Intraocular Pressure (IOP)

Definition

Intraocular Pressure (IOP) is the pressure exerted by the contents of the eyeball (aqueous humor and vitreous humor) against the cornea and sclera. It is a crucial parameter in maintaining the shape and function of the eye. The normal range of IOP is 10–21 mmHg, with an average of 15 ± 2.5 mmHg.

Physiology of IOP Regulation

1. Production of Aqueous Humor

• Aqueous humor is produced by the non-pigmented epithelium of the ciliary body.

• The production is driven by active transport (carbonic anhydrase-dependent) and passive diffusion.

2. Circulation of Aqueous Humor

• Aqueous humor flows from the posterior chamber → pupil → anterior chamber.

• It nourishes the cornea and lens.

3. Drainage of Aqueous Humor

There are two pathways for aqueous humor outflow:

• Trabecular (conventional) outflow (90%)

• Drains through the trabecular meshwork → Schlemm’s canal → episcleral veins.

• Resistance in this pathway determines baseline IOP.

• Uveoscleral (unconventional) outflow (10%)

• Aqueous humor passes through the ciliary muscle and suprachoroidal space, bypassing Schlemm’s canal.

4. Factors Affecting IOP

• Aqueous humor dynamics: Increased production or decreased drainage raises IOP.

• Episcleral venous pressure: Increased venous congestion reduces aqueous drainage, raising IOP.

• Ciliary muscle tone: Relaxation (e.g., during cycloplegia) increases uveoscleral outflow.

• Body position: Supine position increases IOP by 2–4 mmHg.

• External pressure: Tight eyelid squeezing, Valsalva maneuver, or compression can elevate IOP.

Factors Influencing IOP in Anesthesia

1. Drugs Affecting IOP

Drug Class	Effect on IOP	Mechanism
Inhalational agents	↓ IOP	Reduce aqueous humor production, decrease systemic BP, relax extraocular muscles
IV anesthetics	↓ IOP	CNS depression reduces sympathetic tone
Succinylcholine	↑ IOP (by 6–12 mmHg)	Extraocular muscle contraction, choroidal vascular congestion
Non-depolarizing NMBs	↓ IOP	Reduce muscle tone, lowering episcleral venous pressure
Ketamine	↑ IOP (controversial)	Sympathomimetic effects may increase episcleral venous pressure
Opioids	↓ IOP	CNS depression reduces sympathetic activity

2. Effects of Physiological Changes on IOP

Condition	Effect on IOP	Mechanism
Hypercapnia	↑ IOP	Increased choroidal blood flow
Hypoxia	↑ IOP	Sympathetic activation → vasodilation
Hypothermia	↓ IOP	Reduced aqueous humor production
Hypertension	↑ IOP (mild)	Increased choroidal blood volume
Coughing/straining	↑ IOP	Increased intrathoracic and venous pressure

Measurement of IOP

1. Applanation Tonometry (Gold Standard)

• Based on Imbert-Fick principle (pressure = force/area).

• Goldmann Applanation Tonometry (GAT): Most widely used in clinics.

2. Other Methods

• Schiøtz tonometry (indentation principle).

• Non-contact tonometry (air-puff tonometry): Uses a jet of air to flatten the cornea.

• Tono-Pen: Portable, used in trauma or anesthesia settings.

Clinical Significance of IOP in Anesthesia

1. IOP and General Anesthesia

• Most anesthetic agents lower IOP, except for ketamine and succinylcholine.

• Succinylcholine-induced rise in IOP can be problematic in open globe injuries.

• Deep anesthesia and controlled ventilation reduce IOP.

2. IOP and Regional Anesthesia

• Peribulbar and Retrobulbar Blocks: Reduce IOP by relaxing extraocular muscles.

• Epidural and Spinal Anesthesia: Lower IOP due to systemic hypotension.

3. IOP in Ocular Surgery

• Cataract Surgery: Requires stable IOP to avoid intraoperative complications.

• Glaucoma Surgery: Aims to lower IOP through drainage procedures (trabeculectomy).

• Vitreoretinal Surgery: Gas tamponade (e.g., SF6, C3F8) can raise IOP significantly.

MCQs on IOP

1. What is the normal range of intraocular pressure?

a) 5–10 mmHg

b) 10–21 mmHg

c) 15–30 mmHg

d) 20–35 mmHg

• Answer: (b) 10–21 mmHg

2. Which anesthetic agent causes the greatest increase in IOP?

a) Propofol

b) Sevoflurane

c) Succinylcholine

d) Fentanyl

• Answer: (c) Succinylcholine

3. How does hypercapnia affect IOP?

a) Increases IOP

b) Decreases IOP

c) No effect

d) First increases, then decreases

• Answer: (a) Increases IOP

4. What is the gold standard method for measuring IOP?

a) Schiøtz tonometry

b) Goldmann applanation tonometry

c) Air-puff tonometry

d) Digital palpation

• Answer: (b) Goldmann applanation tonometry

5. Which factor reduces IOP during anesthesia?

a) Ketamine administration

b) Hypercapnia

c) Deep inhalational anesthesia

d) Succinylcholine administration

• Answer: (c) Deep inhalational anesthesia