Intraocular Pressure Lowering Can Change Optic Nerve Head Structure

When intraocular pressure (IOP) is lowered, the lamina cribrosa (LC) moves, contracts in radius, and expands in thickness, according to findings published in Ophthalmology Glaucoma. The change is related to the degree of IOP change, researchers report.

Patients (N=26; n=27 eyes) with primary open angle glaucoma (POAG) who underwent trabeculectomy were recruited for this study at the Wilmer Eye Institute in the United States. Prior to, and 20 minutes after, laser suturelysis in the weeks after the procedure, patents were evaluated by spectral domain optical coherence tomography (SD-OCT). They evaluated the participants for changes to the biomechanical strain of the LC after the IOP lowering procedure.

The patients (mean age 67.2±8.5 years, 15 men, 14 women) had a mean baseline IOP of 25.0±8.7 mm Hg, retinal nerve fiber layer (RNFL) thickness of 63.0±12.2 µm, and axial length of 25.0±8.7 µm. After surgical intervention, IOP lowered by 11.9±8.9 mm Hg.

Overall, the LC depth did not change with IOP lowering (mean, 1.33 µm; P =.26); however, for some patients, the LC moved more inside the eye. In others, it moved more outside the eye (range, +14.14 to -18.20 µm). The position of the LC was not associated with the magnitude of IOP lowering (R², 0.027; P =.39).

LC compliance, defined as dividing displacement by IOP change, was greater in the nasal (P =.026) and inferior (P =.043) quadrants with thinner RNFL (R², 0.14).

The decrease in IOP resulted in tensile anterior-posterior strain (mean E_zz, 0.94%; P =.0002) and comprehensive radial strain (mean E_rr, -0.19%; P =.004), indicating LC expansion in thickness and contraction in radius.

Larger decreases in IOP produced more tensile E_zz (P <.0001) and greater E_max (P <.0001) and Γ_max (P <.0001) strains.

Longer axial length associated with greater hoop strain E_ΘΘ (P =.008) and in-plane shear strain E_rz (P =.02). Axial length did not change significantly in 16 eyes (P =.11) but change in axial length did associate with the change in IOP (R², 0.50; P =.0033).

The LC depth change associated with shear strains, in which displacement into the eye produced greater maximum Γ_max (P =.014) and positive E_rΘ (P <.0001) shear strain.

Mapping changes using the face of the clock, more anterior LC displacement produced greater E_max at 3-6 hours and E_rr at 4-5 hours (P <.004) and greater Γ_max at 3, 5, and 6 hours and E_rΘ at 2, 5, 7, and 8 hours (P <.0004). More posterior LC displacement produced greater E_max at the 10 hour and E_rr and E_zz at 10-11 hours (P <.004).

A potential limitation of this study was the in-plane strains likely had lower error and uncertainty than other strains due to imaging resolution.

“The LC can move either into or out of the eye when IOP is reduced, and the change is related to the degree of IOP change. Strains within the LC are uniformly tensile at lower IOP and independent of the direction of LC border movement. The LC strains are greater in eyes with worse clinical measures of glaucoma damage. Shear strains are nearly as substantial as in-plane strains and were associated with movement of the LC anterior border,” researchers explain.

This study found that the LC moved and changed in thickness and radius after IOP lowering among patients with POAG. LC strain tended to be greater in eyes with more glaucoma damage.

Reference

Czerpak CA, Kashaf MS, Zimmerman BK, Quigley HA, Nguyen TD. The strain response to intraocular pressure decrease in the lamina cribrosa of glaucoma patients. Ophthalmol Glaucoma. 2022;S2589-4196(22)00120-X. doi:10.1016/j.ogla.2022.07.005