Gunter tonometry notes

• Tonometry


  Measurement of the tension of the eye, which includes the combined resistance to deformity of its coats and the intraocular pressure (IOP)
• Tonometry


  • Accurate measurement would be a cannula (small tube) into the eye – not practical
  • The major clinical reason is the association between IOP and glaucoma
  • Requires consideration of other investigations and risk factors to formulate more meaningful criteria than simply relying on IOP
• The method is based on the assumption that the effect this external pressure has on the corneal shape will depend on the eye's internal pressure
• There are many physiological and measurement variables which affect the relationship between external pressure and IOP
• Intraocular pressure (IOP)


  Depends on the relationship between aqueous production and the resistance against its outflow from the eye
• Aqueous humour production and drainage

  1. Produced by the ciliary body in active (80%) and passive (20%) secretion

  2. Flows from the posterior chamber into the anterior chamber through the pupil

  3. Majority (90%) drained via the trabecular route, through the sieve like trabeculum into Schlemm's canal, before being drained away by the episcleral veins
• Factors affecting IOP


  • Rate of aqueous production
  • Resistance encountered in the outflow channels
  • Level of episcleral venous pressure
• Normal IOP values: 10-21mmHg, mean of 16mmHg
• Physiological factors affecting IOP


  • Fluctuations through day - heartbeat, blood pressure, respiration and time of day
  • Diurnal curves - higher in morning, lower in afternoon/evening
  • Increase of 1-2mmHg between 30 and 70 years old
• IOP by itself cannot be used to reliably discriminate between 'normal' and pathological cases
• IOP measurements are a critical component in Glaucoma management as reducing IOP is currently the only known way to slow down the progression of the disease
• Glaucoma


  Group of optic neuropathies involving progressive optic nerve atrophy associated with loss of visual function, frequently accompanied by raised IOP
• Glaucoma results in


  • Progressive defect in visual field (arcuate scotoma)
  • Extension of optic cup into vertical oval
  • IOP that fluctuates until permanently on wrong side of normal
• Theories for glaucoma pathogenesis


  • Mechanical theory: abnormal IOP compresses laminar cribosa, damaging optic nerve
  • Vascular theory: high IOP reduces blood flow to optic nerve, causing ischaemia
• Damage depends on individual susceptibility of optic nerve head
• Classification of glaucomas


  • By appearance of anterior chamber angle: open angle, closed angle
  • By aetiology: primary, secondary, congenital
• Primary open-angle glaucoma (POAG)


  Bilateral, glaucomatous optic nerve damage, visual field defects, IOP >21mmHg at some point, adult onset, open and normal appearing angles, absence of secondary causes
• About 15% of POAG have IOP consistently <21mmHg (normal tension glaucoma)
• There is a great deal of overlap and redundancy in retinal ganglion cells, so one cell dying may be covered by others</b>
• Tests to detect retinal ganglion cell loss earlier than conventional perimetry


  • Short wavelength perimetry, frequency doubling perimetry, central 10 degree visual fields
  • Structural measures of retinal nerve fibre layer (RNFL) using OCT
• Purpose of glaucoma management


  • Slow down disease progression to delay visual impairment as much as possible
  • Detect disease early when little visual function has been lost
  • Treat disease efficiently and effectively to slow progression
• There is an idiosyncratic susceptibility for glaucoma such that some patients with high IOP do not develop glaucoma, while others with low IOP do
• Sensitivity


  Likelihood that a diseased patient will have a positive test result
• Specificity


  Likelihood that a healthy subject will have a negative test result
• POAG benefits from a sensitive test as it is a serious but treatable disease
• Suspicious IOP is consistently >21mmHg on two or more occasions using Goldmann applanation tonometry
• IOP on its own is not reliable for glaucoma diagnosis, it must be used in combination with other tests and patient demographics
• Major criteria for POAG diagnosis


  • Visual fields, optic disc appearance, IOPs
• Optic disc changes associated with POAG


  • Cup-to-disc ratio >0.6, unequal C/D ratios between eyes (difference ≥0.2), changing C/D ratio, disc pallor, disc haemorrhages, nasal displacement of vessels, neuro-retinal rim changes, negative ISNT rule, reduced rim to disc ratio
• IOP findings associated with POAG


  • Repeated measurements consistently >21mmHg, discrepancies of 5mmHg or more between eyes, larger than normal diurnal fluctuation (≥10mmHg)
• Early visual field defects in POAG


  • Scotomas between 10-20 degrees of fixation, baring of blind spot, isolated paracentral nasal scotoma, nasal step, arcuate defects arching from blind spot around macula
• Cup-to-disc ratio


  • ≥0.4
• Observations that should be treated with suspicion


  • C/D ratio greater than 0.6
  • Unequal C/D ratios between the eyes (C/D ratio difference ≥0.2)
  • Changing C/D ratio
  • Disc pallor (atrophic change)
  • Disc haemorrhages
  • Nasal displacement of vessels
  • Neuro-retinal rim (NRR)
  • Negative ISNT (Inferior-superior-nasal-temporal) rule
  • Rim to disc ratio
• Observations indicating high IOP


  • Repeated measurements consistently above 21mmHg
  • Discrepancies of 5mmHg or more between the two eyes
  • Larger than normal diurnal fluctuation (about 90% of POAG show 10mmHg or more)
• Earliest clinically significant field defect


  1. Scotoma that develops between 10° and 20° of fixation, in areas that constitute downward or, more commonly, upward extensions from the blind spot

  2. Baring of the blind spot

  3. Isolated paracentral nasal scotoma

  4. Nasal step

  5. Arcuate defects which arch from the blind spot around the macula, reaching to within 5° of fixation nasally
• Pattern of visual field defects is due to the layout of the nerve fibres at the optic disc and the fact that some are affected earlier by raised IOP than others
• Family history


  10% of first order relatives of a POAG sufferer have, or will develop, the disease
• In patients with family history of POAG, the presence of the disease should be considered even when only criteria are less supportive
• Clinical techniques for measuring intraocular pressure


  • Digital palpation
  • Schiøtz tonometer – indentation tonometry
  • Applanation Tonometry
  • Goldmann Applanation Tonometry (GAT)
  • Perkins Tonometry
  • Tonopen Tonometer
  • Mackay-Marg Tonometer
  • Icare Tonometer
• Digital palpation


  Eyes closed and gaze down, superior sclera above the tarsal plate is palpated by the forefingers of both hands