SYMPTOMS

1. Painful blurred vision.

2. Poor night vision.

3. Diplopia in affected eye.

4. Need for bright light while reading.

5. Fading of colors.

TREATMENT

• Surgery is the only treatment for cataract.• During surgery, the cloudy lens is removed from eye through a surgical incision.

• The natural lens is replaced with a permanent clear and plastic intra ocular lens (IOL) implant.

• Different procedures were followed to remove the cloudy lens. The common methods are;

1) Extra capsular extraction• This is a rather old technique.• A 12 mm incision is made in eye under an operating microscope to remove the lens as a hole.

• The post. capsule of the lens is left in place to hold the IOL implant.

• Multiple sutures are required to seal the eye after surgery. The sutures must be perfect otherwise astigmatism may develop.

2) Phacoemulsification• This is current technique.• It is the procedure in which ultrasonic vibrations are used to break the cataract into smaller fragments. It is done through a small (3mm) incision.

• An u/s (or laser) probe is used to break the lens material without damaging the capsule.

• A foldable IOL is then introduced through the incision. Once inside the eye, the lens unfolds to take position inside the capsule.

• No sutures are needed, as the incision is self-sealing.

FUNDUS OCULI

• The post. part of interior of the eyeball is called fundus oculi or fundus.

• Fundus is examined by ophthalmoscope.• The fundus has two parts;

1. optic disk

2. macula lutea

FUNDUS OCULI

OPTIC DISK ( OPTIC PAPILLA)

• Situated near the center of the post. wall of eyeball.• It appears like pale disk.• Formed by the convergence of axons from ganglion cells, while forming the optic nerve.

• It contains all layers of retina except rods and cones.

• It is insensitive to light, in other words; it is the blind spot.

• The central blood vessels of the retina are located right in the center of the optic disk.

The retinal arteries are end-arteries except at the optic disk.• It is important to know that ophthalmoscope examination of the eye enables one to see into the eye through the pupil to the retina.

• The most obvious feature of retina viewed through ophthalmoscope is the blood vessels on its surface.

• The retinal blood vessels originate from the optic disk; where the optic nerve fibers exit the retina.

• The examination of optic disk (also called optic nerve head) provides valuable and important information about the state of optic nerve head, arteries and veins of the retina, disorders of retina, pigment epithelium and choroids.

Shape of optic disc

The normal disc is rounded or slightly oval.

Colour of optic disc• Pale-pink. It is distinctly pale than the surrounding fundus.

• Temporal side of the disc is paler than the nasal side.

• In optic atrophy, the disc becomes more pale than normal.

• In inflammation of the optic nerve or raised intra- cranial pressure, the disc becomes hyperaemic and more pink.

Physiological cupping• It is the depression in the central part of optic disc.

• The cup is paler than the surrounding disc, where from retinal vessels enter and leave the eye.

• In glaucoma (increased IOP ) the cupping is greatly increased and the retinal vessels get kinked as they cross the edge of the rim.

Blood vessels of the optic disc• Radiate from the disc, dividing into many branches.

• The retinal arteries are narrower than the veins and bright red in colour.

• It is the artery that crosses the vein.• Spontaneous retinal artery pulsation is an abnormal finding which may occurs if IOP is high while venous pulsation is seen frequently in normal eye and is absent in papilloedema.

MACULA LUTEA

• M. Lutea or yellow spot is small yellowish area of retina.

• Situated a little lateral to the optic disk.• The yellow color of macula lutea is due to the presence of a yellow pigment.

• Fovea centralis; is a minute depression in the center of macula lutea where all layers of retina are very thin.

MACULA LUTEA

• Diameter of fovea is only about 0.5 mm.• Fovea is the region of most acute vision because it contains only the cones which number about 35,000.

• The part of the macula lutea surrounding the fovea has rods also which however are much less in number.

• Degeneration of macula lutea is common cause for blindness.

CAUSES OF ABNORMAL FUNDUS

1) PAPILLOEDEMA

• This is a swelling of the optic nerve head due to raised intracranial pressure (ICP).

• Absence of inflammatory changes.

• Little or no disturbance of visual function.• There is an increased redness of the disc with blurring of its margins in initial stages of papilloedema.

• The blurring appear first at the upper and lower margins, particularly in the upper nasal quadrant.

PAPILLOEDEMA

• The physiological cup becomes filled in and disappears.

• Retinal veins are slightly distended.• Spontaneous pulsation of the retinal veins is usually absent.

• The disc becomes definitely swollen.• If papilloedema develops rapidly, there will be marked engorgement of the retinal veins with haemorrhages and exudates on and around the disc; but with slow onset there will be little or no vascular change.

Causes of papilloedema.

1) Space-occupying lesions particularly.

liable to occur in children with tumors of cerebellum and 4th ventricle.

2) Malignant hypertension.• Arterial changes characteristic of this condition.• Haemorrhages, exudates and cotton wool spots extending far beyond the region of the disc.

2) OPTIC NEURITIS• Inflammatory or demyelinating disease may affect any part of the optic nerve, producing an optic neuritis.

• The characteristic symptom is loss of vision.• There is a pain on moving the eye.• The pupil on the affected side shows a diminished and ill-sustained contraction to a bright light.

3) RETINAL HAEMORRHAGES.

These occur in a number of different conditions and are due to one or more of the following factors;

a. Hypertension in which there is increased blood pressure within the retinal vessels.

b. Decreased retinal pressure due to proximal large vessel disease.

c. Abnormalities in the walls of the retinal vessels, as in diabetes or occlusion of the retinal veins.

d. Abnormalities in the circulating blood, as in severe anaemia, leukaemias, and bleeding diatheses.

e. Haemorrhages are elongated and flame shaped when superficial, within the nerve fiber layer of the retina, whereas when deep they are round blotches or spots.

4) RETINAL ARTERIOSCLEROSIS.

This occurs either as an exaggeration of the general ageing process of the body or in association with hypertension. It is characterized by;

a. Tortuosity of the vessels.

b. Nipping, identation or deflection of the veins where they are crossed by the arteries.

c. Copper wire or silver wire appearance of the fundus of the eye.

d. In hypertension, flame-shaped haemorrhages and cotton wool spots in the region of macula.