Intraocular*pressure*(IOP)*measuring*devices*–from*the...

Intraocular  pressure  (IOP)  measuring  devices  –  from  the  past  to  the  future  

Frances  Meier-­‐Gibbons,  MD1;  Sylvain  Roy,  MD,  PhD2,  3  

1Glaucoma  Prac>ce,  Rapperswil,  Switzerland,  2Montchoisi  Clinic,  Lausanne,  Switzerland,  3Swiss  Federal  Ins>tute  of  Technology,  Lausanne,  Switzerland    

Purpose  The  purpose  of   this  presenta>on   is   to  give  an  overview  of   the   IOP  measuring  devices  from  the  medieval  >mes  to  the  future.  Main  focus  on  the  comparison  of   Goldmann   Applana>on   Tonometry   (GAT)   with   Pascal   Tonometry   with   the  presenta>on  of  a  new  clinical  study.  

Methods  A   clinical   prospec>ve   monocentric   study:   Office   based   study   of   consecu>ve    144  pa>ents   (271   eyes)   suffering   from  glaucoma  or   ocular   hypertension.   The  IOP  measurements  were  performed  using  GAT  and  Pascal  tonometers.  Central  corneal  thickness  (CCT)  was  also  measured.  

Introduc;on  The   importance  of  the   intraocular  pressure  (IOP)   is  known  since  the  medieval  >mes.  In  the  19.  century  Albert  von  Graefe  (1828-­‐70)  invented  the  first  device  that   measured   approxima>vely   the   IOP.   It   was   an   indenta>on   tonometer  measuring   the   eye   pressure   through   the   closed   eye   lid.   Schiotz   (1905)   and  Maklakoff  (1885)  further  refined  the  concept.    In   1957,   Goldmann   and   Schmidt   published   the   revolu>onary   inven>on   of   an  applana>on  tonometer  (Goldmann  applana>on  tonometer  „GAT“)  that  became  –  and  s>ll  is-­‐  the  gold  standard  for  measuring  the  IOP  worldwide.  Because  of  the  progress   in  corneal  surgery  since  the  end  of  the  1970  and  the  growing  knowledge   that   the   corneal   thickness  and   the   corneal   structure  vary  individually,   the  accuracy  of   the  GAT  was  more  and  more  ques>oned.   It   took  another  decade  un>l  other  devices  for  measuring  the  IOP  independently  of  the  the  cornea  were  developed.  One  of  the  most  accurate  measuring  devices  is  the  dynamic  contour  tonometer,  (DCT)  which  will  be  presented  in  an  overview  of  the  new  IOP  measuring  devices  worldwide.  

Results  From  the  144  pa>ents  (271  eyes)  included  in  this  office  based  clinical  study  the  mean   IOP  GAT  was  16.2  mm  Hg,  whereas   the  mean   IOP  Pascal  was  19.5  mm  Hg.   The   pa>ents   treated   with   Prostaglandin   analogs   alone   (93   eyes)   or  Prostaglandinanalogs   in  combina>on  with  other  medica>ons  (178  eyes)  had  a  mean   difference   GAT-­‐Pascal   of   3.2   mmHg   and   3.3   mm   Hg,   respec>vely.   The  mean  CCT  of  the  271  eyes  was  544  micrometers.  

Conclusions  The   GAT   is   s>ll   the   gold   standard   but   in   the   future   IOP   measuring   devices,  which   are   measuring   the   IOP   independent   of   the   CCT   (for   example   Pascal  Tonometry),  will  become  more  important  in  the  glaucoma  prac>ce.  Caveat:  The  so  called  „Triggerfish“  contact  lens  measures  a  change  of  an  eye  and  probably  not  the  IOP!  Too  many  ques>ons  persist  and  need  to  be  answered  before  this  device  can  be  used.  

Indenta;on  tonometry  by  Von  Graefe  (1828  –  1870)  

Picture  Ch  Kniestedt,  MD  

Indenta;on  tonometry  by  Schiotz  (1905)  

First  tonometry  by  Alexei  Maklakoff  (1885)  

W  =  Counterpressure  

A  =  Flafened  surface  

P  =  Pressure  in  the  ball  

S  =  Tension  of  the  surface  of  the  tear  liquid  

B  =  S>ffness  of  the  cornea  

An   external   force  indents   a   ball   filled  with   a   liquid.   The  indenta>on   is   deeper  when   the   pressure   in  the  ball  is  lower  

Goldmann  H,  Schmidt  T:  Ueber  Applana>onstonometrie.    Ophthalmologica  134,  221-­‐242,  1957  

Goldmann  and  Schmidt  (1957)  

Applana>ontonometry:  

W  +  S  =  P  x  A  +  B,  but  

W  =  P  x  A  

With  diameter  A  =  3.06  mm  

 applana>on  area  7.35  mm2  

 corneal  thickness  =  500  µm  

A  plunger   indents   the  cornea   and   the   IOP   is  d e t e r m i n d e d   b y  measuring   how   much  the  cornea  is  indented  by  a  given  weight  

Ocular  Response  Analyzer  

The  cornea  biomechanic  proper>es,  the  corneal  hysteresis  is  es>mated  by  measuring  the  force  to  indent  the  cornea  in  a  double  fashion  with  a  pulse  of  100  µs  using  a  burst  of  air.  The  difference  between  the  onward  and  backward  applana>on  reflects  the  corneal  hysteresis.    

Y  Robert,  H  Kanngiesser,  Ch  Kniestedt  

The   applana>on   is  performed   using   a  steady   force.   The  measured   area   i s  converted   into   the  IOP,   a   small   area  corresponds   to   a  high  IOP  

Dynamic  Contour  Tonometry  (DCT)  

In   the   center   of  the   >p   a   piezo-­‐electric   pressure  sensor   measures  IOP  100  x/sec.  

The   DCT   is   adapted   to   the   slitlamp   and  shows  a  dynamic  con>nous  IOP  recording  

The  ocular  pulse  amplitude  is  recorded  

The   measurement   of  t h e   I O P   i s  independent   of   the  cornea   thickness   and  b i o m e c h a n i c s  proper>es  

Glaucoma  diagnosis  

Primary  Open  Angle  Glaucoma  

Pseudoexfolia>on  Glaucoma  

Primary  Angle  Closure  Glaucoma  

Other  types  of  Glaucoma  

An>glaucoma  medica>on   Number  of  eyes  

Prostaglandin  analogues   93  

Beta  blockers   12  

Prostaglandin  analogues/Beta  blockers   45  

Carbonate  anhydrase  inhibitor   4  

Carbonate  anhydrase  inhibitor/Beta  blockers  

19  

Alpha-­‐agonists   2  

Alpha-­‐agonists/Beta  blockers   2  

Other  Prostaglandin  analogues/  combina>on  

39  

None   55