Accessory visual structures
Accessory visual structures are the adnexa of the eye, including the eyebrow and lacrimal apparatus. One source defines "ocular adnexa" as the orbit and eyelids
The lacrimal apparatus is the physiological system containing the orbital structures for tear production and drainage. It consists of: The lacrimal gland, which secretes the tears, its excretory ducts, which convey the fluid to the surface of the human eye; this article incorporates text in the public domain from page 1028 of the 20th edition of Gray's Anatomy Diagram at hixie.ch
Photorefractive keratectomy and laser-assisted sub-epithelial keratectomy are laser eye surgery procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. LASEK and PRK permanently change the shape of the anterior central cornea using an excimer laser to ablate a small amount of tissue from the corneal stroma at the front of the eye, just under the corneal epithelium; the outer layer of the cornea is removed prior to the ablation. A computer system tracks the patient's eye position 60 to 4,000 times per second, depending on the specifications of the laser, used; the computer system redirects laser pulses for precise laser placement. Most modern lasers will automatically center on the patient's visual axis and will pause if the eye moves out of range and resume ablating at that point after the patient's eye is re-centered; the outer layer of the cornea, or epithelium, is a soft regrowing layer in contact with the tear film that can replace itself from limbal stem cells within a few days with no loss of clarity.
The deeper layers of the cornea, as opposed to the outer epithelium, are laid down early in life and have limited regenerative capacity. The deeper layers, if reshaped by a laser or cut by a microtome, will remain that way permanently with only limited healing or remodelling. With PRK, the corneal epithelium is removed and discarded, allowing the cells to regenerate after the surgery; the procedure is distinct from LASIK, a form of laser eye surgery where a permanent flap is created in the deeper layers of the cornea. LASEK and PRK are two different procedures. While both procedures interact with the epithelium atop the cornea, the PRK procedure removes this while LASEK brushes the material away for the procedure, before being placed back for healing after laser surgery; the procedure can be used to treat astigmatism and farsightedness. During the procedure, the epithelium is displaced using a diluted alcohol solution. LASEK has advantages over LASIK in that it avoids added complications associated with the flap created during surgery.
The procedure may reduce the chances of dry eye symptoms after surgery. Due to the LASEK procedure not requiring a surgical flap, athletes or individuals concerned with trauma introduced by the flap may see benefits to LASEK. Patients that wear contact lenses will need to stop wearing these for a specified time before the procedure. LASEK disadvantages include a longer recovery time for vision in contrast to LASIK. Post-surgery patients are required to wear a "bandage" contact lens over the eye, not required for LASIK post-surgery. Another disadvantage is that patient may be required to apply steroid eye drops for a few weeks longer than that of a LASIK procedure. Vision after the LASEK procedure has a longer recovery than LASIK which may be between five days and two weeks for blurred vision to properly clear; when LASEK is compared to LASIK, LASIK can have better outcomes with corneal haze while LASEK has a lower rate of flap complications than LASIK. There are a number of basic criteria which a person should satisfy: Normal ocular health Age 18 years or older Stable refraction error correctable to 20/40 or better Between −1.00 to −12.00 diopters of Myopia Not pregnant at the time of surgery Realistic expectations of the final results Pupil size 6 mm or less in a dark room is ideal Assessment of allergies, where allergy may complicate the eyelid margins following surgery leading to dry eye.
There are some pre-existing conditions that may complicate or preclude the treatment. Collagen vascular disease Ocular disease Systemic disorders History of side effects from steroids Granular corneal dystrophy type II Some complications that can be temporary or permanent include: Dry eyes Recurrent erosions during sleep Long healing period Pain Glare, halos, or starburst aberrations Increased ocular straylight Under- or overcorrection Recurrence of myopia Corneal haze Scarring Reduced best corrected visual acuity Reduced acuity in low light Increased sensitivity As with other forms of refractive surgery, keratoconjunctivitis sicca, colloquially referred to as'dry eye,' is the most common complication of PRK, can be permanent. In more advanced cases, recurrent erosions occur during sleeping from adherence of the corneal epithelium to the upper eyelid with rapid eye movement. Adjuvant polyunsaturated fatty acids with high Omega-3 content before and after surgery improves sicca due to their anti-inflammatory effects.
