Clinical Skills Lecture 2

NON-CONTACT TONOMETRY

Non contact tonometry

an instrument used to measure the intraocular pressure

Applanation Tonometry

Measures pressure needed to flatten part of cornea,

applanation tonometry instruments

NCT

Goldman

Perkins

Tonopen

Indentation tonometry

amount of indentation by a probe

schiotz

Rebound tonometry

measures how hard the bouncing ball rebounds off of the cornea

icare

Basic principle of non-contact tonometry

uses a puff of air (force) to flatten (applanate) a portion of the cornea

the more force it takes --> the more time --> increased IOP

Parts of the NCT

alignment system

transmitter

receiver/detector

pneumatic system

Alignment system

optically aligns the patient's cornea with instrument

Transmitter

directs the light beam to cornea vertex

receiver/detector

accepts parallel rays reflected from the cornea once the cornea is flattened

pneumatic system

generates the puff of air

How does the NCT work

light is reflects from the flattened cornea and received by the detector

the time at which the peak is reached is related to the force of the puff

NCT applantes ___ mm of corneal diameter

3.6 mm

1.4 gm of force/10mm hg

Goldmann applanates ---- mm of corneal diameter

3.06

1gm/10 mmHg

What may NCT be inaccurate

Ocular pulse

Blink - flase high

Misalignment - false high

irregular corneas

Why do we test IOP

Screening

Primary Care

Kids

Glaucoma evaluations - Goldmann

If IOP is elevated with NCT we must

retest with Goldmann

Normal IOP

10-21 mmHg

If greater than ___ or asymmetric by _____we must retest with goldman

21

4 or more

Importance of IOP

monitoring of glaucoma

Procedure

IOPcc

corneal compensated

people with thicker corneas have falsely high IOP

IOPg

Goldmann correlated

Score

how accurate

closer to 10 the more reliable

When to repeat the testing

asterik or data isorgane = low confidence

low score

IOP> 21 --> goldmann

difference of 4 or more

Glaucoma suspect --> Goldmann

AUTOMATED OBJECTIVE REFRACTION

define automated objective refraction

an estimation of the refractive error of an eye without the need of a response from a patient

may also provide cornea curvature measurement (keratometry )

Uses of Auto refractors

- estimate starting point of subjective refraction

- screening

- confirm a difficult or unusual refraction

- refraction for nonverbal or uncooperative patients

- Research

Accommodation

Your eyes make more plus!

Accommodation control

most use fogging or plus lenses

provide a distance target to prevent near accommodation!

Uses near ____ light

infrared radiation (NIR)

800 +- 80 nm

not visible to the eye

fundus reflects back 35%

Dioptric range: sphere

+- 25 - 30 D

0.12 or 0.25 D increments

Dioptric range: cylinder power

12 D

0.12 or 0.25 D steps

Dioptric range: cylinder axis

180 degrees

1 degree increments

Auto refractors are most accurate when

you are closest to emmetropia

as increase refractive error, some units lose accuracy

Pupil limits

samples ove 2.5-3 mm

if pupil is too small will affect

Vertex distance

the distance between the cornea and spectacle plane

with larger refractive errors there is an adjustment necessary

Achievement of endpoint

Nulling or Open loop

Nulling

neutralizing refractive error

Open loop

measures radiation exiting the eye

What settings do we want on autorefector

0.25 D

12 mm vertex distance

(-) cylinder form

refraction and keratometry

Procedure

When may we be unable to obtain reading

small pupils

young

media opacities

refractive error outside range

uncontrolled blinking or ptosis

improper aligment

fixation

posterioir segment abn=normalities

Precision/ Reliability

closeness of agreement between independent test results

repeatabiloty

same instrument same examiner same results

reproducibly

same patient different pt and differnt maxhine

repeatability is better if there is _____ cylinder

more

reasons for variability

changes in fixation

changes in pupil diameter

changes in accommodation

corneal and ocular surface

ptosis

media opacities

amblyopia

instrument or operator varibalilty

Validity

the extent to which a test measures or predicts what it is supposed to

Sphere validity

high percentage of agreement

Cylinder power

reasonable agreement

cylinder axis

lower agreement

Reasons for variations in validity

measurement bias

accommodative abnormalities - better agreement when cyclopledged

IOLS

anterior segment abnormalities

posterior segments abnormalities

refractive surgery abnormalities

Advantages of autorefraction

speed

efficiency

technician can perform

impress pt with tech

less examiner bias

easy to use

disadvantage of auo refractionv

cost

accuraacy may not be satisfortory

advancements with auto refraction

keratoemery readings

wavefront technology

nay include acuity charts