Conj and cornea and refractive error Flashcards
Which of the following organisms is MOST commonly associated with the formation of “true” membranous conjunctivitis?
Streptococcus pyogenes
Staphylococcus aureus
Neisseria meningitidis
Haemophilus influenzae
Corynebacterium diphtheriae
Corynebacterium diphtheriae
Explanation - The formation of a true membrane in cases of acute membranous conjunctivitis is most commonly associated with a Corynebacterium diphtheria infection. Several other organisms can also cause membranous conjunctivitis (Strep hemolyticus, Strep pneumonia, N. gonorrhoeae, S. aureus, H. aegypticus, E. coli, adenoviruses, and herpes simplex); however, membranes are more or less synonymous with diphtheric conjunctivitis.
The difference between a true membrane and pseudomembrane is that in a pseudomembrane, the coagulum is deposited on the surface of the epithelium, while true membranes infiltrate the superficial layers of the conjunctival epithelium. A pseudomembrane can typically be easily peeled off, leaving the epithelium intact; while attempted removal of a true membrane can lead to bleeding and tearing of the conjunctival epithelium.
While performing keratometry on your patient, you notice that the reflected corneal mires appear elliptical ( oval )with the long axis located vertically. Which of the following keratometry values is this patient MOST likely to possess?
- 75@098 / 41.00@008
- 62@045 / 40.00@135
This is the expected result and the patient does not have astigmatism
49.00@180 / 43.75@090
49.00@180 / 43.75@090
thinK oval elliptical around vertical around 090 = flatter
The patient’s cornea is steeper horizontally and therefore minimizes the image in this meridian, creating an ellipse with its axis located vertically. Because the flatter meridian is vertical, this patient is most likely to exhibit against-the-rule astigmatism in which the flat meridian of the cornea is located at 90 degrees +/- 30 degrees.
Upon slit lamp examination of your 37 year-old male patient, you notice fine pigment dusting on the corneal endothelium that appears to form a triangular shape (Krukenberg’s spindle). Which of the following structures of the eye is this pigment derived from?
Ciliary body
Zonules
Iris stroma
Iris epithelium
Retinal pigment epithelium
The release of pigment that occurs in patients with pigment dispersion syndrome is thought to result from the chronic rubbing of the pigmented iris epithelium on the crystalline lens zonules. This causes radial iris transillumination defects, pigment release into the anterior chamber, and subsequent deposition onto the endothelial layer of the cornea (Krukenberg’s spindle), the anterior surface of the lens and iris, and in the trabecular meshwork.
Which of the following vitamins, in addition to UV light, is utilized in the process of corneal collagen cross-linking?
Folic acid
Niacin
Riboflavin
Ascorbic acid
Beta-carotene
Thiamine
Correct answer Riboflavin B2
Corneal collagen cross-linking is a surgical procedure in which UVA light and riboflavin are utilized to stiffen the collagen matrix of the corneal stroma. The procedure varies between surgeons, but typically involves either full or partial debridement of the corneal epithelium to allow for maximal absorption of isotonic riboflavin into the stromal tissue. Drops of riboflavin are instilled until the yellow-color of the riboflavin is apparent in the anterior chamber (by use of a lit-lamp and blue light). The thickness of the cornea is then measured, as it must be greater than 400 microns to continue with the UV light exposure. Once this is confirmed, UVA radiation is then applied to the cornea (with riboflavin drops still being instilled) until the desired treatment time has elapsed. The cornea is then irrigated thoroughly, and a bandage contact lens is placed on the eye.
All else being equal, cells found in which layer of the cornea consume the GREATEST amount of oxygen?
Epithelium
Stroma
Endothelium
Bowman’s membrane
Endothelium
has most mitochondria needs energy to work on th epumps
The cells of the endothelium require the greatest amount of oxygen. This is due to the fact that endothelial cells maintain a steady state of corneal clarity and hydration. They actively pump out ions into the anterior chamber, which sets up an osmotic gradient that causes water to flow down its concentration gradient, thus preventing corneal swelling and opacification. However, because the endothelium is only one layer thick, in total this layer consumes 21% of the oxygen provided to the cornea. The stroma utilizes 39% of oxygen made available to the cornea, which is a low consumption rate considering that it makes up the bulk of the cornea. The epithelium is responsible for 40% of the oxygen consumed by the cornea. However, all else being equal, the endothelial cells consume the greatest amount of oxygen (140 X 10-5 ml of oxygen per sec), vs. stromal cells (2.85 X 10-5 ml of O2/sec) and epithelial cells (26.5 X 10-5 ml of O2/sec).
