cornea Flashcards
Which cells of the corneal epithelium comprise a single layer of dome-shaped germinal cells?
Superficial cells
Stromal cells
Basal cells
Wing cells
Basal cells
The corneal epithelium can be further subdivided into three zones. The most posterior zone is comprised of a single layer of basal cells which adhere to the epithelial basement membrane. These cells give rise to all of the other zones. Moving exteriorly, the basal cells move more anteriorly and flatten out to become wing cells. This zone is two to three layers thick. The most anterior zone is one to two layers thick and is made up of superficial cells. These cells are flat, plate-like cells . Superficial cells are constantly sloughed off.
Your patient calls you at home because they have lost their pre-LASIK patient instructions and they have forgotten all the restrictions. Which of the following actions is permitted?
Eating or drinking the day of the surgery
Wearing contact lenses the day before the surgery
Swimming the day after surgery
Driving home after the surgery
Eating or drinking the day of the surgery
Explanation
There are no restrictions placed upon the ingestion of food or drink the day of the surgery; however, the consumption of alcohol the day of surgery is not recommended. The patient should absolutely not wear their contact lenses since a clear, clean and defect-free cornea is essential the day of the surgery. Contact lenses can alter the shape of the cornea (and the prescription) which will alter the final visual outcome. Legally, the patient will not be allowed to drive home after the surgery. In case of an accident, the surgeon may be held liable. Patients are not allowed to swim or go into a hot tub for at least one week post-LASIK (two weeks is preferable). Patients are also advised not to get water or shampoo in their eyes while showering for a week post-surgery.
A 36 year-old female patient presents with a concern of foreign body sensation, burning, tearing, and redness of both eyes. Slit lamp examination reveals radial injection of the superior bulbar conjunctiva that results in a fold of redundant tissue when light downward pressure is placed on the upper eyelids. Based on these clinical findings, what is the MOST likely diagnosis for this patient?
Superior limbic keratoconjunctivitis Episcleritis Pinguecula Thygeson's superficial punctate keratitis Vernal keratoconjunctivitis Marginal keratitis
Superior limbic keratoconjunctivitis Your Answer
Explanation
Patients presenting with superior limbic keratoconjunctivitis (SLK) typically complain of non-specific symptoms such as redness, foreign body sensation, burning, tearing, photophobia, pain, frequent blinking, and mild mucoid discharge. Clinical signs that are commonly observed in SLK are superior bulbar conjunctival thickening and radial injection, especially near the limbus. When light downward pressure is placed on the upper eyelids, a fold of conjunctival tissue will commonly cross the upper limbus. Staining is usually observed on the superior cornea, limbal region, and bulbar conjunctiva. Another important clinical feature is the presence of fine papillae on the superior tarsal conjunctiva that results in a velvety appearance. Papillae are also occasionally observed at the limbus, as well as superior filamentary keratitis, in more severe cases of SLK.
Which of the following components make up the majority of the dry weight of the cornea?
Keratocytes
Proteoglycans
Corneal nerves
Collagen
Collagen
Which of the following statements is TRUE in regards to gonococcal keratoconjunctivitis?
Gonococcal infections do not result in pseudomembrane formation
Neisseria gonorrhoeae is a Gram-positive organism
Neisseria gonorrhoeae is incapable of invading an intact corneal epithelium
Lymphadenopathy in gonococcal infections is typically prominent
Lymphadenopathy in gonococcal infections is typically prominent
Explanation
Gonorrheal infections typically have the following characteristics:
- Profuse conjunctival purulent discharge
- Eyelid tenderness and edema
- Severe conjunctival chemosis and hyperemia
- Pseudomembrane formation can occur
- Lymphadenopathy is typically prominent
- N. gonorrhoeae can invade an intact epithelium; therefore peripheral corneal ulceration can occur if conjunctivitis is not treated properly
- In severe cases, the ulceration can extend centrally, and eventual corneal perforation and endophthalmitis is possible
- Gram staining will reveal a Gram-negative organism with a kidney-shaped diplococcic appearance
Inflammation of the corneal stroma without the involvement of the epithelium or endothelium is known as which of the following conditions?
Corneal hydrops Interstitial keratitis Bullous keratopathy Neurotrophic keratitis Infiltrative keratitis
Interstitial keratitis (IK) is an inflammation of the corneal stroma in the absence of primary involvement of the corneal epithelium or endothelium. It is most often associated with congenital syphilis but may occur with other conditions such as tuberculosis, leprosy, Lyme disease, and other viral infections.
