Structure, Function, and Examination Techniques of the External Eye and Cornea

Question 1

How often are cilia replaced?

How long to regrow after cut? After epilation?

Answer 1

every 3-5 months
2 weeks
2 months

Question 2

What are the glands of Moll?

Answer 2

apocrine seat glands near the eyelid margin

Question 3

where are corneal epithelial stem cells located?

Answer 3

palisades of Vogt, at the limbus

Question 4

blood supply of palpebral conj? bulbar conj?

Answer 4

shared with eyelids

anterior ciliary arteries

Question 5

what nerves make up the afferent and efferent limbs of the corneal blink reflex?

Answer 5

afferent: CN V (via long ciliary nerve)
efferent: CN VII (orbicularis)

Question 6

what is the normal blink rate in adults?

Answer 6

15 blinks/minute

Question 7

dimensions of normal cornea?

Answer 7

10-11 mm vertical height, 11-12 mm horizontal width

Question 8

refractive index of air? aqueous and tears? of the corneal stroma? what is the derived refractive index of the entire cornea? why is it different?

Answer 8

air: 1
aqueous and tears: 1.336
stroma: 1.376
entire cornea: 1.3375.
negative contributions of posterior corneal surface

Question 9

average refractive power of central cornea? posterior cornea? total corneal unit?

Answer 9

49D, -6D, 43D

Question 10

Where does the cornea get glucose and oxygen?

Answer 10

glucose diffuses form aqueous. oxygen diffuses from tears, and peripheral cornea receives oxygen form limbal vessels.

Question 11

What protects penetration of tear fluid into corneal stroma?

Answer 11

tight junctions between corneal epithelial cells

Question 12

How long does it take for a corneal epithelial cell to differentiate from a basal to a superficial epithelial cell?

Answer 12

7-14 days

Question 13

what are keratocytes and how are they arranged?

Answer 13

flattened fibroblasts located between collagen lamellae in the corneal stroma that are arranged uniformly to maintain corneal clarity

Question 14

effect of age and corneal surgery on density of keratocytes?

Answer 14

decreases in both cases

Question 15

Types of collagen in corneal stroma?
Prominent collagen type in Bowman’s layer?
Prominent collagen type in sclera?

Answer 15

I, V, VI
I
I

Question 16

Major corneal proteoglycans?

Answer 16

decorin (a/w dermatan sulfate, which is more prominent in anterior cornea), and lumican (a/w keratin sulfate, which is more prominent in posterior cornea)

Question 17

What is the average water content of the corneal stroma? Which has higher water concentration: anterior or posterior stroma?

Answer 17

78%. posterior

Question 18

What type of telescope is the slit lamp?

Answer 18

Galilean telescope (one minus lens and one plus lens)

Question 19

List methods of direct and indirect illumination using the slit lamp

Answer 19

direct: diffuse illumination, focal illumination, speculr reflection
indirect: proximal illumination, sclerotic scatter, retroillumination

Question 20

what is considered an abnormal tear break up time?

Answer 20

<10 sec

Question 21

what does fluorescein stain?

what do rose Bengal and lissamine green stain?

Answer 21

disruptions of epithelial cell junctions

dead and devitalized cells

Question 22

Provide causes for the following fluorescein staining patterns:

1. diffuse
2. inferior cornea
3. interpalpebral
4. superior
5. 3 and 9 o’clock
6. inferior conjunctiva

Answer 22

1. viral or toxic keratoconjunctivitis, trauma
2. blepharoconjunctivitis, lag, trichiasis
3. DES, exposure, neurotrophic keratopathy
4. superior limbic keratoconjunctivitis, foreign body, trichiasis
5. contacts
6. meibomitis, mechanical

Question 23

Name the following tests, indicate which type(s) of secretion each test measures, and indicate the cutoff for an abnormal value.

1. After installation of topical anesthetic and light blotting of the tear meniscus, filter paper is placed in the inferior fornix and wetting is measured after 5 minutes.
2. Without topical anesthetic, a filter paper is placed in the inferior fornix and wetting is measured at 5 minutes.
3. After installation of topical anesthetic, filter paper is placed in the inferior fornix. A CTA is sued to stimulate the nasal mucosa, and wetting is measured after 2 minutes.

Answer 23

1. basic secretion test, measures only basic tearing, <3mm after 5 minutes is abnormal, 3-10mm is equivocal
2. Schirmer I, measures basic + reflex tearing, <5.5mm after 5 minutes is abnormal
3. Schirmer II, reflex tearing only, <15.5mm after 2 minutes

Question 24

In the dry eye state, indicated whether each of the following tear values is increased or decreased:

1. osmolality
2. lactoferrin
3. lysozyme

Answer 24

1. increased (hyperosmolar)
2. decreased
3. decreased

Question 25

describe how impression cytology is performed and name a few examples of its use

Answer 25

A filter paper is applied to the conjunctiva or cornea to harvest epithelial cells which can then be examined directly in morphologic or histologic studies, or processed further as free cells for flow cytometry. Conjunctival impression cytology can be used to monitor ocular surface changes such as decreased goblet cell density, squamous metaplasia, or keratinization

Question 26

where is the thinnest area of a normal cornea?

Answer 26

1.5mm temporal to the geographic center

Question 27

what is the average central corneal thickness?

Answer 27

540 um

Question 28

What percentage increase in CCT is needed to see D-folds? What thickness is epithelial edema seen at? What are the name of the faint stromal wrinkles seen in early corneal edema?

