Anatomy of the Eye
Many structures in the human eye, such as the cornea and fovea, process light so it can be deciphered by rods and cones in the retina.
Explain how eyes have evolved to benefit organisms
- The cornea and the lens bend light to focus the image on the retina; the iris and pupil regulate the amount of light entering the eye.
- The aqueous humour maintains the convex shape of the cornea; the vitreous humour supports the lens and maintains the shape of the entire eye.
- Presbyopia occurs because the image focuses behind the retina; it is similar to hyperopia (farsightedness), which is caused by an eyeball that is too short.
- Myopia (nearsightedness) occurs when an eyeball is elongated; images in the distance appear blurry, but images nearby are clear.
- Rods are used for peripheral and nighttime vision; cones are used for daytime and color vision.
- The fovea is responsible for acute vision because it has a high density of cones.
- rod: a rod-shaped cell located in the outer retina of the eye that is extremely sensitive to light
- retina: the thin layer of cells at the back of the eyeball where light is converted into neural signals sent to the brain
- cone: cell located near the center of the retina that is weakly photosensitive and is responsible for color vision in relatively bright light
Anatomy of the Eye
The retina, a thin layer of cells located on the inner surface of the back of the eye, consists of photoreceptive cells, which are responsible for the transduction of light into nervous impulses. However, light does not enter the retina unaltered; it must first pass through other layers that process it so that it can be interpreted by the retina.
The cornea, the front transparent layer of the eye, along with the crystalline lens, refract (bend) light to focus the image on the retina. After passing through the cornea, light passes through the aqueous humour, which connects the cornea to the lens. This clear gelatinous mass also provides the corneal epithelium with nutrients and helps maintain the convex shape of the cornea. The iris, which is visible as the colored part of the eye, is a circular muscular ring lying between the lens and the aqueous humour that regulates the amount of light entering the eye. Light passes through the center of the iris, the pupil, which actively adjusts its size to maintain a constant level of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil. In conditions of low light, the iris relaxes and the pupil enlarges.
The main function of the lens is to focus light on the retina and fovea centralis. The lens is a transparent, convex structure located behind the cornea. On the other side of the lens is the vitreous humour, which lets light through without refraction, maintains the shape of the eye, and suspends the delicate lens. The lens focuses and re-focuses light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through to focus it sharply on the retina. With age comes the loss of the flexibility of the lens; a form of farsightedness called presbyopia results. Presbyopia occurs because the image focuses behind the retina. It is a deficit similar to a different type of farsightedness, hyperopia, caused by an eyeball that is too short. For both defects, images in the distance are clear, but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry, but images nearby are clear.
There are two types of photoreceptors in the retina: rods and cones. Both are named for their general appearance. Rods, strongly photosensitive, are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones, weakly photosensitive, are located near the center of the retina. They respond to bright light; their primary role is in daytime, color vision.
The fovea is the region in the center back of the eye that is responsible for acute (central) vision. The fovea has a high density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object’s image falls on the fovea. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina, rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the fovea.
Normal Eye Microbiota
A small number of bacteria are normally present in the conjunctiva.
Give examples of the microorganisms found in the normal eye microbiota
- The lachrymal glands continuously secrete tears keeping the conjunctiva moist, while intermittent blinking lubricates the conjunctiva and washes away foreign material.
- Tears contain bactericides such as lysozyme, so that microorganisms have difficulty in surviving the lysozyme and settling on the epithelial surfaces.
- Some pathogens able to infect the conjunctiva, such as Neisseria gonorrhoeae and Chlamydia trachomatis are thought to have special processes allowing them to attach to the conjunctival epithelium.
- lachrymal gland: The lacrimal glands are paired almond-shaped glands, one for each eye, that secrete the aqueous layer of the tear film.
- conjunctiva: A clear mucous membrane that lines the inner surface of the eyelid and the exposed surface of the eyeball or sclera.
- lysozyme: A bacteriolytic (or antibiotic) enzyme found in many animal secretions and in egg white.