Patients diagnosed with vernal keratoconjunctivitis have an increased incidence of which of the following corneal diseases?
Thygeson’s superficial punctate keratitis
Terrien’s marginal degeneration
Keratoconus Your Answer
Superior limbic keratoconjunctivitis
Keratoconus Your Answer
Patients with vernal keratoconjunctivitis are more likely than the general population to develop keratoconus. This is due to the association of atopic disease in both conditions. Additionally, patients with VKC and keratoconus tend to have a more severe form of keratoconus that is commonly complicated by corneal hydrops and a greater tendency for corneal neovascularization.
A 23-year old patient is seen at your office complaining of a sudden onset of red, irritated eyes that itch and burn along with a mild, clear, watery discharge. Biomicroscopy reveals palpebral conjunctival follicles inferiorly along with mild eyelid edema and bilateral tender pre-auricular nodes. The cornea does not reveal any epithelial defects. He states that his co-worker had this same condition a week ago. Which of the following is the BEST diagnosis?
Phlyctenulosis
Vernal conjunctivitis
Epidemic keratoconjunctivitis
Bacterial conjunctivitis
Epidemic keratoconjunctivitis Your Answer
Epidemic keratoconjunctivitis (EKC) is a very common and contagious infection of viral etiology. EKC is caused by the adenovirus, of which there are many different strains, but the most common to cause ocular infections are serotypes 8 and 19. EKC is said to follow the “rule of 8s” because type 8 is the type most frequently isolated, on the 8th day the patient will present with diffuse superficial punctate keratitis (SPK), followed 8 days later (16 days from inoculation) by the formation of sub-epithelial infiltrates (SEIs). Once SEIs are present, the patient is no longer considered contagious. Signs of EKC include follicular conjunctivitis, positive lymphadenopathy, and mild lid edema. Small sub-conjunctival hemorrhages, pseudo-membranes, and iritis may also be present. Treatment for this condition is generally palliative and consists of ocular lubrication, topical vasoconstrictors, cool or warm compresses, topical NSAIDs, and sunglasses. Some clinicians use a Betadine® (5%) ophthalmic solution off-label treatment in office, which seems to work rather well. The use of steroids is still controversial because EKC and the Herpes simplex virus (HSV) can initially present similarly, and steroid use on HSV can lead to horrendous corneal damage. Topical steroids are very effective if the patient suffers from SEIs that are visually debilitating but be sure to taper the steroid use.
Vernal conjunctivitis is a severe form of allergic conjunctivitis and is generally observed in young male patients who suffer from some form of atopy like eczema, asthma, or hay fever. The patient will complain of itching as their main symptom. Pre-auricular nodes will not be affected in this condition.
Bacterial conjunctivitis usually presents in one eye first and then the other eye becomes infected via self-inoculation. This condition is contagious and is easily spread to other individuals in office settings, child-care facilities, or other institutions due to a lack of hand washing. Bacterial conjunctivitis usually causes a mucopurulent discharge (depending on the causative organism) and will not affect the pre-auricular nodes. Hyperacute bacterial conjunctivitis caused by N. gonorrhoeae will cause swelling of the pre-auricular nodes.
Phlyctenulosis is caused by a hypersensitive reaction to either microbes or their by-products, most commonly Staphylococcus exotoxins. This condition presents as a pink elevated nodule of tissue either on the conjunctiva or encroaching from the conjunctiva onto the cornea. Patients with phlyctenules will complain of pain and irritation, but their pre-auricular nodes will remain un-involved.
All else being equal, cells found in which layer of the cornea consume the GREATEST amount of oxygen?
Endothelium
Bowman’s membrane
Stroma
Epithelium
Endothelium
The corneal stroma is composed mainly of what component?
Proteoglycans
Keratocytes
Collagen
Keratin
Collagen
The corneal stroma is comprised mostly of collagen and water. The stroma itself makes up roughly 90% of the thickness of the cornea. This layer is made up of around 250 lamellae organized in a precise orthagonal arrangement to ensure corneal transparency. The space between the collagen bundles that make up the lamellae is maintained by proteoglycans which are composed of protein and carbohydrate. The proteoglycans have a negative charge and thus repel each other. The collagen is upheld and maintained by keratocytes.
Which of the following is the MOST appropriate action to take if you see red blood cells in the interface during a 1-day laser assisted in-situ keratomileusis (LASIK) post-operative visit?