In which of the following corneal disorders is the entire corneal thickness (central and peripheral) abnormally thin?
Post-LASIK ectasia
Keratoglobus
Keratoconus
Pellucid marginal degeneration
Terrien’s marginal degeneration
Keratoglobus
Ectasia is generalized in keratoglobus, and abnormal corneal thinning occurs over the entire cornea.
Corneal thinning in keratoconus and post-LASIK ectasia typically occurs centrally, or just inferior to the central region of the cornea.
Peripheral corneal thinning occurs in cases of pellucid marginal degeneration (PMD) 4-8 o’clock and Terrien’s marginal degeneration (superior thinning just inside the limbus).
A patient with against-the-rule astigmatism in the right eye will exhibit which of the following bowtie configurations on corneal topography imaging?
Horizontal bowtie
Vertical bowtie
Bowtie slanted right
Bowtie slanted left
Horizontal bowtie
ATR = flatter at 090 so green at 180 = Horizontal Bowtie = green
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).
Which of the following BEST describes the average central radius of curvature of the anterior and posterior cornea, respectively?
- 5mm, 7.8mm
- 7mm, 8.2mm
- 5mm, 6.8mm
- 2mm, 7.7mm
- 8mm, 7.5mm
- 8mm, 6.5mm
7.8mm, 6.5mm
The average central radius of curvature for the anterior surface of the cornea is 7.8mm, while the posterior radius of curvature averages 6.5mm. This means that the anterior corneal curvature is flatter than the posterior curvature. In addition to this difference between the two corneal surfaces, the anterior cornea tends to flatten more at the periphery as compared to the posterior surface.
Which of the following layers of the cornea is MOST susceptible to thinning in patients with keratoconus?
Stroma
All layers of the cornea
Bowman’s membrane
Endothelium
Epithelium
Descemet’s membrane
Stroma
Histological studies conducted on individuals diagnosed with keratoconus show that the layer of the cornea that experiences significant central or paracentral thinning is the corneal stromal layer. Other layers of the cornea also undergo changes, resulting in characteristic clinical observations such as Vogt’s striae (thin striations in the deep stroma in the area of ectasia). Additionally, brown pigment deposition of iron in the epithelial layer can be seen at the base of the ectasia during the course of the disease. The appearance of prominent corneal nerves, endothelial guttata, and posterior shagreen are also common observations viewed on slit-lamp examination. Furthermore, acute corneal hydrops is an associated condition that occurs when breaks in Descemet’s membrane occur.
The average cornea possesses approximately how much refractive power?
+12 diopters
+43 diopters
+36 diopters
+56 diopters
+43 diopters
Which of the following systemic conditions is MOST commonly associated with the development of interstitial keratitis?
Herpes simplex
Lyme disease
Chlamydia
Syphilis
Sarcoidosis
Human immunodeficiency virus
Syphilis
The presence of interstitial keratitis (IK) is essentially synonymous with congenital syphilis in most cases but may also occur in association with other infective causes such as Lyme disease, leprosy, and other viral diseases. Due to the common connection with congenital syphilis, all patients diagnosed with interstitial keratitis must undergo treponemal serology testing irrespective of the presence or absence of other associated clinical findings.
Which one of the following statements is FALSE regarding pellucid marginal corneal degeneration (PMCD)?
Acute hydrops can occur leading to scarring and corneal vascularization
PMCD is characterized by a narrow band of inferior corneal thinning 1-2 mm in width
Correct answer The cornea protrudes in the area of thinning
Differential diagnosis PMCD includes Terrien’s degeneration and Mooren’s ulcer
High amounts of irregular or against-the-rule astigmatism typically appears with PMCD
The cornea protrudes in the area of thinning
Unlike keratoconus, the cornea in PMCD protrudes superior to the area of corneal thinning. The other statements presented are all true. Two of the hallmark diagnostic signs of pellucid are the kissing birds/gull-wing pattern exhibited on corneal topography and the extreme inferior positioning of a GP lens on the eye. Oftentimes the GP diagnostic lens will cross the inferior limbus onto the sclera
What is the approximate average refractive power of the human cornea at birth?
38 diopters
56 diopters
32 diopters
48 diopters
48 diopters
At the time of birth, the approximate average refractive power of the human cornea is 48 diopters. As emmetropization occurs, the cornea will typically lose about 4 diopters of refractive power by the time a child reaches the age of 2.
which of the following values BEST describes the average imbibition pressure (IP) of the corneal stroma?
