2000B06 Briefly outline the physiological control of intraocular pressure.

Summary

·   Definition: force per unit area within the globe

·   Normal value: 15mmHg

·   Physiological role: maintaining corneal refraction

·   Similar physiology to intracranial pressure

·   Monroe-Kellie doctrine: relatively fixed volume, poor compliance; increase in one substance comes at the expense of another or else pressure rises rapidly

·   Excessive IOP -> compression on optic nerve, blindness

·    

Anatomy

·   Anterior chamber: aqueous humour

·   Canal of Schlemm = lymphatic-like vessel around outside anterior chamber

·   Posterior chamber: vitreous humour

·   In between chambers: iris, lens, ciliary body and muscle

·   Blood supply: retinal artery and choroidal arteries

·    

Amount of vitreous humour

·   Not under control

·   Minimally changing

Amount of aqueous humour

·   Production: by ciliary body, constant at mAP > ~70mmHg

o  Decreased by acetazolamide, mannitol

·   Reabsorption: via trabecular meshwork into canal of Schlemm
(most important factor)

o  Mydriasis -> ↑resistance to reabsorption

o  Glaucoma: -> ↑resistance to reabsorption

o  Mannitol: ↑reabsorption

Amount of blood

·   Supply: via retinal and choroidal arteries

·   Drainage: via retinal and ? other veins

·   ↑mAP -> ↑vol

·   ↑CVP -> obstruction to drainage -> ↑volume (e.g. coughing, Valsalva, PPV)

·   ↑PaCO₂ -> vasodilatation -> ↑volume

·   ↓PaO₂ -> vasodilatation -> ↑volume

Volume of globe

·   Contraction of extra-ocular muscles -> ↓volume -> ↑pressure
(e.g. suxamethonium)

·   External compression in prone surgery -> ischaemia