Have the patient rinsed immediately
Increase the dosing frequency of the topical steroid
Watch the patient, as the red blood cells will usually reabsorb on their own
Use a topical vasoconstrictor to stop the progression
Watch the patient, as the red blood cells will usually reabsorb on their own
Red blood cells are occasionally seen after a LASIK procedure when neovascular vessels are severed during the creation of the corneal flap. Red blood cells are seen more often when a microkeratome is used versus a femtosecond laser. They have not been shown to cause an increase in inflammation or progress, nor have any effect on the patient’s visual outcome.
Neovascularization associated with contact lens wear is generally located in which layer of the cornea?
Posterior to the endothelium
Endothelium
Epithelium
Posterior stromal layers
Epithelium Correct Answer
Neovascularization associated with soft contact lens wear is generally located superficially and presents as an extension of vessels from the superficial marginal arcade beyond the limbus into the cornea. Most commonly, neovascularization is the result of over-wear from a contact lens that possesses a low Dk/t.
Stromal neovascularization can occur with contact lens wear but this is not the norm. Most stromal neovascularization is typically the result of infections such as chronic blepharoconjunctivitis, keratitis, phlyctenulosis, trachoma, or graft rejection.
The stem cells of the corneal epithelium originate from which location on the eye?
Aqueous
The palisades of Vogt
Goblet cells
Axenfeld loops
The palisades of Vogt Your Answer
The epithelial stem cells are located within the palisades of Vogt. These white, linear structures traverse radially around the cornea and are more prominent inferiorly than superiorly. They are clearly evident right on the edge of the limbus and extend to blend invisibly with the cornea. The palisades serve as housing for blood vessels, nerves, connective tissue and lymphatics. Most importantly, the palisades have been proven to contain stem cells that can be stimulated for renewal of aging/damaged epithelial cells. Axenfeld loops are generally seen in the sclera as sometimes raised areas of pigment. These loops are formed by nerves that initially course one way and then turn back on themselves. Goblet cells secrete the mucous layer of the tear film and are found primarily in the conjunctiva.
Hypoxic conditions cause the cornea to become cloudy and lose its transparency. Which layer of the cornea is the first to become edematous?
Epithelium Correct Answer
Endothelium
Bowman’s membrane
Stroma
Epithelium Correct Answer
The corneal epithelium is the first to swell due to the fact that this layer is bathed by the tear film which supplies the epithelium with oxygen. During periods of hypoxia, mitochondrial function slows and the cornea switches to glycolysis for ATP production which is termed the Pasteur Effect. Anaerobic metabolism will cause a build-up of lactic acid and hydrogen ions which changes the pH of the cornea causing osmotic swelling.
You are performing manual keratometry on your post-LASIK patient and realize that his corneal curvature is so flat that it falls outside of the range of the keratometer. Which of the following trial lenses would extend the range in the proper direction, and what adjustment do you need to make to the reading to obtain the true value?
Add a -1.00 lens and add 6D to the drum reading Your Answer
Add a +1.00 lens and subtract 9D from the drum reading
Add a -1.00 lens and subtract 9D from the drum reading
Add a -1.00 lens and subtract 6D from the drum reading Correct Answer
Add a +1.00 lens and add 9D from the drum reading
Add a +1.00 lens and subtract 6D from the drum reading
Add a -1.00 lens and subtract 6D from the drum reading Correct Answer
Explanation
When measuring the keratometry values utilizing a manual keratometer, there are certain circumstances in which the reading may be out of the range of the drum values. In these cases, one will need to add a trial lens to keratometer in order to extend the ranges (lenses are added to the patient’s side of the keratometer). Cases in which the curvature is steeper than the drum reading, plus trial lenses are required, and when the reading is flatter than the drum reading, minus trial lenses are necessary.
In the case of steeper curvatures, typically a +1.25 trial lens is tried first. If a measurement can be found with this lens, one will need to add about 8-9D to the drum reading in order to obtain the true value. A +1.25 lens will extend the range from about 52D to 60 or 61D.
If keratometry values are even steeper (in the 60-68D range), a +2.25 trial lens can be utilized to extend the drum range even more. In these cases, about 16D is added to the drum reading to reach the true keratometry value.
If the curvature is flatter than can be measured with the manual keratometer, a minus lens can be added to extend the range in the opposite direction. Typically a -1.00 trial lens will encompass a keratometry reading from about 32-38D. If a -1.00 lens is added, one will need to subtract 6D from the drum reading. This is the case for the above patient.
Which of the following corneal conditions occurs as a result of excessive amyloid deposition?
