The eyes are complex vision receptors situated in a pair of bony cavities in the skull—the orbits—which open anteriorly to the exterior and posteriorly for the entry and exit of nerves and blood vessels. The main nerve—the optic nerve—carries visual impulses from the retina to the brain.
The anterior covering of the eyeball is the transparent cornea, which permits entry of light into the eyeball through the lens, which is the focusing mechanism. The cornea is continuous at the limbus with the sclera. The conjunctiva lines the inner surface of the eyelids (palpebral conjunctiva) and is reflected onto the sclera (bulbar conjunctiva). When the eyelids are closed, the conjunctiva forms a sac that is lubricated by tears, the secretion of the lacrimal gland, situated in the lateral part of the orbit.
The eyeball is separated from orbital bone by connective tissue, muscles, nerves, and blood vessels. The eyelids, which protect the front of the eye, are covered with skin on the outside and conjunctiva on the inside.
Many different types of pain may occur in eye diseases. In conjunctival and corneal inflammation, burning or itching of the eye is commonly associated with increased sensitivity to light (photophobia). Deep aching pain occurs in angle-closure glaucoma and inflammation of the uveal tract. Pain in acute glaucoma may be so severe as to cause vomiting. Headache may accompany conditions of disturbed vision.
Decreased visual acuity (amblyopia) is a feature of many ocular diseases. Spots and halos before the eyes occur in early cataract. Halos may also occur in glaucoma. Diminution of the visual field may signify disease of the retina, the optic disk, or the visual neural pathways, which include the optic nerve, chiasm, radiation, and visual cortex. Night blindness may result from vitamin A deficiency and retinal degenerative diseases. Double vision (diplopia) is a feature of eye muscle dysfunction.
Eye discharge may represent increased tearing (eg, in allergy) or inflammation of the conjunctiva. Microscopic examination of the discharge shows the type of inflammatory cells present and the presence of viral and chlamydial inclusions when these agents are involved. The presence of numerous eosinophils is typical of allergic conjunctivitis, while neutrophils dominate in acute infectious conjunctivitis. Gram stain for bacteria and potassium hydroxide preparations for fungi are of value in some cases.
Inspection of the eyes may disclose evidence of strabismus (muscle imbalance), hemorrhage, congestion, jaundice, swelling, displacements of the eye such as proptosis (forward displacement), and the presence of tumors. Ophthalmoscopic examination may reveal abnormalities of the anterior chamber (eg, hypopyon and hyphema—pus and blood, respectively, in the anterior chamber), lens (eg, early cataract, dislocation), vitreous (eg, hemorrhage), retina (eg, diabetic and hypertensive retinopathy, retinal degenerative diseases, detachment, hemorrhages, exudates, changes in ...