O2B Tonometry

• dynamic balance between inflow and outflow of aqueous
• AH exerts pressure on the globe-> inflation of the globe

• trabecular meshwork is clogged
• or high production of AH at the pars plicana

• high myopia
• diabetes
• hereditary
• vascular disorders

• glaucoma
• damage to ocular structures
• blinding

10-21mmHg

• 15+/-3mmHg
• 68% of population

• <3
• >2 than it should be investigated

• 3-5mmHg
• 40% of population
• the eye is more hydrated in the morning

• supine +2-4mmHg
• this is because the heart is at the same level as the head

• 2-4mmHg
• influenced by heart rate

• more rigid surface too applnate
• requires more pressure
• >IOP readings

• and birth ~10
• increases to adult values at around 4yo
• slightly increases with age

• more fatter= >5mmHg
• fat pushing on globe
• hypertension

• high myopia 
• >1.5-2mmHg

3-8mmHg

• female >40% 
• during menstruation
• more bloated
• increased weight

• young: 3 years
• borderline results: last appointment of the day, return 1-2 weeks

Area deformed kept constant- measure applied force

force kept constant: measure deformation

• measures depth of area of constant size that deforms the cornea
• forces aqueous out at the trab mesh: lower IOP reading
• ocular rigidity: affects indentation
• >rigidity: >IOP reading
• >age, ocular disease, high myopia, eye surgery-scar tissue tensive, miotics thyroid disease will provide a higher IOP reading

• uses 5.5g/7.5/10g weights
• mechanical plunger on a sleeve

• Adv: 
• mech simple
• inexpensive, no slitlamp needed

• Disadv
• supine
• small scale errors= > IOP errors
• scale not linear: use conversion table
• corneal curvature assumed average
• must be held exactly vertical
• foot plate bears some weight/pressure
• parallel errors in reading the scale

uses empiracally derived nomograms to get a better estimate of IOP

• indentation tonometer
• modern schiotz with disposable rubber sheaths

• adv: 
• quick
• no anaesthetic required

• disav:
• <IOP readings
• average readings are inaccurate so you have to write down each one

• central cornea applanated with air of known pressure increasing linearly with time
• at the point of applanation, infrared beam is projceted onto the area and reflected maximally to a symmetrically placed telecentric receiver
• time elasped for beam to return is the IOP

• Pneumatic system: delivers air pulse
• Applanation monitoring system: detects when cornea is flattened
• Opticoelectronic instrument to cornea monitoring alignment system: autopuff triggering system, recognises good readings

hand held NCT

• ADV:
• doesnt touch eye
• no alteration of corneal integrity/IOP
• no anaesthetic
• extremely rapid
• no risk of microbial contamination
• objective

• DISAD:
• irregular cloudy cornea- inaccurate beam cannot bounce properly of non flat surface
• inaccurate if no proper fixations
• 1st reading 1mmhg apprenhension
• lid squeezing >IOP
• off axis measurement= overestimation of IOP
• stage of  ocular pulse

• take 3 readings- dont take averages
• record time of day
• record instrument used

• slitlamp mounted
• touches the cornea for a few seconds
• measures pulsatile IOP directly and continuously
• numeric output of IOP OCA
• independent of corneal characteristics

• 10mm head moulds to cornea: constant curvature
• less affected by corneal thickness and age
• >2mmHg than Goldmann:

Pascal is 2mmHg more 

• moving object momentarily collides iwth eye and motion parameters are monitored
• >IOP: >deceleration of probe
• no anaesthetic

Measures force per unit area required to flatten cornea= IOP

surface tension addes to the force but bending force of the cornea opposed force when area applantaed is 3.06mm

• 3 readings
• +/-1mmHg

inner mire touches other inner mire

• used as a screening tool
• mean value same as goldmann but more variability
• risk of false positives