Lattice dystrophy Correct Answer
Macular dystrophy
Fleck dystrophy
Granular dystroph
Lattice dystrophy Correct Answer
A patient with against-the-rule astigmatism in the right eye will exhibit which of the following bowtie configurations on corneal topography imaging?
Bowtie slanted right
Horizontal bowtie
Bowtie slanted left
Vertical bowtie
Horizontal bowtie
The orientation of the bowtie on corneal topography is aligned with the location of the steep keratometric meridian (90 degrees away from the axis). Therefore, in against-the-rule astigmatism (axis near 90 degrees), the bowtie will be oriented horizontally. With-the-rule astigmatism (axis near 180 degrees) will reveal a vertically aligned bowtie pattern on topography. Additionally, patients with oblique astigmatism will have the bowtie slanted either left or right. For example, if a patient reveals astigmatism with an axis of 45 degrees, topography will show a bowtie pattern that is slanted to the right by 45 degrees (aligned with the 135 degree meridian).
A 23 year old male is seen at your office and has been waiting since he was 15 to undergo refractive surgery. His subjective refraction is -16.00 -0.75 x 180 OD and -15.00-0.25 x 167 OS which he reports has been stable for four years. Which type of surgery would be the BEST option for him?
Laser assisted in-situ keratomileusis (LASIK) Photo-refractive keratectomy (PRK) Implantable collamer lens (ICL) Refractive lens exchange (RLE) Conductive keratoplasty (CK)
Implantable collamer lens (ICL) Correct Answe
Due to the nature of this patient’s high prescription, the only viable options of the choices presented would be either ICL or RLE. However, if he were to undergo a refractive lens exchange in which the clear crystalline lens is removed and replaced with a corrective posterior chamber intraocular lens implant, ultimately he would end up presbyopic and would require some form of near correction. RLE procedures are best reserved for presbyopes who are dependent upon reading glasses already. Therefore, the best option for our patient would be implantable collamer lenses. This procedure offers several advantages in that it is not permanent and can be reversed without thinning of the corneal tissue. Essentially, in this technique a corrective lens is implanted either in front of or behind the iris without removal of the natural crystalline lens. One must exercise caution: if the surgeon is not careful and either the corneal endothelium or the natural lens is touched during the procedure, there is a risk of endothelial cell loss or cataract formation. For lenses placed in the anterior chamber, a peripheral laser iridotomy is performed to prevent the development of pupillary block glaucoma.
Conductive keratoplasty is a type of refractive surgery that is performed on low hyperopes (+0.75 to +3.00 D with less than 0.75 D of astigmatism). A thin probe is inserted into the peripheral cornea at specified intervals which delivers radiofrequency energy and causes a shrinking of the surrounding collagen. The circular ring of altered collagen results in a steepening of the central cornea and thus a decrease in hyperopia. The results of CK are temporary and generally do not last more than a few years.
Photo-refractive keratectomy (PRK) requires the use of an excimer laser to ablate the corneal epithelium and then refine the underlying tissue. PRK has been used for thin corneas and can eliminate up to roughly 7.00 D of myopia. This procedure may be quite painful and entails a long recovery time while the epithelium regenerates. PRK also requires the use of a bandage contact lens during the recovery period.
LASIK also utilizes a laser to alter the corneal thickness. Past LASIK procedures utilized a microkeratome to cut a flap and expose the underlying tissue for sculpting. It is now more common for the flap to be created by a laser, which allows for better precision and reproducibility of incision depth. Reportedly, LASIK can be used to correct up to 12.00 D of myopia, 6.00 D of astigmatism, and 6.00 D of hyperopia. LASIK is quite popular due to its good results, increased reliability, quick recovery time and decreased level of discomfort when compared to surface ablation refractive surgeries.
A newborn presenting with symptoms of ophthalmia neonatorum 3 days after birth is MOST likely infected with which of the following organisms?
Chlamydia trachomatis Streptococcus pneumonia Neisseria gonorrhoeae Haemophilus influenza Herpes simplex virus Staphylococcus aureus
Neisseria gonorrhoeae Your Answer
Ophthalmia neonatorum is a conjunctivitis that typically develops within the first 3 weeks after birth as a result of transmission of infection from mother to child during delivery. This condition is particularly serious due to the lack of immunity in infants as well as the immaturity of the ocular surface (poor tear film and undeveloped lymphoid tissue).