Normal Eye Microbiota
The human microbiome (or human microbiota) is the aggregate of microorganisms that reside on the surface and in deep layers of skin, in the saliva and oral mucosa, in the conjunctiva, and in the gastrointestinal tracts. They include bacteria, fungi, and archaea. Some of these organisms perform tasks that are useful for the human host. However, the majority have been too poorly researched to understand the role they play. Those that are expected to be present and do not cause disease (under normal circumstances), but instead participate in maintaining health, are deemed members of the normal flora.
A small number of bacteria are normally present in the conjunctiva. These include: Chlamydia trachomatis, Chlamydophila pneumoniae, Haemophilus aegyptius, Haemophilus influenzae, Moraxella spp, Neisseria spp, Staphylococcus aureus, Staphylococcus epidermidis and Streptococcus viridians. Staphylococcus epidermidis and certain coryneforms such as Propionibacterium acnes are dominant. Staphylococcus aureus, streptococci, Haemophilus sp. and Neisseria sp. sometimes occur. The lachrymal glands continuously secrete tears keeping the conjunctiva moist, while intermittent blinking lubricates the conjunctiva and washes away foreign material. Tears contain bactericides such as lysozyme, so that microorganisms have difficulty in surviving the lysozyme and settling on the epithelial surfaces.
Some pathogens able to infect the conjunctiva, such as Neisseria gonorrhoeae and Chlamydia trachomatis, are thought to have special processes allowing them to attach to the conjunctival epithelium. Newborn infants are particularly prone to bacterial attachment. Chlamydia and Neisseria may be present in an infected mother and show up on the cervical and vaginal epithelium. In such cases the newborn’s eyes may be treated with silver nitrate or antibiotics.
Bacterial Eye Diseases
Conjunctivitis is inflammation of the conjunctiva, most commonly due to an infection.
Describe the various causes of conjunctivitis and keratitis and its symptoms
- Classification can be either by extent of the inflamed area or by cause (allergic, bacterial, viral, or chemical).
- Red eye (hyperaemia), irritation (chemosis) and watering (epiphora) of the eyes are symptoms common to all forms of conjunctivitis.
- Keratitis, a condition in which the eye’s cornea becomes inflamed, is often marked by moderate to intense pain and usually involves impaired eyesight.
- conjunctivitis: An inflammation of the conjunctiva often due to infection.
- keratitis: Inflammation of the cornea.
Common Eye Infections
Conjunctivitis, also called pink eye or Madras eye, is inflammation of the conjunctiva, which consists of the outermost layer of the eye and the inner surface of the eyelids. Conjunctivitis most commonly caused by a viral infection or, less commonly, a bacterial infection, or by an allergic reaction. Classification can be either by extent of the inflamed area or by cause (allergic, bacterial, viral or chemical). Neonatal conjunctivitis is often defined separately due to different organisms.
Symptoms and Diagnosis
An inflamed, red eye (hyperaemia), irritation (chemosis), and watering (epiphora) of the eyes are symptoms common to all forms of conjunctivitis. However, the pupils should be normally reactive and the visual acuity normal. Bacterial conjunctivitis due to common pyogenic (pus-producing) bacteria causes marked grittiness/irritation and a stringy, opaque, greyish or yellowish mucopurulent discharge that may cause the lids to stick together, especially after sleep. Another symptom that could be caused by bacterial conjunctivitis is severe crusting of the infected eye and the surrounding skin.
Contrary to popular belief, discharge is not essential to the diagnosis. Bacteria such as Chlamydia trachomatis or Moraxella can cause a non-exudative but persistent conjunctivitis without much redness. The gritty and/or scratchy feeling is sometimes localized enough for patients to insist they must have a foreign body in the eye. The more acute pyogenic infections can be painful. Like viral conjunctivitis, it usually affects only one eye but may spread easily to the other eye.
Corynebacterium diphtheriae causes membrane formation in conjunctiva of non immunized children. Bacterial conjunctivitis usually resolves without treatment. Antibiotics, eye drops, or ointment may only be needed if no improvement is observed after three days.