-40 mmHg
+40 mmHg
+55 mmHg
+25 mmHg
- 55 mmHg
- 25 mmHg
-40 mmHg
the imbibition pressure of the corneal stroma is due to the anionic (negatively charged) repulsive forces of the glycosaminoglycans (GAGs), which causes the tissue to expand and suck fluid into the cornea. In vitro, the imbibition pressure (IP) and the stromal swelling pressure (SP) are equivalent. However, in vivo, the IP is lower than the SP because the hydrostatic pressure induced by the intraocular pressure must be accounted for. Therefore, the average imbibition pressure of the corneal stroma is approximately -40 mmHg.
Corneal transparency is vital to ensure optimal vision. It is therefore important that the cornea remains strong and able to resist damage. Which type of force is the cornea LEAST resistant to?
Normal force perpendicular to the corneal place
Gravitational force
Lateral shearing force
Direct pressure perpendicular to the globe
Lateral shearing force
Due to the arrangement and architecture of the collagen fibrils that make up the cornea, it is capable of resisting forces of up to 30 atm when applied in the same direction as the visual axis. However, the cornea is not capable of resisting lateral shearing forces. The way that the cornea is structured, the lamellae can slide past one another. This explains why a probe is easily inserted between lamellae into the stroma.
Normal force is a support force that is exerted onto an object by another object that is stable. For example, a car resting on a road pushes down on the road but the road exerts an upward force back on the car to support its weight
In which of the following refractive surgery procedures is the corneal epithelium completely removed prior to ablation?
Laser-assisted epithelial keratomileusis
Laser-assisted in-situ keratomileusis
Conductive keratoplasty
Photorefractive keratectomy
Radial keratotomy
Photorefractive keratectomy
Laser-assisted in-situ keratomileusis (LASIK), laser-assisted epithelial keratomileusis (LASEK), and photorefractive keratotomy (PRK) are all refractive surgery procedures that use an excimer laser to ablate corneal tissue with subsequent correction of refractive error. These techniques differ in the way in which the deeper corneal tissue (stroma) is reached. LASIK involves the creation of a flap of corneal tissue. LASEK involves the removal of the corneal epithelium in a sheet that is then repositioned after ablation is complete. In PRK, the epithelium is completely removed and a bandage contact lens placed over the cornea after ablation to allow the epithelium to heal. The excimer laser used in these refractive procedures alters corneal stromal tissue, thus the need to expose this layer of the cornea.
Conductive keratoplasty is a procedure in which laser burns are placed in the corneal mid-periphery to induce stromal shrinkage and subsequent steepening of the cornea. Radial keratotomy involves the creation of radial incisions formed by a blade to flatten the corneal curvature.
Your patient tells you that he has previously been diagnosed with keratoconus. Which of the following exam findings would you expect to see during your evaluation?
Corneal striae with a horizontal orientation
Correct answer Irregular astigmatism
Limbal girdle of Vogt
Corneal arcus
Keratoconus causes a thinning of the cornea and is generally located centrally/inferiorly. It is not an inflammatory condition, and most patients tend to be asymptomatic other than reporting decreased visual acuity. Keratoconus is progressive and usually begins during puberty but studies have demonstrated that it tends to stabilize in the third or fourth decade of life. Upon examination, one generally observes myopia, irregular astigmatism, and corneal distortion. Some patients manifest a Fleischer’s ring, which is a deposition of iron anterior to Bowman’s layer of the cornea. This ring appears at the base of the cone and is green/brown in color. Fleischer’s rings are best viewed through a Cobalt blue filter. Some patients may also demonstrate Vogt’s striae which appear as fine, gray, parallel, linear wisps with a vertical orientation at the apex of the cornea in the stroma. These are believed to be stress lines caused by stretching of the tissue. If the keratoconus is slightly more advanced, it is possible for the patient to display central corneal scarring caused by ruptures in Bowman’s layer. Prominent corneal nerves may also be exhibited by some patients. Roughly 5% of patients will experience a rupture of Descemet’s membrane, causing hydrops or corneal edema along with a drop in visual acuity. If the cornea is unable to recover from the rupture in Descemet’s, a penetrating keratoplasty (PKP) (corneal transplant) may be warranted.
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?
Presbyopia
Flap dislocation
Post sub-capsular opacification
Epithelial ingrowth
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.
Inflammation of the corneal stroma without the involvement of the epithelium or endothelium is known as which of the following conditions?