Foods containing PUFAs include fish oil. Brush PRK to denude the epithelium, instead of alcohol based techniques result in quantitatively lower ocular dryness after surgery; the amount of corneal hazing after surgery is decreased with brush technique. The platelet activating factor LAU-0901 has shown effect in mitigating dry eye in mouse models. Rabbit models have shown improvement with topical nerve growth factor in combination with docosahexaenoic acid. Mitomycin C worsens post-surgical dry eye. PRK may be performed on one eye at a time to assess the results of the procedure and ensure adequate vision during the healing process. Activities requiring good binocular vision may have to be suspended between surgeries and during the sometimes extended healing periods. PRK can be associated with glare and starburst aberrations, which can occur
Anterior chamber of eyeball
The anterior chamber is the aqueous humor-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium. Hyphema, anterior uveitis and glaucoma are three main pathologies in this area. In hyphema, blood fills the anterior chamber as a result of a hemorrhage, most after a blunt eye injury. Anterior uveitis is an inflammatory process affecting the iris and ciliary body, with resulting inflammatory signs in the anterior chamber. In glaucoma, blockage of the trabecular meshwork prevents the normal outflow of aqueous humour, resulting in increased intraocular pressure, progressive damage to the optic nerve head, blindness; the depth of the anterior chamber of the eye varies between 4.0 mm, averaging 3.0 mm. It tends to become shallower in eyes with hypermetropia; as depth decreases below 2.5 mm, the risk for angle closure glaucoma increases. Determining the anterior chamber depth is important in estimating the risk of angle closure glaucoma. There are various method of measuring ACD, including examination through a slit lamp and Scheimpflug photography.
These methods require expertise. A simpler clinical method of quantitatively estimating ACD using smartphone photography was developed by Dr Ehud Zamir from the Centre for Eye Research Australia, the University of Melbourne, published in 2016; the EZ ratio method is one way. To start, the patient looks at a target in the distance with one eye covered; the examiner takes a digital photograph of the open, examined eye, from the side, perpendicular to the visual axis. The following parameters need to be measured in the photograph, using a personal computer or a smartphone: 1; the pixel distance between the limbus and the front of the cornea. This distance is referred to as Z. 2. The pixel distance between the limbus and the centre of the pupil; this distance is referred to as E. E:Z ratio is the arithmetic ratio between E and Z; this ratio is linearly correlated with the depth of the anterior chamber with the following equation: Anterior chamber depth = -3.3 x EZ ratio + 4.2 This estimate has been shown to be accurate with a 95% confidence interval of +/– 0.33 mm error, when compared to measurements of the anterior chamber depth by Scheimpflug photography.
One peculiar feature of the anterior chamber is dampened immune response to allogenic grafts. This is called anterior chamber associated immune deviation, a term introduced in 1981 by Streilein et al; this phenomenon is relevant to the fact that the eye is considered an "immune privileged site", like the brain and the testis. Glaucoma Hyphema Hypopyon Intraocular pressure Ocular hypertension Anterior segment Anterior chamber IOL Atlas image: eye_2 at the University of Michigan Health System - "Sagittal Section Through the Eyeball"
An optician, or dispensing optician, is a technical practitioner who designs and dispenses corrective lenses for the correction of a person's vision. Opticians determine the specifications of various ophthalmic appliances that will give the necessary correction to a person's eyesight; some registered or licensed opticians design and fit special appliances to correct cosmetic, traumatic or anatomical defects. These devices are called artificial eyes. Other registered or licensed opticians manufacture lenses to their own specifications and design and manufacture spectacle frames and other devices. Corrective ophthalmic appliances may be contact lenses, spectacles lenses, low vision aids or ophthalmic prosthetics to those who are sighted; the appliances are mounted either on the eye as contact lenses or mounted in a frame or holder in front of the eye as spectacles or as a monocle. Opticians may work in any variety of settings such as joint practice, laboratories, eye care centers or retail stores.
