Tonometry

It is important to consider that normal intraocular pressure (IOP) in children ranges from 9-12 mmHg under general anesthesia. IOP measurements of 16-18 mm Hg under general anesthesia already require cautious interpretation. Intraocular pressure measurement in children under general anesthesia should not be performed in the initial phase but during deep anesthesia, in order to avoid the massive fluctuations occurring in the initial phase of general anesthesia [26]. Several studies have demonstrated that intraocular pressure is lowered under general anesthesia, especially halothane anesthesia [4]. Chloral hydrate or ketamine sedation seems to avoid a considerable decrease of IOP, but also requires meticulous monitoring and supervision during and after a child's sedation [28,54].

Tonometry in general anesthesia has to be performed using hand-held devices, e.g., the Schiotz, Perkins, or Tonopen tonometer. Any form of tonometry in the awake child is relatively unreliable owing to unpredictable factors such as pressure of the eye lids or increased episcleral pressure in the crying child. Tonome-try in the young child is usually performed with devices designed for adult patients, thus introducing the risk of systematic measurement errors. Experimental and clinical studies have shown that pathological conditions of the sclera and cornea can lead to considerable tonometric measurement errors. In buphthalmic eyes with corneal scars and opacifications, significantly higher values are obtained with indentation tonometry than with applanation tonometry. As the diameter of the cornea and axial length of the eye are increased in children with congenital glaucoma, the central cornea is significantly thinner in these children than those with nonglaucomatous eyes. [29] This may also contribute to a considerable underestimation of the actual intraocular pressure in buphthalmic eyes.

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