Intra-ocular pressure

• For those fond of equations, IOP = (aqueous production/aqueous outflow) + episcleral venous pressure
• Unless the patient has a hole in their globe (surgical or otherwise), this means IOP is never lower than episcleral venous pressure (~8-10mmHg)

• The volume of the vitreous is relatively fixed and therefore changes to aqueous humour volume are more impactful on IOP
• Vitreous volume can be affected by mannitol administration

• Aqueous humour occupies the anterior chamber of the eye, contributing to IOP
• It is produced by the ciliary bodies in the posterior chamber through a combination of:
• Active secretion (80%) via a Na⁺/K⁺ ATPase, the rate of which is independent of IOP
• Ultrafiltration (20%) of plasma, the rate of which is influenced by IOP, venous and oncotic pressure


• It flows between the iris and anterior lens, through the pupil and into the anterior chamber


• Aqueous humour outflow occurs via two main routes:
  1. The 'conventional' route (80%)
  • Aqueous is reabsorbed into episcleral veins (at the angle between the cornea and iris i.e. iridocorneal angle) down a pressure gradient via:
  • The trabecular mesh network
  • The canal of Schlemm
  
  
  2. The uveoscleral route (20%)
  • There is non-specific outflow through e.g. the suprachoroidal space and choroid itself
  
  
• Increased IOP will increase aqueous drainage (in the otherwise normal eye; this isn't the case e.g. in glaucoma)
• Increased drainage of aqueous humour is therefore the main compensatory mechanism for alterations to other components of the globe
• This compensation occurs over approximately 20mins

• The retinal artery displays autoregulation down to a SBP of 90mmHg
• Choroidal blood vessels themselves do not display myogenic autoregulation
• Otherwise, the factors influencing choroidal blood flow are analogous to that of cerebral blood flow

• The episcleral venous plexus pressure is normally 8-10mmg, only 1-2mmHg below IOP
• A raised CVP will therefore reduce aqueous humour drainage via this route and thus increase IOP

• Such pressure may be:
• Pathological e.g. due to retrobulbar haemorrhage (which itself may be secondary to sharp needle regional anaesthesia)
• Iatrogenic e.g. due to prone positioning or the presence of LA following regional techniques

• Increased aqueous humour volume e.g. glaucoma
• There may be a normal iridocorneal angle but a trabecular meshwork which is clogged with extracellular matrix and other detritus (open angle glaucoma)
• Conversely the iridocorneal angle itself may become narrowed, leading to a physical closure of the trabecular meshwork


• Increased choroidal blood volume e.g. hypoxia, hypercarbia, hypertension, head down (Trendelenburg), laryngoscopy
• Increased CVP causing reduced choroidal venous drainage e.g. coughing, straining, vomiting


• Use of gases to tamponade a detached retina:
• Sulphur hexafluoride (SF₆), especially if it interacts with nitrous oxide thus increasing the gas volume; it typically remains in the eye 3 - 4 weeks post-administration
• Perfluoropropane (C₃F₈); it typically remains in the eye 6 - 8 weeks post-administration

• Prone positioning
• Presence of other fluid in the orbit e.g. blood (retrobulbar haemorrhage), local anaesthetic
• Surgeons using the patient's face as a hand-rest intraoperatively

• Propofol, soidum thiopentone and etomidate all reduce IOP
• Ketamine causes a transient rise in IOP over 1min, before it returns to baseline


• Opioids themselves do directly affect, but can be used to attenuate the pressor response to laryngoscopy
• Unattenuated, laryngoscopy can cause a 10 - 20mmHg increase in IOP
• Avoiding intubation e.g. use of supraglottic devices or other pharmacotherapy such as IV lidocaine can also help


• Neuromuscular blocking drugs
• Suxamethonium increases IOP for approximately 8 - 10mmHg for 5 - 10mins
• Non-depolarising agents conversely reduce IOP by reducing extra-ocular muscle tone


• Volatile agents all decrease IOP
• Nitrous oxide has no effect in and of itself, but can raise IOP if SF₆ used


• Benzodiazepines cause a slight reduction in IOP