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What Is Pressure-Induced Stromal Keratitis?

Pressure-Induced Stromal Keratitis (PISK), also known as Pressure-Induced Interface Keratitis, is a rare but serious complication following LASIK surgery. It typically occurs 10 to 21 days after the procedure, though it can appear months later. PISK is fundamentally a steroid response. Post-operative patients are prescribed steroid eye drops to control healing. In roughly 5% of the population, these steroids cause a significant spike in Intraocular Pressure (IOP). In a LASIK patient, this high pressure forces fluid (aqueous humor) through the endothelium and into the corneal interface, the microscopic space between the flap and the underlying stromal bed.

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What Is Pressure-Induced Stromal Keratitis?

Pressure-Induced Stromal Keratitis (PISK), also known as Pressure-Induced Interface Keratitis, is a rare but serious complication following LASIK surgery. It typically occurs 10 to 21 days after the procedure, though it can appear months later. PISK is fundamentally a steroid response. Post-operative patients are prescribed steroid eye drops to control healing. In roughly 5% of the population, these steroids cause a significant spike in Intraocular Pressure (IOP). In a LASIK patient, this high pressure forces fluid (aqueous humor) through the endothelium and into the corneal interface, the microscopic space between the flap and the underlying stromal bed.

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The Interface Fluid Mechanism

The defining feature of PISK is the accumulation of a thin layer of fluid in the interface. This fluid separates the flap from the bed. Because the fluid is transparent, it can be difficult to see without high-magnification slit-lamp examination. The fluid creates a "cushioning" effect. When a doctor attempts to measure the eye pressure using a standard Goldmann tonometer (which presses on the center of the cornea), the fluid pocket compresses like a soft pillow. This results in a falsely low IOP reading. A patient might have a dangerously high actual pressure of 40 mmHg, but the device might read a normal 12 mmHg because of the fluid damping.

The Diagnostic Trap (PISK vs. DLK)

PISK is notorious for being misdiagnosed as Diffuse Lamellar Keratitis (DLK) or "Sands of Sahara," which is an inflammatory condition. Both conditions present with hazy vision and a cloudy interface. However, the treatments are exact opposites. DLK is treated by increasing steroid drops to kill inflammation. PISK is caused by the steroids. If a doctor mistakes PISK for DLK and prescribes more steroids, the eye pressure will skyrocket further, potentially damaging the optic nerve. The key differentiator is that PISK usually presents with a quiet, white eye, whereas DLK often presents with redness and inflammation.

Visual Symptoms: The Myopic Shift

Patients with PISK often report that their vision was perfect for a week or two, but has suddenly become blurry. Specifically, they experience a myopic shift, meaning they have become nearsighted again. This happens because the fluid accumulation physically pushes the LASIK flap upward and forward, steepening the curvature of the cornea. This change in shape induces myopia. Once the condition is treated and the fluid absorbs, the cornea flattens back out, and the vision typically returns to its previous clarity.

Treatment: Stopping the Steroids

Once correctly diagnosed, the treatment for PISK is straightforward: stop the offending agent. The doctor will immediately discontinue the steroid eye drops. Additionally, they will prescribe aggressive anti-glaucoma medications (like beta-blockers or alpha-agonists) to lower the fluid production inside the eye. As the internal pressure drops, the fluid in the interface is pumped back out by the corneal endothelial cells, the gap closes, and the haze disappears.

FAQs on PISK

How do you measure the real pressure?

To get an accurate reading, the doctor must measure the pressure on the peripheral cornea (the white edge) rather than the center, often using a Tono-Pen. Measuring over the fluid-filled center yields inaccurate data.

Is it permanent?

Rarely. If caught early, PISK resolves completely with no loss of vision. However, if misdiagnosed and treated with more steroids for weeks, the sustained high pressure can cause permanent glaucomatous damage to the optic nerve.

Does it happen with PRK?

No. PRK is a surface ablation and does not create a stromal flap or an interface. Therefore, there is no potential space for the fluid to accumulate. PISK is specific to lamellar surgeries like LASIK and SMILE.

When to See Your Eye Doctor

If your vision was clear after LASIK but starts to get foggy or blurry 2 to 3 weeks later, do not assume it is just "dry eye." It is critical to have your eye pressure checked specifically for interface fluid, especially if you are still taking steroid drops.

References

https://pubmed.ncbi.nlm.nih.gov/15505481/ https://eyewiki.aao.org/Pressure_Induced_Stromal_Keratitis_(PISK https://www.reviewofophthalmology.com/article/the-many-faces-of-pisk https://journals.lww.com/claojournal/Abstract/2007/04000/Pressure_Induced_Interface_Keratitis.