Intraocular pressure (IOP) refers to the fluid pressure maintained within the eye, which is measured in millimeters of mercury (mmHg). This pressure is created by a continuous balance between the production and drainage of the aqueous humor, the clear fluid that nourishes the front structures of the eye. A normal, healthy IOP typically falls within the range of 10 to 21 mmHg. Cataract surgery involves removing the eye’s cloudy natural lens and replacing it with an artificial intraocular lens (IOL). Because the procedure temporarily alters the eye’s fluid dynamics, monitoring IOP is a standard component of post-operative care.
Expected Pressure Changes After Surgery
The eye’s pressure often experiences a temporary fluctuation in the hours immediately following cataract surgery. A transient rise in pressure, known as post-operative ocular hypertension (POHT), is generally considered an expected physiological event. This pressure spike typically peaks within 3 to 7 hours after the procedure and can reach levels as high as 30 or 40 mmHg in some cases.
For most patients, this initial elevation is short-lived and resolves, with pressure returning to near-normal levels within 24 to 48 hours. However, patients with pre-existing conditions, such as glaucoma, are at greater risk of experiencing a more pronounced or sustained spike. The long-term effect of successful cataract surgery is often a slight decrease in baseline IOP, which can be beneficial for individuals with elevated pressure before the procedure.
Conversely, a temporary drop in pressure, called hypotony, can also occur, though it is less common. Hypotony is defined as an IOP of 5 mmHg or lower. This low pressure can be caused by a minor wound leak from the tiny incision used during the surgery. The immediate post-operative check-up, usually scheduled the day after the procedure, monitors these early changes and ensures the pressure is stabilizing.
Underlying Causes of Pressure Fluctuation
The primary cause of the common, immediate pressure spike is the retention of viscoelastic material used during the procedure. Viscoelastic substances are gel-like compounds injected into the eye to protect internal structures and maintain space during the exchange of the lens. If residual viscoelastic material is not completely removed before the surgery is finished, it can physically block the trabecular meshwork, which is the eye’s primary drainage angle. This temporary obstruction of the outflow pathway causes the aqueous humor to build up, leading to the rapid and transient rise in IOP.
Post-operative inflammation is another significant factor contributing to pressure changes in the days and weeks following surgery. The body’s natural healing response involves a release of inflammatory cells and proteins into the aqueous humor. This cellular debris can clog the trabecular meshwork, slowing the drainage of fluid and causing a more sustained elevation in pressure than the initial viscoelastic spike.
A third, later-onset cause is a specific reaction to the prescribed post-operative steroid drops. In a small percentage of susceptible individuals, topical steroid medications, which are used to control inflammation, can trigger a rise in IOP. This “steroid response” typically develops closer to 10 days or two weeks after the drops are started. This phenomenon is due to changes in the trabecular meshwork that decrease fluid outflow in response to the medication.
Recognizing and Managing High or Low Pressure
Patients should be aware of symptoms that indicate the IOP may be dangerously high or low, necessitating immediate medical attention. Significantly elevated pressure can manifest as severe throbbing eye pain, a deep headache, or nausea and vomiting. Visual symptoms can include a noticeable and persistent blurring of vision or seeing rainbow-colored halos around lights.
Management for persistently elevated IOP often begins with the introduction of pressure-lowering medications. These topical hypotensive eye drops, such as beta-blockers or carbonic anhydrase inhibitors, work to either decrease aqueous humor production or increase its outflow. For severe pressure spikes, particularly those exceeding 30 to 40 mmHg, an oral medication like acetazolamide may be temporarily prescribed to rapidly reduce the fluid production inside the eye.
If the pressure elevation is determined to be a steroid response, treatment involves tapering or discontinuing the offending steroid drop and substituting it with a non-steroidal anti-inflammatory medication to control swelling. Conversely, persistent hypotony (very low pressure) usually requires an evaluation to rule out a wound leak. If a leak is confirmed and is not resolving, the ophthalmologist may need to surgically reinforce the incision to allow the eye to maintain its normal pressure.