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Dr. Lalit Verma
Chairman, Scientific Committee - AIOS
 
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Author Topic: Query No: 108 - Formula for IOP calculation after pachymetry  (Read 2563 times)
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« on: September 11, 2009, 08:10:09 AM »

In  glaucoma  pts., after  pachymetry, which  formula  should  we  consider for  IOP  calculation,: @ Ehler, @ Dresdner, or @Doudhty ?

DR.  DHAVAL  B.  RAIJIWALA
DR.  NITA  D.  RAIJIWALA
SURAT

Responses

No:1

Dear Dr Parekh.
 
There is no conclusive evidence to say that one can use any formula for adjusting the Central corneal thickness,
when calculating the IOP. No Nomograms have been identified to this day to be full proof.
It is also not only the CCT that matters. It however is useful in patients with OHT, wher the thickness of the
cornea helps to identify patients at risk to progressing, & also to identify patients who have thin corneas
& therefore normal IOP's despite having typical disc changes, with no diurnal variations who can then be labelled as NTG.
 
Dr. Rita Dhamankar
Laxmi Eye Institute,
Panvel

No:2

Dear dr.dhaval & dr.nita

In glaucoma pts., after pachymetry, I use ehler formula for IOP calculation .

dr.shakun gupta
SNC, chitrakoot

No:3

Dear Dr Raijiwala,
 
I wd make a few humble submissions here.
 
In my opinion,no Glaucoma workup is complete without a CCT measurement.
 
The Ocular HypertensionTreatment Study(OHTS)clearly points out:
 
1)A thin cornea to be a single independent risk factor for POAG.
2)A decrease in CCT by 40 microns increases the risk by 70%.
3)50-60% of pts diagnosed as ocular hypertensives were normal..
4)30-40% patients considered normal were ocular hypertensives.
Conclusion was that 20.2% patients had outcome significant mean IOP adjustments(an adjustment of >/=3mmhg). 
 
Well,a thin cornea can only be dignosed by Pachymetry,not by other means Ehlers nomogram works well for most cases.

Regards
Dr Sunil Gupta
RamAvtar Eye Hospital & Glaucoma Pavilion
C-17,Bhagat Singh Marg,Tilak Nagar,
Jaipur-302 004 (INDIA)
"Glaucoma Never Sleeps"
www.glaucomapavilion.com


Expert Comment:

The issue of measuring central corneal thickness came to the fore with the Ocular Hypertensive Treatment Study, where in a multivariate analysis  of characteristics which might be predictive of development of glaucoma Central corneal thickness showed up as being very significant.  The Ehlers  correction is roughly equivalent to 7mmHg/100um  (about 15 um to 1 mm.Hg.) and Dresdner formula  which suggests 1mm.Hg being  equivalent to 25 microns change in central corneal thickness.  Different models have predicted that a 10% difference in CCT would result in a 1.1 to 3.4 mm.Hg. difference in IOP measurements by applanation tonometry. The Goldmann applanation tonometer was initially thought by its creaters to  provide valid measurements for a CCT of 500 microns till it was demonstrated that it was accurate for 520 microns.  Clinically therefore one may use 15 to 20 microns corneal thickness as being equivalent to 1 mmHg. as a rough average to arrive at the  intraocular pressure reading.  The CCT varies  in different geographical areas. Thus White Americans  were found to have a CCT of 545 microns and the African Americans about 529 microns.  India was probably lucky since  the approximate mean CCT in South India and Central India is  520 microns which is just right to get an accurate IOP measurement with the Goldmann Applanation Tonometer!   The issue with the importance of CCT has been whether simply having a lower CCT predisposes one to the development of glaucoma  by virtue of some tissue characteristics which may affect for example the optic disc and make it more susceptible to glaucoma or is it simply the fact that a false lower IOP measurement may be instrumental in missing  early glaucoma and therefore result in the late detection of glaucoma. Studies have shown that subjects with glaucoma and lower CCT were found to have a greater degree  of glaucomatous optic nerve damage, however no definite correlation of lower CCT with glaucoma progression has been shown.  The real issue clinically is in being able to figure out the impact that measurement of   CCT will have in the clinical management of   subjects with ocular hypertension, normal pressure glaucoma, and in glaucoma patients who progress inspite of control of IOP.  We may also keep in mind,  whether to use the influence of CCT on IOP in  every patient who has a CCT different from the mean or whether we may consider   it   only when the CCT measurement is outside the mean plus one Standard deviation or mean plus two standard deviation, depending on what we may consider and define to be normal.  No one really knows.  In essence CCT may be an additional clinical parameter  to be used in sync with other parameters of glaucoma evaluation, management and follow up.


Dr. Vinay Nangia
Suraj Eye Institute
559, New Colony
Nagpur. 440 001
email: nagpursuraj@gmail.com
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