Answer 28

10%. 700 um. Waite-Beetham lines

Question 29

In the normal eye, which is steeper: central or peripheral cornea

Answer 29

central (by about 3D)

Question 30

# define the following terms with regards to the cornea: 1. optical zone 2. corneal apex 3. corneal vertex 4. apical zone

Answer 30

1. area of cornea that overlies the pupil 2. point of cornea of maximum steepness (usually temporal to geographic center) 3. point on cornea that intersects the line of fixation 4. area comprised of the central zone (1-2mm) plus the paracentral zone (7-8mm donut surrounding central zone) which is important for CL fitting

Question 31

what does keratometry measure, and how does it do so? which patients is keratometry not useful for?

Answer 31

estimates corneal power, and through a vergence formula can also estimate radius of curvature, of the central cornea. not useful for post-refractive surgery patients

Question 32

What do corneal topographers measure and how do they do so? What is the most common type of corneal topographer used? What is a limitation of this technology?

Answer 32

They measure axial curvature by which reflected images of multiple concentric circles are digitally captured and analyzed by computer software. Placido-disc is most common technology. The assumption that the visual axis is coincident with the corneal apex may lead to misinterpretations, such as overdiagnosis of keratoconus.

Question 33

What is the name for the corneal curvature that is determined by taking a perpendicular path through the point in question and the visual axis? What is the term for the power calculated from this curvature? What portion of the cornea is this method better suited at measuring than axial curvature?

Answer 33

Instantaneous curvature. Tangential power. Better for peripheral cornea

Question 34

How does corneal tomography differ from corneal topography? What type of technology does the Pentacam tomographer utilize?

Answer 34

Topography only measures surface corneal curvature where tomography measures anterior and posterior corneal curvature, as well as corneal thickness, anterior chamber depth, and pupil indices. The Pentacam uses a rotating Scheimpflug camera.

Question 35

Match the following anterior segment imaging modalities with their corresponding descriptors: 1. ultrasound biomicroscopy 2. anterior segment OCT 3. confocal microscopy 4. specular microscopy a. measures endothelial cell count and density b. helpful in imaging anterior segment in patient with eight-ball hyphema c. utilizes reflected light to obtain high resolution cross sectional images of ocular tissue d. provides great spacial resolution and high magnification, making it useful for in vivo diagnosis of infectious fungal or amebic keratatitis

Answer 35

1. b 2. c 3. d 4. a

Question 36

Endothelial cell density: 1. at birth 2. adulthood 3. old age 4. cut off for poor prognosis with intraocular surgery

Answer 36

1. 3500 cells/mm^2 2. 2400 3. 2000 4. 1000

Question 37

Mean endothelial cell size? What term describes an increased variation in individual cell area? What unitless factor is used to describe variation in cell size, and what value is considered abnormal?

Answer 37

- 150-350 square micrometers - polymegathism - coefficient of variation. >0.4 has high likelihood for complication with intraocular surgery

Question 38

Normal shape of corneal endothelial cells? What term describes increased variability in shape of these cells? Cutoff for percentage of normally-shaped endothelial cells to tolerate intraocular surgery?

Answer 38

hexagonal. pleomorphism. >50% non-hexagonal has high likelihood for complication with intraocular surgery

Question 39

What is the name of the corneal esthesiometer which uses a retractable filament at various lengths to contact the un-anesthetized eye? What would indicate reduced corneal sensitivity?

Answer 39

Cochet-Bonnet. Lower number/shorter filament threshold for sensation indicates reduced corneal sensitivity (easier to feel short, rigid filament than long, lax filament)

Question 40

What subnucleus do efferent fibers in the lacrimal system originate in, and what ganglion do they pass through?

Answer 40

salivary subnucleus; sphenopalatine ganglion

Question 41

Which of three prominently used corneal stains... 1. stain(s) dead epithelial cells 2. inhibit(s) grown of HSV 3. is used for the dye disappearance test

Answer 41

1. lissamine green and rose bengal 2. rose bengal 3. fluorescein

Question 42

Indicate whether the following characteristics are descriptive of the anterior v posterior corneal stroma: 1. increased water content 2. short, narrow lamellae 3. lamellae extending from limbus to limbus 4. higher scattering 5. higher refractive index 6. higher density of keratocytes

Answer 42

1. posterior 2. anterior 3. posterior 4. anterior 5. anterior 6. anterior posterior is wet, wide (lamellae), and weak (compared to anterior)

Question 43

what accounts for the transparency of the cornea?

Answer 43

lattice arrangement of collagen fibrils where each lattice element is smaller than the wavelength of visible light

Question 44

average thickness of Descemet's at birth? In adulthood?

Answer 44

3 microns; 10-12 microns

Question 45

name 3 changes that can lead to scleral translucency

Answer 45

1. thinning 2. dehydration below 40% (ex. senile scleral plaques from calcium phosphate deposits that dehydrate sclera) 3. hydration above 80%

Question 46

Which illumination method is demonstrated by the corneal light reflex seen in flash photography? Which principle allows for this illumination method

Answer 46

``` Specular reflection (normal light reflexes bouncing off a surface) Total internal reflection. ```

Question 47

which illumination method is best for visualization of each the following pathologies: 1. endothelial dystrophy 2. map-dot-fingerprint 3. stromal infiltrates 4. corneal verticillata

Answer 47

1. specular reflection 2. retroillumination 3. focal illumination 4. sclerotic scatter

Question 48

A CCT greater than ___ is associated with a higher risk of symptomatic corneal edema after intraocular surgery

Answer 48

640 microns

Question 49

Compare ultrasound biomicroscopy and B-scan with regards to the following: 1. frequency 2. resolution 3. penetration

Answer 49

UBM has higher frequency, higher resolution, and lower penetration, making it useful for anterior segment imaging.