Nutritional Deficiency Blindness

Dr. Mohanad R. Alwan

Vitamin A serves varied functions in the human body

A major role of vitamin A is as part of the visual pigment rhodopsin

Forms of vitamin A are also involved in: Gene expression Maintenance of epithelial tissue Regulation of growth and differentiation of cells,

including some cells of the immune system

Vitamin a

As part of rhodopsin, retinol binds with the protein opsin

Rhodopsin absorbs light, which signals visual cortex of brain

Reconversion and replenishing of rhodopsin must occur before it can respond again to light

When amt. of rhodopsin is limited, difficult to see in dim light “night blindness”

When A pool is low, dark adaptation is slowed down

Vitamin A occurs in several forms

Retinoids, including retinol, retinal and retinoic acid, are common in animal tissues; they can serve vitamin A functions directly

Beta-carotene is a precursor of vitamin A found in plant tissues; it is also an antioxidant Converted/split by intestine into retinol and

retinal retinol then oxidized to retinoic acid and retinal

Absorption, transport and storage of vitamin A

Vitamin A is a fat-soluble vitamin and is absorbed and transported with lipids

Vitamin A is stored in the liver; recorded instances of toxicity involve excess consumption of polar bear liver by Arctic explorers

Good dietary sources of vitamin A

Best sources include: beef liver carrots (as beta-carotene) mustard greens (as beta-carotene) eggs apricots (as beta-carotene)

Note that beta-carotene is much less toxic in higher doses than is the preformed animal forms of vitamin A

Food Sources of Vitamin A

What are the symptoms of vitamin A deficiency?

Dry, hard skin Dry cornea and eventual blindness

(Xerophthalmia) Night blindness (insufficient retinal

for rhodopsin formation) Impaired immune function

Xeropthalmia – Sequence of Events

Deterioration of the eye results from bacterial invasion Role of A in resistance to infection

Conjunctival xerosis (abnormal dryness of the lining of the eyelids and the outer surface of the eyeball) and Bitots spots (drying out of the eye and appearance of hardened epithelial cells) appear as Vit. A deficiency worsens

Xeropthalmia – Sequence of Events

Finally, corneal ulcerations and keratomalacia (softening of the cornea) results in scarring Can see effects from barely detectable to

blindness

Treating Vitamin A Deficiency

What about food? Vegetables are the major source of pro-

vitamin A in children where animal products are limited

Kids don’t like these foods Insufficient dietary fat intake limits

absorption of what they do get

Terms- Visual field defect

Visual field defect - a portion of visual field missing. This may be:

central (e.g. optic disc or nerve problem) peripheral (along the visual pathways from

the optic chiasm back).

Common causes of VF defect

Central field loss occurs with: Optic neuropathy Macular degeneration Macular hole Cone dystrophies A number of rare conditions like Best’s

disease, Stargardt's disease and achromatopsia.

A careful hydrodissection frees the nucleus, which is then rotated to elevate the superior pole. The bent 30 gauge needle is then inserted behind the nucleus and rotated. The nucleus is then pulled out of the eye

Peripheral field loss occurs with: Retinitis pigmentosa Chorioretinitis Glaucoma Retinal detachment Leber's optic atrophy

Lesions before the chiasm

These will produce a field deficit in the ipsilateral eye (On the same side).

Field defects from damage to the optic nerve tend to be central, asymmetrical and unilateral.

Lesions just before the chiasm can also produce a small defect in the upper temporal field of the other eye

Lesions at the chiasm

These classically produce a bitemporal hemianopia.

If they spread up from below, for example, pituitary tumours, the defect is worse in the upper field.

If the tumour spreads down from above , e.g. craniopharyngioma, the lesion is worse in the lower quadrants.

Lesions after the chiasm

These produce homonymous field defects. A lesion in the right optic tract produces left

visual field defect. Lesions in the main optic radiation cause

complete homonymous hemianopia without macular sparing.

Lesions in the temporal radiation cause congruous upper quadrantic homonymous hemianopia commonly with macular sparing.

Lesions in the anterior visual cortex (common) produce a contralateral homonymous hemianopia with macular sparing .

Lesions in the macular cortex produce congruous homonymous macular defect

Lesions of the intermediate visual cortex produce a homonymous arc scotoma, with sparing of both macula and periphery.

Glaucoma

Glaucoma is a disease in which the optic nerve is damaged, leading to progressive, irreversible loss of vision. It is often, but not always, associated with increased IOP pressure of the fluid in the eye (Intraocular pressure (IOP) is the fluid pressure of the aqueous humor inside the eye).

The nerve damage involves loss of retinal ganglion cells in a characteristic pattern. There are many different sub-types of glaucoma but they can all be considered a type of optic neuropathy.

Influence of Glaucoma on Visual Function

The visual field defects that are caused by loss of retinal nerve fiber bundles are the most common and familiar change, the central vision is typically one of the last region to be lost

Cataract- Symptoms

Blurred vision Increasing difficulty with vision at night Glare, especially at night Halos around lights The need for brighter light for reading Double vision in a single eye Fading or yellowing of colors

Pain, redness, discharge, or irritation in the eye are usually not signs or symptoms of a cataract, but may be signs and symptoms of other eye disorders