Structure, Function, and Examination Techniques of the External Eye and Cornea Flashcards
How often are cilia replaced?
How long to regrow after cut? After epilation?
every 3-5 months
2 weeks
2 months
What are the glands of Moll?
apocrine seat glands near the eyelid margin
where are corneal epithelial stem cells located?
palisades of Vogt, at the limbus
blood supply of palpebral conj? bulbar conj?
shared with eyelids
anterior ciliary arteries
what nerves make up the afferent and efferent limbs of the corneal blink reflex?
afferent: CN V (via long ciliary nerve)
efferent: CN VII (orbicularis)
what is the normal blink rate in adults?
15 blinks/minute
dimensions of normal cornea?
10-11 mm vertical height, 11-12 mm horizontal width
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?
air: 1 aqueous and tears: 1.336 stroma: 1.376 entire cornea: 1.3375. negative contributions of posterior corneal surface
average refractive power of central cornea? posterior cornea? total corneal unit?
49D, -6D, 43D
Where does the cornea get glucose and oxygen?
glucose diffuses form aqueous. oxygen diffuses from tears, and peripheral cornea receives oxygen form limbal vessels.
What protects penetration of tear fluid into corneal stroma?
tight junctions between corneal epithelial cells
How long does it take for a corneal epithelial cell to differentiate from a basal to a superficial epithelial cell?
7-14 days
what are keratocytes and how are they arranged?
flattened fibroblasts located between collagen lamellae in the corneal stroma that are arranged uniformly to maintain corneal clarity
effect of age and corneal surgery on density of keratocytes?
decreases in both cases
Types of collagen in corneal stroma?
Prominent collagen type in Bowman’s layer?
Prominent collagen type in sclera?
I, V, VI
I
I
Major corneal proteoglycans?
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)
What is the average water content of the corneal stroma? Which has higher water concentration: anterior or posterior stroma?
78%. posterior
What type of telescope is the slit lamp?
Galilean telescope (one minus lens and one plus lens)
List methods of direct and indirect illumination using the slit lamp
direct: diffuse illumination, focal illumination, speculr reflection
indirect: proximal illumination, sclerotic scatter, retroillumination
what is considered an abnormal tear break up time?
<10 sec
what does fluorescein stain?
what do rose Bengal and lissamine green stain?
disruptions of epithelial cell junctions
dead and devitalized cells
Provide causes for the following fluorescein staining patterns:
- diffuse
- inferior cornea
- interpalpebral
- superior
- 3 and 9 o’clock
- inferior conjunctiva
- viral or toxic keratoconjunctivitis, trauma
- blepharoconjunctivitis, lag, trichiasis
- DES, exposure, neurotrophic keratopathy
- superior limbic keratoconjunctivitis, foreign body, trichiasis
- contacts
- meibomitis, mechanical
Name the following tests, indicate which type(s) of secretion each test measures, and indicate the cutoff for an abnormal value.
- 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.
- Without topical anesthetic, a filter paper is placed in the inferior fornix and wetting is measured at 5 minutes.
- 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.
- basic secretion test, measures only basic tearing, <3mm after 5 minutes is abnormal, 3-10mm is equivocal
- Schirmer I, measures basic + reflex tearing, <5.5mm after 5 minutes is abnormal
- Schirmer II, reflex tearing only, <15.5mm after 2 minutes
In the dry eye state, indicated whether each of the following tear values is increased or decreased:
- osmolality
- lactoferrin
- lysozyme
- increased (hyperosmolar)
- decreased
- decreased
describe how impression cytology is performed and name a few examples of its use
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
where is the thinnest area of a normal cornea?
1.5mm temporal to the geographic center
what is the average central corneal thickness?
540 um
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?
10%. 700 um. Waite-Beetham lines
In the normal eye, which is steeper: central or peripheral cornea
central (by about 3D)
define the following terms with regards to the cornea:
- optical zone
- corneal apex
- corneal vertex
- apical zone
- area of cornea that overlies the pupil
- point of cornea of maximum steepness (usually temporal to geographic center)
- point on cornea that intersects the line of fixation
- area comprised of the central zone (1-2mm) plus the paracentral zone (7-8mm donut surrounding central zone) which is important for CL fitting
what does keratometry measure, and how does it do so? which patients is keratometry not useful for?
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
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?
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.
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?
Instantaneous curvature. Tangential power. Better for peripheral cornea
How does corneal tomography differ from corneal topography? What type of technology does the Pentacam tomographer utilize?
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.
Match the following anterior segment imaging modalities with their corresponding descriptors:
- ultrasound biomicroscopy
- anterior segment OCT
- confocal microscopy
- 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
- b
- c
- d
- a
Endothelial cell density:
- at birth
- adulthood
- old age
- cut off for poor prognosis with intraocular surgery
- 3500 cells/mm^2
- 2400
- 2000
- 1000
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?
- 150-350 square micrometers
- polymegathism
- coefficient of variation. >0.4 has high likelihood for complication with intraocular surgery
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?
hexagonal. pleomorphism. >50% non-hexagonal has high likelihood for complication with intraocular surgery
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?
Cochet-Bonnet. Lower number/shorter filament threshold for sensation indicates reduced corneal sensitivity (easier to feel short, rigid filament than long, lax filament)
What subnucleus do efferent fibers in the lacrimal system originate in, and what ganglion do they pass through?
salivary subnucleus; sphenopalatine ganglion
Which of three prominently used corneal stains…
- stain(s) dead epithelial cells
- inhibit(s) grown of HSV
- is used for the dye disappearance test
- lissamine green and rose bengal
- rose bengal
- fluorescein
Indicate whether the following characteristics are descriptive of the anterior v posterior corneal stroma:
- increased water content
- short, narrow lamellae
- lamellae extending from limbus to limbus
- higher scattering
- higher refractive index
- higher density of keratocytes
- posterior
- anterior
- posterior
- anterior
- anterior
- anterior
posterior is wet, wide (lamellae), and weak (compared to anterior)
what accounts for the transparency of the cornea?
lattice arrangement of collagen fibrils where each lattice element is smaller than the wavelength of visible light
average thickness of Descemet’s at birth? In adulthood?
3 microns; 10-12 microns
name 3 changes that can lead to scleral translucency
- thinning
- dehydration below 40% (ex. senile scleral plaques from calcium phosphate deposits that dehydrate sclera)
- hydration above 80%
Which illumination method is demonstrated by the corneal light reflex seen in flash photography? Which principle allows for this illumination method
Specular reflection (normal light reflexes bouncing off a surface) Total internal reflection.
which illumination method is best for visualization of each the following pathologies:
- endothelial dystrophy
- map-dot-fingerprint
- stromal infiltrates
- corneal verticillata
- specular reflection
- retroillumination
- focal illumination
- sclerotic scatter
A CCT greater than ___ is associated with a higher risk of symptomatic corneal edema after intraocular surgery
640 microns
Compare ultrasound biomicroscopy and B-scan with regards to the following:
- frequency
- resolution
- penetration
UBM has higher frequency, higher resolution, and lower penetration, making it useful for anterior segment imaging.