Ophthalmia neonatorum secondary to N. gonorrhoeae typically develops within 2-5 days postpartum as hyperacute conjunctivitis. Most cases present bilaterally with periorbital edema, conjunctival chemosis, and excessive amounts of purulent discharge. It is extremely important to quickly and aggressively treat this infection due to the ability of N. gonorrhoeae to penetrate an intact corneal epithelium.
When C. trachomatis is the organism responsible for ophthalmia neonatorum, mild to moderate symptoms of unilateral or bilateral conjunctivitis commonly occur between 5 to 14 days after birth. C. trachomatis is the most common cause of ophthalmia neonatorum. These patients present with lid edema, conjunctival chemosis, punctate corneal opacities, and occasionally micropannus formation.
Other etiologies of ophthalmia neonatorum can include S. aureus, Haemophilus species, S. pneumoniae, E. coli, and P. aeruginosa. These pathogens are part of the normal bacterial flora of the female genital tract and are likely acquired as the newborn travels through the birth canal.
Your patient had LASIK over one year ago, and her prescription has regressed slightly by 1.25 D of myopia. She wishes to get an enhancement and asks you what is the MOST common complication associated with a LASIK enhancement?
Flap dislocation
Epithelial ingrowth
Post sub-capsular opacification
Presbyopia
Epithelial ingrowth
The most common complication associated with an enhancement after previously undergoing LASIK is epithelial ingrowth. Although this condition can occur after the first LASIK procedure due to poor flap adhesion or from stray epithelial cells remaining under the flap, it is far more common with enhancements. Some surgeons are attempting to decrease the incidence of ingrowth occurrence with enhancements by re-cutting a new flap or via refractive keratotomy (RK). In general, ingrowth does not cause a problem if it is isolated to a small area and if there is little elevation or change with time. If the ingrowth is significant and vision is compromised, treatment requires that the flap be lifted and the offending cells removed.
Posterior capsular opacification only occurs after cataract surgery when residual lenticular epithelial cells proliferate, causing opacification of the posterior aspect of the space between the posterior lens implant and the posterior capsule. If vision is compromised, treatment requires that the cells be removed via a YAG laser.
Arlt’s lines and Herbert’s pits are associated with which of the following ocular conditions?
Recurrent corneal erosion
Vernal limbic keratoconjunctivitis (VKC)
Epidemic keratoconjunctivitis (EKC)
Trachoma
Trachoma Your Answer
Trachoma is more common in lesser-developed countries and can cause blindness if not treated appropriately. Trachoma presents in several stages, initially starting with mucopurulent discharge, lymphadenopathy, red eye, small superior tarsal follicles, and mild superior pannus. This infection eventually progresses to horrible scarring of the eyelid and cornea, causing extremely poor visual acuity. Arlt’s lines denote the characteristic linear scarring that occurs on the palpebral conjunctival surface. This scarring of the eyelids can cause entropion and trichiasis which abrade the cornea leading to scarring and/or ulceration. Herbert’s pits are conjunctival depressions or excavations caused by scarring of limbal follicles that occurs along the limbocorneal junction. Treatment includes oral doxycycline, tetracycline, azithromycin, or erythromycin along with topical tetracycline or erythromycin ointment.
A recurrent corneal erosion generally occurs in response to a corneal abrasion incurred by something organic (like a finger-nail or a tree branch). The initial abrasion heals but a short time afterwards the patient will experience another episode without any new incidence of trauma. The second occurrence tends to transpire first thing in the morning; the eyelids stick to the unstable flap of tissue and rip it off like a band-aid when the eyes open. The best way to treat a recurrent corneal erosion is through the use of a topical antibiotic (unpreserved is best) to ensure sterility (as the cornea is exposed) as well as a bandage contact lens to speed up the healing process. Some argue that a bandage contact lens can pose more problems than it solves as the lens must be worn continually for a minimum of 10-12 weeks (the lens should be changed intermittently). Hyperosmotic drops or artificial tears should be prescribed for roughly 10-12 weeks to ensure healing and to allow for proper formation of hemidesmosomes to alleviate future episodes. Other treatments include stromal micropuncture or debridement.
