lecture 3: Tonometry and Pachymetry Flashcards
Why is early detection of glaucoma needed?
*Treatment is more effective (lowering IOP is the only treatment that is effective)
*Allows identification and follow up of at-risk people
What demographic and clinical factors can affect IOP?
*AGE: IOP increases with age
*RACE: higher in black population
*GENETICS
*GENDER: IOP is 1-2 mmHg Higher in older women. If they take HRT (taken in menopausal women), IOP decreases. Open angle glaucoma more common in men.
*MYOPIA: associated with IOP
*SYSTEMIC DISEASES: systemic hypertension, diabetes can lead to high IOP
*OCULAR DISEASE: pigment dispersion syndrome. Retinal detachment can lower IOP
*CORNEAL CHARACTERISITCS
How can corneal characteristics influence IOP?
-Corneal thickness (if you have a thin cornea, it’s a risk factor for OAG)
-Corneal curvature
STEEP cornea: over-estimate true IOP
FLAT cornea: under-estimate true IOP
What does clinical research show for corneal characteristics and glaucoma?
Ocular hypertensives have thicker corneas
Patients with normal tension glaucoma have thinner corneas
How can you measure central corneal thickness?
Pachymetry
no correct factor is agreed on
average CCT: 535 um
How do ultrasound pachymeters work?
*Operates at frequencies of 20-50 MHz
*Emits short acoustic pulses and detect reflections from the anterior and posterior surfaces of the cornea
*Corneal thickness is then calculated from the measured time-of-flight between these reflections and the accepted speed of sound in the cornea of 1636-1640 m/s.
What are the sources of error in pachymetry?
*Not centered on cornea (thicker values if off centre)
*Not perpendicular to cornea (thicker values if probe not at 90 degrees to corneal surface)
*Corneal compression by probe if too vigorous
*Possible effect of local anaesthetic on corneal up to 10um difference (epithelial oedema)
*Variation in the speed of sound between health and diseases tissue
*Inter and intra observer variability (2 different optometrists taking measurement)
What short-term factors effect IOP?
*Time of day
*Cardiac cycle
*Body position
*Blinking, forced blinking and hard squeezing
*Fluid intake
*Patient holding their breath
*Accommodation
What is the normal diurnal range?
*Normal range is 3-5mmHg
*Glaucomatous average range of up to 13 mmHg
*Anything over 10mmHg range is considered pathological
When is IOP highest throughout the day?
How can you make sure this doesn’t give u a false high reading?
two types:
*Pressure is highest in the morning with afternoon dip.
*Pressure peaks in the afternoon
*Nearly everyone had nocturnal dip (lowest pressure during the night)
*Repeat pressure at a different time of day. Always record time of day pressure was recorded.
*Phasing: measuring pressure over 24 hours.
How does the cardiac cycle effect IOP?
How does body position effect IOP?
*Average variation is 2-3 mmHg
*Crucial in non-contact tonometry, hence we take 3 readings to average variation.
*If you go from standing up to laying down pressure increase happens by 0.3-6 mmHg
*Inversion (upside down): increase to 30-35 mmHg in normal and glaucomatous patients. Blood pressure in head also goes up witch provides protection.
How does blinking effect IOP?
How does drinking effect IOP?
*Blinking: increase of 3mmhg
*Forced blinking: increase of 10mmHg
*Hard squeezing: increase of up to 50 mmHg. -Repeated squeezing potentially lowers IOP.
*Water increases IOP by up to 3mmHg with maximum effect at 20 minutes
*Alcohol decreases IOP by up to 3mmHg with maximal effect after 5 minutes
*Coffee increases IOP by up to 3mmHg with maximal effect at 20 minutes.
How does holding your breath effect IOP?
How does accommodation effect IOP?
*Increases IOP is majority of px
*IOP can double (increase of 5-20mmHg)
*Advise px to breath normally
*Accommodation and increase and decrease pressure
*Sustained accommodation can cause reduction of up to 3mmhg. Greater in younger people.
*Onset of accommodation can increase IOP.
What are the advantages and disadvantages of non-contact tonometry?
advantages:
-Repeat measures do not effect IOP
(if you repeat it with GAT, pressure goes down)
-no anaesthetic
-can be used by paraprofessionals
-little if any risk of cross infection
-quicker
disadvantages:
-accuracy has been challenged
-must take multiple measurements
-initial costs
-ophthalmologists claim they cause unnecessary referrals
What are the the NICE 2017 guideline about non-contact tonometry?
-don’t refer soley on IOP’s measured by NCT
-if IOP>/ 24mmHg is the only suspicious finding, do goldmans to confirm NCT reading before referral
-if IOP is below 24mmHg and normal disc and field, continue with routine examinations