However, registered opticians have to meet standards of practice and training, commit to ongoing education, hold professional liability insurance and are held to these standards by their respective regulating bodies. A credentialed optician in the United States is college educated in Optical Science and is known as an Ophthalmic Optician® and they are credentialed by the Society to Advance Opticianry. To achieve this nationally registered title an optician must achieve a combination of a college education, American Board of Opticianry and National Contact Lens Examiners advanced certifications, or maintain their state license in both eyewear dispensing and contact lens fitting when applicable. In the United Kingdom, an ophthalmic optician is known as an optometrist and is regulated by the General Optical Council under the Opticians Act 1989. Like many health care providers, opticians are regulated professionals in certain countries; the profession is regulated by optician-specific agencies, as in Canada and some states of the U.
S. or jointly with optometry such as the New Zealand Optometrist and Dispensing Opticians Board or the United Kingdom General Optical Council. Opticians may work independently or dependently with an optometrist or ophthalmologist although some opticians may work in an optical laboratory as a laboratory technical optician. Opticians convert a prescription for the correction of a refractive error into an ophthalmic lens or some other device, such as reading aids or telescopic lenses; the first known artistic representation of eyeglasses was painted by Tommaso da Modena in 1352. He did a sequence of frescoes of brothers efficiently replicating manuscripts. Once Tommaso had established the example, other painters positioned spectacles on the noses of many of subjects certainly as a representation of wisdom and respect. One of the most noteworthy developments in spectacle production in the 15th century was the introduction of concave lenses for the myopic or nearsighted. Pope Leo X, myopic, wore concave spectacles when hunting and professed they enabled him to see clearer than his cohorts.
The first spectacles utilized quartz lenses. The lenses were set into bone and leather mountings fashioned like two small magnifying glasses with handles riveted together and set in an inverted V shape that could be balanced on the bridge of the nose; the use of spectacles extended from Italy to Germany, Spain and Portugal. From their inception, eyeglasses posed a dilemma that wasn't solved for 350 years: how to keep them on the bridge of the nose without falling. Spanish spectacle makers of the 17th century experimented with ribbons of silk that could be attached to the frames and looped over the ears. Spanish and Italian missionaries carried the new models to spectacle wearers in China; the Chinese attached little ceramic or metal weights to the strings instead of making loops. In 1730 a London optician named Edward Scarlett perfected the use of rigid sidepieces that rested atop the ears; this perfection spread across the continent. In 1752 James Ayscough publicized his latest invention, spectacles with double hinged side pieces.
These became popular and appear more than any other kind in paintings and prints of the period. Lenses were fabricated of tinted glass as well as clear. Ayscough felt. In Spain in 1763 Pablo Minguet recommended green, or yellow lenses but not amber or red. Europeans, in particular the French, were self-conscious about the use of eyeglasses. Parisian aristocrats used reading aids only in private; the gentry of England and France used a "perspective glass” or monocular which could be concealed from view easily. In Spain, spectacles were popular amongst all classes since they considered eyeglasses made them look more important and dignified. Far-sighted or aging colonial Americans imported spectacles from Europe. Spectacles were for the affluent and literate colonists, who required a valuable and precious appliance. Benjamin Franklin in the 1780s developed the bifocal. Bifocal lenses advanced little in the first half of the 19th century; the terms bifocal and trifocal were introduced in London by John Isaac Hawkins, whose trifocals were patented in 1827.
