Tonometry

• normal pressure to maintain shape of eye

• Chronic raised IOP= damage to RNFL = primary open angle glaucoma

• if raised IOP + no other symptoms = ocular hypertension = risk factor for glaucoma

• Acute raised IOP= from narrow angle= angle closure glaucoma

ocular hypotony- IOP<8mmHg

occurs as result of ret detachment, trauma, surgery

• narrow AC angles,

• sus optics discs,

• VF loss

• sudden pain,nausea, photophobia, halos around light, red eyes - acute closed angle glaucoma

• findings of inflammation in anterior chamber

• px on long term steroids- increase IOPs

• family history of glaucoma

• age- above 35

• afro-caribbean + SE asian px

• bz cornea is not perfectly spherical, dry, flexible, elastic and infinitely thin

• moisture creates surface tension (S) - attracts weight towards eye

• corneal rigidity (B) - resists incoming weight

• Goldmann said, S and B cancel out each other if internal area being applanated is 7.35mm²- acheived when diameter of external area is 3.06mm.

• 1g applied force = 10mmHg of internal pressure

• Thicker corneas ( measured with pachymeter) = elevated IOP

• Age= increased IOP in 40-60 and post 70

• Post menopausal = higher IOP

• Diurnal variation of 3 to 6mmHg

• sitting to lying = increased IOP

• increase in ocular rigidity ( esp hyperopic eyes) = increased IOP

• OG method, allows for continuos measurement of IOP

• most accurate BUT highly invasive

• needs general anaesthetic

• a plunger will indent a soft eye more than a hard eye ( more sinks = less IOP)

• measures depth of indentation in response to known weight

• topical anaesthetic

• measures IOP using force which flattens the cornea

• area of cornea being flattened is constant

• force is variable- higher force needed = higher IOP

• uses bi-prism

• topical anaesthetic - assess risk

• diamter of probe is 3.06mm

• egse of cornea is visible with NaFl and cobalt blue light

• dial calibratd in grams

• mires have to be just touching

• rings too narrow ( insufficient NaFl) = understimate IOP

• ring to wide( too much NaFl) = overestimates IOP

• if probe not on central cornea, one mire appear larger - need to withdraw tonometer and re-position in direction of lager mire. DO NOT RE-POSITION PROBE IN SITU - SCRATCH CORNEA

• in astigmatic eyes, mires = elliptical - rotate prism

  • 4D of WTR astig= IOP underestimated by 1mmHg

  • 4D of ATR astig= IOP overestimated by 1mmHg

• uses same bi-prism as GAT

• handheld- ideal for domicilliary

• battery operated light - ideal for domicilliary

Ads:

• accurate, reliable, repeatable

• easy to perform

• not very expensive

• gentler on eye than non contact air puff method

Disads:

• anaesthesia may be contraindicated in some pxs

• risk of corneal abrasion

• instruments need regular check of calibration- don manually

• same as Goldmann - use 2 known weights and observe mvt of probe in repsonse

• weights applied onto probe itself- so change probe before using on px

• remove batteries when calibrating becasue it adds to the opposition weight

• rapid air pulse to applanate cornea

• IOP estimated by detecting force of air jet at applanation

• 3 components:

  • alighmnet system - to align px eyes

  • optoelectric applan monitering system = transmitter, receiver , timer

  • pneumatic system- generates air pulse

• easy to use

• desktop and heldhand

• no anaesthetic required

• multiple readings required

• prefer to use non contact as screening, then refine with contact