2000B06 Briefly outline the physiological control of intraocular pressure.

List:

· Intro: summary and relevant anatomy

· Determinants of IOP

Intro:

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

Determinants of IOP:

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