Disorders of Visual Function

Causes of Alterations in Vision
Disorders of the eyelids and optic globe (conjunctiva, cornea, and uvea)
Intraocular pressure (glaucoma)
Lens (cataract)
Vitreous humor and retinas
Dual pathways and visual cortex
Extraocular muscles and eye movement

Layers of the Wall of the Eyeball
Sclera
Outer supporting layer
Choroid
Middle vascular layer
Retina
Composed of the neuronal retinal layer and the outer pigmented layer
Components of the Functions of Vision
Visual receptor function of the eyeball
The optic nerve
Visual pathways that carry and distribute sensory information from the optic glove to the CNS
Primary and visual association cortices that translate the sensory signals into visual images
The Eyeball
Description
A hollow spherical structure that functions in the reception of the light rays that provide the stimuli for vision
Mechanism of vision
The refractive surface of the cornea and accommodative properties of the lens focus the light signals from near and far objects on the photoreceptors in the retina


Structures of the Upper and Lower Eyelids
Palpebrae
Modified folds of skin that protect the eyeball
Palpebral fissure
The oval opening between the upper and lower eyelids
Canthus
Found at the corners of the eye, where the upper and lower lids meet
Structures of the Upper and Lower Eyelids (cont.)
Tarsus (plate of dense connective tissue)
Gives the lid its shape
Contains modified sebaceous glands, called meibomian glands, the ducts of which open onto the eyelid margins

Type Main Types of Anterior Blepharitis
Seborrheic form
Usually associated with seborrhea (i.e., dandruff) of the scalp or brows
Staphylococcal blepharitis
May be caused by Staphylococcus epidermitis or S. aureus
The lesions are often ulcerative
Components of the Lacrimal System
Major lacrimal gland
Produces the tears
The puncta and tear sac
Collect the tears
Nasolacrimal duct
Empties the tears into the nasal cavity
Disorders of the Conjunctiva
Conjunctivitis
Infectious conjunctivitis
Bacterial conjunctivitis
Viral conjunctivitis
Chlamydial conjunctivitis
Opthalmia neonatorum
Allergic conjunctivitis
Layers of Tissue Forming the Cornea
An extremely thin outer epithelial layer
Continuous with the bulbar conjunctiva
A middle layer called the substantia propria or stroma
Composed of regularly arranged collagen bundles embedded in a mucopolysaccharide matrix
An inner endothelial layer
Lies next to the aqueous humor of the anterior chamber
Disorders of the Cornea
Corneal trauma
Keratitis
Herpes simplex keratitis
Abnormal corneal deposits
Arcus senilis

Pupillary Reflex
Controls the size of the pupil
Is controlled by the autonomic nervous system
The parasympathetic nervous system controls pupillary constriction
The sympathetic nervous system controls pupillary dilation

Control of Intraocular Pressure
The aqueous humor
Serves to maintain the intraocular pressure and
Provides for the nutritive needs of the lens and posterior cornea
Mediates the exchange of respiratory gases
Contains a low concentration of protein and high concentrations of ascorbic acid, glucose, and amino acids

Glaucoma
Definition
An optic neuropathy characterized by optic disk cupping and visual field loss
Causes
An increase in intraocular pressure that results from abnormalities in the balance between aqueous production and outflow
Most common cause is an interference with aqueous outflow from the anterior chamber, rather overproduction of aqueous humor
Classifications of Glaucoma
Angle-closure (narrow-angle) vs. open-angle (wide-angle)
Depends on location, circulation, and resorption
Congenital vs. acquired condition
Primary vs. secondary disorder
Primary: no evidence of preexisting ocular or systemic disease
Secondary: results from inflammatory processes affecting the eye, tumors, or blood cells of trauma-produced hemorrhage
Symptoms of Congenital or Infantile Glaucoma
Excessive lacrimation and photophobia
Affected infants tend to be fussy, have poor eating habits, and rub their eyes frequently
Diffuse edema of the cornea usually gives the eye a grayish-white appearance
Enlargement of the entire globe (buphthalmos)
Structure of the Lens
The lens is an avascular, transparent, biconvex body, the posterior side of which is more convex than the anterior side
A thin, highly elastic lens capsule is attached to the surrounding ciliary body by delicate suspensory radial ligaments called zonules, which hold the lens in place
When changing lens shape, the tough elastic sclera acts as a bow, the zonule and the lens capsule act as the bowstring
Structure of the Lens (cont.)
The suspensory ligaments and lens capsule are normally under tension, causing a flattened shape for distant vision
Contraction of the muscle fibers of the ciliary body produces a more spherical or convex shape for near vision


Disorders of Refraction
Hyperopia or farsightedness
The anterior-posterior dimension of the eyeball is too short, the image is focused posterior to (behind) the retina
Myopia or nearsightedness
The anterior-posterior dimension of the eyeball is too long, the focus point for an infinitely distant target is anterior to the retina
Presbyopia
Refers to changes in vision that occur because of aging
Cataract
Lens opacity that interferes with the transmission of light to the retina
The most common cause of age-related visual loss in the world
Risk Factors for Cataracts
Effects of aging
Genetic influences
Environmental and metabolic influences
Drugs
Injury
Vitreous Humor
Definition
A colorless, amorphous biologic gel that fills the posterior cavity of the eye
Composition
Approximately 99% water, some salts, glycoproteins, proteoglycans, and dispersed collagen fibrils
Location
Attached to the ciliary body and the peripheral retina in the region of the ora serrata and to the periphery of the optic disk

Function of the Retina
Receive visual images
Partially analyze them
Transmit this modified information to the brain
Composition of the Neural Retina
Three layers of neurons
A posterior layer of photoreceptors
A middle layer of bipolar cells
An inner layer of ganglion cells that communicate with the photoreceptors
Types of Photoreceptors Present in the Retina
Rods, capable of black–white discrimination
Cones, capable of color discrimination
Disorders of the Retina
Ischemia of the retina
Papilledema
Central Retinal Artery Occlusion
Central Retinal Vein Occlusion
Retinopathies
Diabetic retinopathy
Hypertensive retinopathy
Atherosclerosis of retinal vessels
Retinopathy of prematurity
Types of Age-Related Macular Degeneration
‘Dry” form
An atrophic non-exudative
“Wet” form
Exudative

Disorders of Neural Pathways and Cortical Centers
Visual field defects
Retinal defects
Disorders of the optic pathways
Disorders of the Visual Cortex
Disorders of Eye Movement
Amblyopia
A condition of diminished vision in which no detectable organic lesion of the eye is present
Strabismus
Any abnormality of eye coordination or alignment that results in loss of binocular vision
Paralytic strabismus
Non-paralytic strabismus
Extraocular Eye Muscles and Their Innervation
Innervated by three cranial nerves
The abducens nerve (CN VI) innervates the lateral rectus
The trochlear nerve (CN IV) innervates the superior oblique
The oculomotor nerve (CN III) innervates the remaining four muscles