Corneal hydrops
Infiltrative keratitis
Bullous keratopathy
Interstitial keratitis
Neurotrophic keratitis
Interstitial 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.
Which of the following best describes the pathophysiology of corneal guttata commonly observed in patients with Fuchs’ endothelial dystrophy?
Irregular focal thickening of Bowman’s membrane
Focal areas of corneal endothelial cell loss
Abnormal deposition of hyaloid material in the posterior corneal stroma
Irregular excrescences of Descemet’s membrane
Persistent epithelial edema resulting in the formation of microcysts
Irregular excrescences of Descemet’s membrane
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.
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?
Vernal keratoconjunctivitis
Pinguecula
Superior limbic keratoconjunctivitis
Marginal keratitis
Thygeson’s superficial punctate keratitis
Episcleritis
Superior limbic keratoconjunctivitis
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.
You are attempting to determine the etiology of your patient’s symptoms of ocular dryness. Which of the following tests could you perform to confirm the diagnosis of evaporative dry eye?
Tear break-up time
Schirmer test
Tear osmolarity
There is no clinical test to confirm this specific diagnosis
At this time, there is no clinical test that can definitively determine the presence of evaporative dry eye in a patient with ocular dryness signs and symptoms. Evaporative dry eye is considered a presumptive diagnosis based on clinical findings of meibomian gland disease.
Tear film break-up time measures tear film stability and will be decreased in both aqueous deficiency and evaporative dry eye. Schirmer testing, fluorescein clearance, and tear osmolarity tests measure tear production and are used in determining a diagnosis of aqueous deficiency dry eye.
Your patient has keratoconus and is pregnant. She would like to know if this condition is hereditary. What is the MOST appropriate response to this question?
Yes, keratoconus is hereditary 100% of the time
No, keratoconus has a high correlation with hypertension
No, there is no hereditary component associated with keratoconus
Yes, there is a hereditary component to keratoconus but it presents with incomplete penetrance
Yes, there is a hereditary component to keratoconus but it presents with incomplete penetrance
Although there still remains a lot of debate regarding the genetic link to keratoconus, according to the collaborative longitudinal evaluation in keratoconus study (CLEK) it is believed that the condition is autosomal dominant with incomplete penetrance. Therefore, there is a slightly higher chance of her child manifesting keratoconus; however, the odds are still quite small. Most patients with diagnosed cases of keratoconus do not profess a family history of the condition. Keratoconus does appear to be linked to eye rubbing, although whether this is a cause or simply a symptom is unclear at this point. There also appears to be a very high correlation between keratoconus and atopy. People who suffer from eczema, allergies, and hay fever tend to display a higher incidence of keratoconus compared to the rest of the general population. Again, this subset of people has a high correlation of eye rubbing associated with ocular irritation, which may contribute to the formation of keratoconus.
Which of the following refractive errors is associated with a deeper anterior chamber depth?
Presbyopia
Myopia
Hyperopia
Emmetropia
Myopia
Patients with myopia tend to possess increased anterior chamber depths and axial lengths when compared to patients who are emmetropic or hyperopic.
You are measuring the curvature of your keratoconic patient’s cornea using a manual keratometer. You are experiencing difficulty measuring the steep keratometry value as the drum reading does not go high enough. You extend the range by adding a +1.25 trial lens and are able to obtain a reading of 50.50D. Which of the following MOST closely represents the true keratometry value in his steep meridian?
- 75D
- 75D
- 50D
- 50D
- 00D
- 50D
59.00D
if pt is too steep –> drum can not read it –> use +1.25 trial
if pt is too flat –> drum can not read it –> use -1.00 D trial
trial
1- +1.25 –> add 8-9D to drum reading
2. +2.25 –> add 16 D TO
3- IF TOO FLAT (-1.00 )–> SUBSTARCAT 6 D FROM THE READING
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 the 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 patients with 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. In the case of the above patient, adding about 8.5D will result in a true value of 59.00D.
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.
