CS3: Lecture 2 Central Corneal Thickness

Glaucoma

a group of eye conditions that results in damage to the optic nerve resulting in VF loss and eventual blindness if not treated

IOP

intraocular pressure

the pressure of the fluid in the eye

Pachymetry

the measurement of the corneal thickness

microns or micrometers (um)

one millionth of a meter

CCT

central corneal thickness

GAT

Goldmann applanation tonometry

Types of Glaucoma

- Primary open angle (most common)

- Acute angle closure glaucoma

- Pigmentary glaucoma

- Childhood glaucoma

- Secondary Glaucoma (trauma, disease, diabetes, reaction to meds)n

- Normal tension glaucoma

- Angle closure glaucoma

Glaucoma affects

3.3 million in USA

2% of population

Predicted cases of glaucoma in 2050

sunbelt (old people retire to warm weather)

New Mexico !

Worldwide prevalance

64.3 million

3.54%

Africa has highest

china and SE asia

highest amount of narrow angle glaucoma

(narrow angle)

1954 Goldmann theory

For a tonometer tip with a diameter of 3.06 mm and for cornea of average thickness (520um) the elastic resistance of the cornea to applanation and the surface tension of the rear film were balance and cold be ignored

The theory fails when

cornea is not of average thickness

cornea has disease (changes elastic properties)

IOP measurements become ---- with variable CCT

less reliable

Normal IOP

10-21 mmHg

15.5 +/- 5 mmHg

Skew distrubution of pressire

5-7% of population > 21 mmHg instead of 2.5%

Ocular hypertension

- IOP greater than 21 mm Hg in one or both eyes measured with applanation tonometry on 2 or more occasion

- absence of glaucomatous defects on VF testing

- Normal appearance of the optic nerve head and nerve fiber layer

- normal open angles on gonio

- no conditions contributing to elevation of pressure such as narrow angloes, neovascular conditions, uveitis

The central cornea thickness is directly associated w

measuring the IOP

Goldmann tonometry was based on an average thickness of

520 um

When the cornea is not of average thickness nthe tonometry readings are

less reliable!

tonometry reading on thicker corneas are

falsely high

true reading would be lower ***********

Tonometry readings on thinner corneas are

falsely low

true reading would be higher

Mean CCT of normal eyes is

544 +/- 34 um

meta analysis of 80 studies

Numbers are

study dependent

Ocular Hypertension Treatment Study OHTS

Concluded: first large-scale study to demonstrate that lowering eye pressure can safely and effectively delay and possibly prevent the disease

"corneal thickness provides new information about the risk of developing POAG - recommend its measurement in the clinical evaluation of patients with ocular hypertension."

OHTS 6 predictive factors of developing glaucoma

person risk factors:

- older age

- african descent

Ocular risk factors:

- higher eye pressure

- cup to disc ratio of the ONH

- Thinness of the cornea***

- VF pattern standard deviation

US population by race

White: 64%

Hispanic: 16%

Black: 12%

rates of glaucoma were higher in every age group in ____

blacks

Glaucoma by race

White: 66%

Hispanic: 8%

Blacks: 19%

african americans make up 12% of the population makes up 19% of glaucoma pxs

high rate of glaucoma

Patients with ____ corneas were at higher risk of developing glaucoma

thinner

CCT < 555um and a baseline IOP greater than 25.75 mmHG has a 5 year risk of

36% of developing glaucoma

CCT > 588um and a baseline IOP greater than 25.75 mmHG has a 5 year risk of

6% of developing glaucoma

CTT has a 6 fold increase in thinner CCT

GAT _______ the true IOP in thinner corneas

underestimates

GAT _______ the true IOP in thicker corneas

overestimates

Applanation IOP sources of error

Thin cornea --> falsely low IOP

Thick cornea --> falsely high Iop

Should there be a correction factor

no!

Afrtcan americans have ____ corneas

thinner

High risk glaucoma

IOP > 25.75 mmHg

CCT < 555 um

Vertical C/D >0.50

20 % reduction in IOP

Tx: 4.4% in 5 years

No Tx: 9.5% in 5 years

Glaucoma meications

Prostaglandin Analogs 1x day - increase outflow of aqueous

Non-PGAs - 2-3x daily

PGAs

Xalatan (Latanoprost)

Travastan Z (Trovoprost)

Lumigan (Bimatprost)

Non-PGAs

Beta blockers

BB fixed combo

Alpha angonists

Carbonic Anhydrase Inhibitors

Pachymetry

process of measuring the thickness of the cornea

common measured risk factor for glauoma

Pachymetry is an essential step prior to

LASIK

Two forms of Pachymetry

1. Optical (user dependent varies by 20um or more)

2. Ultrasound (more reliable and repeatable)

Optical Pachymetry 1 Optical slit lamp techniques

device mounted on the slit lamp through which the observer aligns the anterior surface and endothelial surface of the cornea through image doubling

thickness is then estimated using an equation (refractive index and anterior radius of curvature of the cornea)

Optical Pachymetry - Orbscan

noncontact optical scanning slit instrument that provide pachymetry in addition to topographic analysis

Optical Pachymetry -3: OCT

non contact technique that acquire pachymetry measurements based on optical interferometry

Optical Pachymetry: Confocal Microscopy

contact* technique that acquire measurements by focusing though confocal microscope through thickness of the cornea

precise location of corneal layers with 5 microns of accuracy

Optical pachymetry 5 Pentacam Scheimpflug Camera

noncontact technique

rotating scheimpflug camera to rapidly capture images of the anterior segment of the eye

allows measurements of cornea thickness frmo limbus to limbus

Ultrasound Pachymetry

low frequency of 10-20 MHz

dry contact

accuracy dependent on the perpendicularity of the pribes application to the cornea

High frequency (50 MHz) and very-highfrequency ultrasound (70 MHz) have the disadvantage of requiring a water bath, but allow for determination of corneal sublayer detail and pachymetry

Preferred method for CCT

Ultrasound!

Ultrasound principle

instrument measures the amount of time (transit time) needed for ultrasound pulse pass from one end of transducer (epithelium) to Descemet's membrane and back to the transducer (endothelium not counted)

Performed at Speed of sound wave in cornea = 1640 m/s

CCT uses in pt care

- glaucoma risk

- refractive surgery

- corneal disease

LASIK

laser in-site keratomileusis

corneal flap is lifted and laser is applied to deeper layers of the cornea

excimer laser

need 160 um for the corneal flap

need 250 um remaining for structural integrity

removal of 12-14 um per diopter of vision correction

need minimum of 452 um for surgery on 3D px

PRK

photorefractive keratectomy

excime laser

works on thin corneas (no flap)

more discomfprt

longer recovery tome

more fu visits

safer than LASIK

outer layer of cornea is removed chemically for laser surgery and needs to regrow

Corneal disease- Thicker CCT

Edema

- endothelial disorders --> Fuch's distrophy

- inflammatory or infectious process --> HSV or HZO

- ocular surgery --> after corneal transplants

- Trauma --> corneal abrasions/blunt trauma

- Toxins --> from prescription medicaiton

Corneal disease- tinner CCT

Keratoconus - progressive thinning of the cornea

1.2000 in USA population

middle of cornea thins anbd bulges outward into a cone

genetic, enviormental, homronal

slit lamo exam, keratometry, red reflex

may cause corneal scarring

munsons sign

In lab: Ulrtasound pachymeter

press and hold!

25 readings

IOP offset (correction facotr no longer used)

SD ( how much variation is there from average measured value)

If orange - SD> 10 unreliable