Chlamydiaconjunctivitis or trachoma was once the most important cause of blindness worldwide. The infection can be spread from eye to eye by fingers, shared towels or cloths, coughing and sneezing, and by eye-seeking flies. Newborns can also develop chlamydia eye infection through childbirth. Chlamydia can affect infants by causing spontaneous abortion, premature birth, and conjunctivitis, which may lead to blindness and pneumonia. Conjunctivitis due to chlamydia typically occurs one week after birth (compared with chemical causes (within hours) or gonorrhea (2–5 days)).
Keratitis is a condition in which the eye’s cornea, the front part of the eye, becomes inflamed. The condition is often marked by moderate to intense pain and usually involves impaired eyesight. Superficial keratitis involves the superficial layers (i.e. the epithelium) of the cornea. After healing, this form of keratitis does not generally leave a scar. Deep keratitis involves deeper layers of the cornea (i.e. the epithelium, Bowman’s membrane and often stroma), and the natural course leaves a scar upon healing that impairs vision if it occurs on or near the visual axis. This can be reduced or avoided with the use of topical corticosteroid eyedrops.
Causes and Treatment
Keratitis has multiple causes. Bacterial infection of the cornea can follow from an injury or from result from wearing contact lenses. The bacteria involved are Staphylococcus aureus and, for contact lens wearers, Pseudomonas aeruginosa. Pseudomonas aeruginosa contains enzymes that can digest the cornea. Treatment depends on the cause of the keratitis. Infectious keratitis can progress rapidly, and generally requires urgent antibacterial, antifungal, or antiviral therapy to eliminate the pathogen. Treatment is usually carried out by an ophthalmologist and can involve prescription eye medications, systemic medication, or even intravenous therapy. It is inadvisable to use over-the-counter eye drops as they are typically not helpful in treating infections; using them could also delay crucial correct treatment, increasing the likelihood of sight-threatening complications. In addition, contact lens wearers are typically advised to discontinue contact lens wear and replace contaminated contact lenses and contact lens cases.
Other Infectious Eye Diseases
Fungi and viruses such as herpes simplex can cause eye infections.
Summarize the various types of herpes simplex keratitis: dendritic ulcer (epithelial keratitis) and disciform keratitis (stromal keratitis)
- Improper hygiene is a major cause of fungal contamination of contact lenses.
- Herpetic simplex keratitis is a form of keratitis caused by recurrent herpes simplex virus in cornea.
- Primary infection most commonly manifests as blepharoconjunctivitis i.e. infection of lids and conjunctiva that heals without scarring.
- ulcer: An open sore of the skin, eyes, or mucous membrane, often caused by an initial abrasion and generally maintained by an inflammation and/or an infection.
- keratitis: Inflammation of the cornea.
- conjunctivitis: An inflammation of the conjunctiva often due to infection.
Microbial corneal infection is the most serious and “most common vision threatening” complication of wearing contact lenses, which is believed to be strongly associated with contact lens cases. Such infections “are being increasingly recognized as an important cause of morbidity and blindness” and “may even be life-threatening. ” While the cornea is believed to be the most common site for fungal eye infections, other parts of the eye such as the orbit, sclera, and eyelids may also be involved.
Fungal Infections of the Eye
Factors that contribute to fungal contamination of contact lenses include, but not limited to, hygiene negligence such as: improper sterilization and disinfection of contact lenses, use of contaminated lenses, contaminated contact lens case, contaminated contact lens solution, wearing of contact lenses during eye infections and introduction of micro-organisms from the environment.
Diagnosis is determined”by recognition of typical clinical features and through direct microscopic detection of fungi in scrapes, biopsy specimens, and other samples. “Ultimately, cultures that are made from the samples isolated from patients is what “confirms diagnosis. ” Other tests that may also be used if needed include “histopathological, immunohistochemical, or DNA -based tests. Pathogenesis of the fungal contaminants includes a wide range of factors such as invasiveness, toxigenicity, and host factors. Once diagnosis is accessed, specific anti-fungal therapy can be administered. One of the most popular and common treatments used”for life-threatening and severe ophthalmic mycoses is amphotericin B which is a specific anti-fungal drug. For the treatment for filamentous fungal keratitis, “topical natamycin is usually the first choice. For the treatment of yeast keratitis, topical amphotericin B is usually the first choice. Current advances in further treatments include evaluations of triazoles such as itraconazole and fluconazole” as therapeutic options in ophthalmic mycoses.