Which 2 of the following statements regarding laser-assisted in-situ keratomileusis (LASIK) and contact lens wear are TRUE? (Select 2)
Rigid contact lenses should be removed for 1 week per decade of wear prior to the procedure
Correct answer Soft contact lenses should be removed 2 weeks prior to the procedure
Soft multifocal contact lenses must be discontinued 4 weeks prior to the procedure
Correct answer Extended wear contact lenses should be removed for a minimum of 2 weeks prior to the procedure
Daily disposable contact lenses may be worn until 2 days prior to the procedure
Toric contact lenses should be removed for a minimum of 6 weeks prior to the procedure
Contact lenses of all types can have an effect on a patient’s tear film, corneal topography, and refractive error. Therefore, it is important to have a patient discontinue wear of their contact lenses for a certain timeframe prior to surgery. The FDA requires patients to be out of soft contact lenses for a minimum of 2 weeks prior to any customized procedure. Because rigid gas-permeable lenses can have a more dramatic effect on corneal topography and thus on refractive error, they require a longer time for the cornea to normalize. The FDA requirement is a minimum of 1 month with the recommendation of 1 month for every decade of wear or until the corneal topography is stable.
Salzmann’s nodular degeneration results from what type of deposition and occurs between which layers of the cornea?
Amyloid deposition between Descemet’s and the endothelium
Mucopolysaccharide deposition between Bowman’s membrane and the stroma
Amyloid deposition between the stroma and Descemet’s membrane
Hyaline deposition between the epithelium and Bowman’s membrane
Hyaline deposition between the epithelium and Bowman’s membrane
Salzmann’s nodular degeneration appears as blue/white hyaline plaque deposits between the epithelium and Bowman’s membrane, generally around the pupillary area of the cornea. This condition stems from other pathologies, primarily old phlyctenula. Treatment is generally not required unless vision is affected.
Your 37 year-old male patient with a previous history of basement membrane disease reports that he began experiencing severe pain, watering, and photophobia upon wakening. Slit-lamp examination reveals a large epithelial defect in the interpalpebral zone that stains with sodium fluorescein. What is the MOST likely diagnosis of this patient?
Corneal abrasion
Corneal erosion
Corneal ulcer
Corneal laceration
Corneal erosion
Recurrent corneal erosions occur secondary to an abnormally weak attachment between the basal cells of the corneal endothelium and their basement membrane. Symptoms most commonly occur upon wakening, in which minor injuring resulting from opening the eyes after sleep causes shearing forces that are sufficient enough to tear the corneal epithelium. Patients typically present with severe ocular pain, watering, and photophobia that occurs during the night or shortly after wakening. There is usually a history of prior trauma or surgery (that could have occurred several years prior), or in association with an epithelial membrane dystrophy or anterior stromal dystrophy. Slit-lamp examination of these patients will usually reveal a frank epithelial defect, particularly in the interpalpebral zone or lower half of the cornea. There may also be a larger area surrounding the epithelial defect that represents the extent of the loose epithelium and may be highlighted with pooling of sodium fluorescein.
The corneal stroma is composed mainly of what component?
Keratocytes
Keratin
Proteoglycans
Collagen
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 values BEST describes the average imbibition pressure (IP) of the corneal stroma?
+25 mmHg
- 55 mmHg
- 25 mmHg
+40 mmHg
-40 mmHg
+55 mmHg
-40 mmHg
The imbibition pressure of the corneal stroma is due to the anionic (negatively charged) repulsive forces of the glycosaminoglycans (GAGs), which causes the tissue to expand and suck fluid into the cornea. In vitro, the imbibition pressure (IP) and the stromal swelling pressure (SP) are equivalent. However, in vivo, the IP is lower than the SP because the hydrostatic pressure induced by the intraocular pressure must be accounted for. Therefore, the average imbibition pressure of the corneal stroma is approximately -40 mmHg.
Which of the following conditions is NOT a contraindication to LASIK surgery?
Thin corneas
Active ocular disease
A keloid former
Retinal detachment
Controlled type II diabetes
Keratoconus
Controlled type II diabetes
LASIK is a procedure that results in thinning of the cornea; therefore, a person who has thin corneas to begin with or any corneal conditions resulting in irregular astigmatism or ectasia is not a good candidate. The presence of an active ocular disease such as conjunctivitis, microbial keratitis, or corneal erosions would also contraindicate a patient, as these diseases can lead to very serious post-LASIK infections and unpredictable surgical outcomes. A keloid former is a patient who is prone to producing a lot of scar tissue in the event of trauma or tissue manipulation. If a lot of scar tissue is present after surgery, this can increase the perception of light scatter and haloes, thereby decreasing acuity levels. Also, extreme scarring may also alter the shape of the cornea and offset the desired final outcome causing over- or under-correction or undesired astigmatism.
Arlt’s lines and Herbert’s pits are associated with which of the following ocular conditions?
Vernal limbic keratoconjunctivitis (VKC)
Epidemic keratoconjunctivitis (EKC)
Recurrent corneal erosion
Trachoma
Trachoma
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.