VKC is a condition of the young and presents with an increased frequency in males. This type of allergy typically develops before age 14 and lasts for 4-10 years before the child outgrows it; it occurs predominantly in the spring and summer. The condition progressively improves, with the first episode being the most severe. Usually VKC is seen in patients who are prone to atopy and therefore they suffer from eczema, asthma or hay fever. Patients typically suffer from itchy eyes and photophobia. The condition basically presents as a very severe type of allergic conjunctivitis. Signs include cobblestone papillae of the upper lid, lid swelling, and ropy discharge that is worse in the morning. Corneal defects (usually superiorly) known as keratitis of Togby may also be present. Occasionally, patients will develop a shield ulcer and Tranta’s dots, which are calcified eosinophils seen circumlimbally (they appear as chalky concretions) and may lead to the feeling of an associated foreign body sensation. Treatment includes mast cell stabilizers that should be started several weeks prior to re-occurring episodes, pulse steroid therapy, cool compresses, and sunglasses to help alleviate ensuing photophobia.
Epidemic keratoconjunctivitis (EKC) is a very common and contagious infection of viral etiology. EKC is caused by the adenovirus; there are many different strains, but the two most common to cause ocular infections are serotypes 8 and 19. EKC is said to follow the “rule of 8s” because type 8 is the type most frequently isolated; on the 8th day, the patient will present will diffuse superficial punctate keratitis (SPK), followed 8 days later (16 days from inoculation) by the formation of sub-epithelial infiltrates (SEIs). Once SEIs are present, the patient is no longer considered contagious. Signs of EKC include follicular conjunctivitis, positive lymph adenopathy, and mild lid edema. Small sub-conjunctival hemorrhages, pseudo-membranes, and iritis may also be present. Treatment for this condition is generally palliative and consists of ocular lubrication, topical vasoconstrictors, cool or warm compresses, topical NSAIDs, and sunglasses. Some clinicians use a Betadine® (5%) ophthalmic solution off-label treatment in office, which seems to be rather effective. The use of steroids is still controversial because EKC and the Herpes simplex virus (HSV) can initially present similarly, and steroid use on HSV can lead to horrendous corneal damage. Topical steroids are very effective if the patient suffers from SEIs that are visually debilitating, but be sure to taper the steroid use.
Which of the following best describes the pathophysiology of corneal guttata commonly observed in patients with Fuchs’ endothelial dystrophy?
Abnormal deposition of hyaloid material in the posterior corneal stroma
Irregular excrescences of Descemet’s membrane
Focal areas of corneal endothelial cell loss
Persistent epithelial edema resulting in the formation of microcysts
Irregular focal thickening of Bowman’s membrane
Irregular excrescences of Descemet’s membrane Your AnswerExplanation
Corneal guttata are abnormal excrescences or bumps of Descemet’s membrane that are secreted by anomalous corneal endothelial cells. Slit-lamp examination in patients with this finding will show a classic “beaten metal” appearance, which is best viewed by specular reflection. Guttata typically present in the central cornea and seldom reach the periphery. Patients are not considered having Fuchs’ endothelial dystrophy unless stromal and epithelial edema occurs in association with the finding of corneal guttata. When persistent epithelial edema produces the formation of microcysts and bullae, this is known as bullous keratopathy.
Which of the following values most closely corresponds to the average eccentricity of the human cornea?
- 50
- 00
- 10
- 00
- 50
0.50
Eccentricity is the measurement of the asphericity of a curved surface. In the case of the human cornea, it indicates the way in which the cornea changes from a more curved central portion to a flatter periphery (also known as a prolate shape). Normal corneal eccentricity values (e values) range between +0.50 and +0.60 in humans. A higher corneal eccentricity value indicates that the cornea flattens more rapidly in the periphery; a lower eccentricity measurement would occur in a patient whose cornea flattened more slowly in the periphery.
Another way to think of eccentricity is that the ‘e’ value designates that amount by which the cornea deviates from a perfect sphere (which has an eccentricity value of 0). A parabola has an eccentricity value of 1; a prolate ellipse (as in the normal cornea) will have an eccentricity between 0.1 and 0.9 depending on how fast the peripheral flattening occurs. Lastly, an oblate ellipsoid (as in some post myopic refractive surgical corneas) will have an eccentricity value of between -0.1 and -0.9, depending on how fast the curvature steepens in the periphery.
According to the Food and Drug Administration (FDA), what amount of corneal neovascularization is considered to be significant?
Vessel penetration in excess of 1.5mm
Any amount of vessel penetration is considered to be significant
Vessel penetration in excess of 0.7 mm
Vessel penetration in excess of 0.2 mm
Vessel penetration in excess of 1.5mm Your Answer
Explanation
The FDA has deemed that vessel penetration into the cornea in excess of 1.5 mm is considered to be of significance. A small amount of neovascularization (especially with soft contact lens wear) is to be expected, but if the vessels penetrate too far into the cornea, it is likely a sign of hypoxia and should be dealt with accordingly.
