Instruments

1. Schiotz Tonometer :

• Indentation tonometer
• Principle- The cornea is indented in the shape of a truncated cone by standard weights (5.5gms,  7.5gms, 10gms, 15gms) which are applied via a plunger which moves freely within a shaft in the footplate.
• Technique: The patient is laid supine with the cornea anesthetized. The fingers of the examiner spread the eyelid without putting pressure on the globe while patient is asked to fixate at a distant object. The needle on the  tonometer scale oscillates with the ocular pulse and the mid-point of the excursion is used as the scale reading (Friedenwald table). If the value of the scale reading is not greater than four units, additional weights are added.

   

Uses : Intraocular pressure (IOP) is measured

Source of errors :

• Errors inherent to the instrument- Due to difference in weights of different parts of tonometer; difference in size, shape and curvature of footplate; friction arising in movement of plunger and pointer of the scale.   
• Errors due to contraction of the extra-ocular muscles-- Errors due to accommodation.
• Errors due to variation of corneal curvature: steep and thick corneas- record false high IOP.- Errors due to high scleral/ ocular rigidity. 
• Observer’s errors. 
• Mose’s effect: False high IOP recorded if the cornea gets sucked into the space between plunger and hole in footplate. 

Limitations :

• In cases of high ocular rigidity, the conversion tables overestimates IOP and in cases of low ocular rigidity, the conversion tables underestimates IOP. 
• Corneal variables: False high IOP is obtained with thick corneas or very steep corneas. Measurements are unreliable in significant corneal pathologies.