Viral Infections of the Eye
Herpes Simplex Virus
Herpetic simplex keratitis is a form of keratitis caused by recurrent herpes simplex virus in cornea. Herpes simplex virus (HSV) infection is very common in humans. HSV is a double-stranded DNA virus that has icosahedral capsid. HSV-1 infections are found more commonly in the oral area and HSV-2 in the genital area. Primary infection most commonly manifests as blepharoconjunctivitis i.e. infection of lids and conjunctiva that heals without scarring. Lid vesicles and conjunctivitis are seen in primary infection. Corneal involvement is rarely seen in primary infection. Recurrent herpes of the eye in turn is caused by reactivation of the virus in a latently infected sensory ganglion, transport of the virus down the nerve axon to sensory nerve endings, and subsequent infection of ocular surface. The following classification of herpes simplex keratitis is important for understanding this disease:
Dendritic Ulcer (Epithelial Keratitis)
This classic herpetic lesion consists of a linear branching corneal ulcer (dendritic ulcer). During eye exam the defect is examined after staining with fluorescein dye. The underlying corneal has minimal inflammation. Patients with epithelial keratitis complain of foreign-body sensation, light sensitivity, redness, and blurred vision. Focal or diffuse reduction in corneal sensation develops following recurrent epithelial keratitis. In immune deficient patients or with the use of corticosteroids the ulcer may become large and in these cases it is called geographic ulcer.
Disciform Keratitis (Stromal Kieratitis)
Stromal keratitis manifests as a disc-shaped area of corneal edema. Longstanding corneal edema leads to permanent scarring. It is the major cause of decreased vision associated with HSV. Localized endotheliis (localized inflammation of corneal endothelial layer) is the cause of disciform keratitis.
Diagnostic testing is seldom needed because of its classic clinical features and is not useful in stromal keratitis as there is usually no live virus. Laboratory tests are indicated in complicated cases when the clinical diagnosis is uncertain and in all cases of suspected neonatal herpes infection. Corneal smears or impression cytology specimens can be analyzed by culture, antigen detection, or fluorescent antibody testing. Demonstration of HSV is possible with viral culture. Serologic tests in turn may show a rising antibody titer during primary infection but are of no diagnostic assistance during recurrent episodes.
Treatment of herpes of the eye is different based on its presentation. Epithelial keratitis is caused by live virus. Stromal disease is an immune response. Metaherpetic ulcer results from inability of the corneal epithelium to heal. Epithelial keratitis is treated with topical antivirals, which are very effective with low incidence of resistance. Acyclovir ophthalmic ointment and Trifluridine eye drops have similar effectiveness but are more effective than Idoxuridine and Vidarabine eye drops. Topical antiviral medications are not absorbed by the cornea through an intact epithelium, but orally administered acyclovir penetrates an intact cornea and anterior chamber.
Cytomegalovirus retinitis, also known as CMV retinitis, is an inflammation of the retina of the eye that can lead to blindness. Caused by human cytomegalovirus, it occurs predominantly in people whose immune system has been compromised.
Parasitic Infections of the Eye
Acanthamoeba is a microscopic, free-living ameba (single-celled living organism) commonly found in the environment that can cause rare, but severe, eye illness. Acanthamoeba causes three main types of illness involving the eye (Acanthamoeba keratitis), the brain and spinal cord (Granulomatous Encephalitis), and infections that can spread throughout the entire body (disseminated infection).
A single-celled parasite called Toxoplasma gondii causes a disease known as toxoplasmosis. While the parasite is found throughout the world, more than 60 million people in the United States may be infected with the Toxoplasma parasite. Of those who are infected, very few have symptoms because a healthy person’s immune system usually keeps the parasite from causing illness.
Signs and symptoms of ocular toxoplasmosis can include reduced vision, blurred vision, pain (often with bright light), redness of the eye, and sometimes tearing. Ophthalmologists sometimes prescribe medicine to treat active disease. Whether or not medication is recommended depends on the size of the eye lesion, the location, and the characteristics of the lesion (acute active, versus chronic not progressing). An ophthalmologist will provide the best care for ocular toxoplasmosis.