In-Vivo Laminar and Scleral Canal Displacements During Acute Intracranial and Intraocular Pressure Elevation in Monkeys

Abstract

 

Purpose

 

Recent epidemiological evidence suggests that intracranial pressure (ICP) may play an important role in glaucoma. However, the effects of controlled modulation of ICP and IOP on the optic nerve head (ONH) have not been measured in-vivo and are not well understood. Our goal was to quantify the deformations of the anterior lamina cribrosa (ALC) and scleral canal due to acute elevation of ICP and/or IOP.

 

Methods

 

Four eyes from three monkeys were imaged in-vivo with SD-OCT (Bioptigen, Durham, NC) under four controlled conditions of ICP and IOP. ICP and IOP were set to either baseline (ICP=10mmHg, IOP=15mmHg) or elevated levels (ICP=27mmHg, IOP=30mmHg) through cannulas inserted into the lateral ventricle or anterior chamber, respectively. Scleral canal, defined by Bruch’s membrane opening (BMO), and ALC were marked in equally spaced B-scans (every 31 µm and 62 µm, respectively). Maximal horizontal canal diameter was measured in cross sectional view of BMO plane. Mean ALC depth, relative to BMO best-fit plane, was measured in 3D in regions where ALC was visible in all four pressure conditions for an eye.

 

Results

 

Elevated IOP decreased canal diameter in most cases, except when both pressures were elevated in eyes 1 and 3R (Figure 1 a, c). Elevated IOP decreased mean ALC depth in 5 out of 6 cases (Figure 2). Elevated ICP increased canal diameter in 3 of 4 eyes at baseline IOP (Figure 1 a, b, d) and in 2 eyes at elevated IOP (Figure 1 a, b). Elevated ICP decreased mean ALC depth in 3 of 4 eyes at baseline IOP (Figure 2 a, c, d) and in 3 eyes at elevated IOP (Figure 2 a-c).

 

Conclusions

 

Overall there were strong effects of ICP and IOP on both canal diameter and ALC depth, with clear interactions between the pressures. In a majority of cases, acute elevation of either ICP or IOP resulted in shallower ALC (closer to the BMO plane), whereas elevated ICP caused expansion of the canal, and elevated IOP caused contraction of the canal. Our in-vivo results are evidence that acute ICP elevation can cause deformations of the ONH and alter the effects of IOP.  

 

Figure 1: Maximal horizontal canal diameter with acute variations in ICP and IOP

 

 

Figure 2: Mean ALC depth with acute variations in ICP and IOP