In 1884 B. M. Hanna was granted patents on two forms of bifocals which become commercially standardized as the "cemented" and "perfection" bifocals. Both had the serious faults of ugly appearance and dirt-collection at the dividing line. At the end of the 19th century the two sections of the lens were fused instead of cemented At the turn of the
Refractive error known as refraction error, is a problem with focusing light onto the retina due to the shape of the eye. The most common types of refractive error are near-sightedness, far-sightedness and presbyopia. Near-sightedness results in far away objects being blurry, far-sightedness and presbyopia result in close objects being blurry, astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision and eye strain. Near-sightedness is due to the length of the eyeball being too long, far-sightedness the eyeball too short, astigmatism the cornea being the wrong shape, presbyopia aging of the lens of the eye such that it cannot change shape sufficiently; some refractive errors occur more among those whose parents are affected. Diagnosis is by eye examination. Refractive errors are corrected with contact lenses, or surgery. Eyeglasses are the safest method of correction. Contact lenses can provide a wider field of vision. Refractive surgery permanently changes the shape of the cornea.
The number of people globally with refractive errors has been estimated at one to two billion. Rates vary between regions of the world with 80 % of Asians affected. Near-sightedness is the most common disorder. Rates among adults are between 15-49% while rates among children are between 1.2-42%. Far-sightedness more affects young children and the elderly. Presbyopia affects most people over the age of 35; the number of people with refractive errors that have not been corrected was estimated at 660 million in 2013. Of these 9.5 million were blind due to the refractive error. It is one of the most common causes of vision loss along with cataracts, macular degeneration, vitamin A deficiency. An eye that has no refractive error when viewing distant objects is said to have emmetropia or be emmetropic meaning the eye is in a state in which it can focus parallel rays of light on the retina, without using any accommodation. A distant object in this case is defined as an object located beyond 6 meters, or 20 feet, from the eye, since the light from those objects arrives as parallel rays when considering the limitations of human perception.
An eye that has refractive error when viewing distant objects is said to have ametropia or be ametropic. This eye needs accommodation to do so; the word "ametropia" can be used interchangeably with "refractive error". Types of ametropia include myopia and astigmatism, they are categorized as spherical errors and cylindrical errors: Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refractive error have blurry vision. Nearsightedness: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness; this can arise from a cornea or crystalline lens with too much curvature or an eyeball, too long. Myopia can be corrected with a concave lens, which causes the divergence of light rays before they reach the cornea. Farsightedness: When the optics are too weak for the length of the eyeball, one has hyperopia or farsightedness; this can arise from a cornea or crystalline lens with not enough curvature or an eyeball, too short.
This can be corrected with convex lenses, which cause light rays to converge prior to hitting the cornea. Presbyopia: When the flexibility of the lens declines due to age; the individual would experience difficulty in near vision relieved by reading glasses, bifocal, or progressive lenses. Cylindrical errors cause astigmatism, when the optical power of the eye is too powerful or too weak across one meridian, such as if the corneal curvature tends towards a cylindrical shape; the angle between that meridian and the horizontal is known as the axis of the cylinder. Astigmatism: A person with astigmatic refractive error sees lines of a particular orientation less than lines at right angles to them; this defect can be corrected by refracting light more in one meridian than the other. Cylindrical lenses serve this purpose. There is evidence to suggest genetic predilection for refractive error. Individuals that have parents with certain refractive errors are more to have similar refractive errors; the Online Mendelian Inheritance in Man database has listed 261 genetic disorders in which myopia is one of the symptoms.
Myopia may be present in heritable connective tissue disorders such as: Knobloch syndrome. Myopia has been reported in X-linked disorders caused by mutations in loci involved in retinal photoreceptor function such as: autosomal recessive congenital stationary night blindness. Many genes that have been associated with refractive error are clustered into common biological networks involved in connective tissue growth and extracellular matrix organization. Although a large number of chromosomal localisations have been associated with myopia, few specific genes have been identified. In studies of the genetic predisposition of refractive error, there is a correlation between environmental factors and the risk of developing myopia. Myopia has been observed in individuals with visually intensive occupations. Reading has been found to be a predictor of myopia in children, it has been reported that children