Ultramicroscopic study of the Optic Nerve Sheath in Patients with Severe Vision Loss from Idiopathic Intracranial Hypertension - Results

Abstract

Purpose: The objective of our study is to describe microscopic changes in the optic nerve sheath (ONS) of patients with severe and/or progressive vision loss from IIH.

Methods: ONS specimens were obtained at ONS fenestration in IIH (cases) and enucleation for painful blind eye (controls). Both procedures were performed by a single surgeon. After fixation and staining, specimens were examined by masked experts using light (LM), transmission electron (TEM) and polarization (PM) microscopy for specimen quality and tissue anatomy (cellularity and collagen structure).

Results: Of 12 specimens, 7 (6 cases, 1 control) met diagnostic and quality criteria. For cases: Mean age was 31.25y; mean disease duration 57 days; Mean CSF OP 40.25cm water. Papilledema grades were II (3 eyes); III (1 eye); IV (2 eyes). 5/6 patients showed severe visual field loss (MD> 15 dB); 3/6 had acuity worse than 20/40 and 2/6 had RAPD. All had normal CSF contents. Control ONS was obtained from an enucleated eye of a 18y with childhood ocular trauma.Collagen abnormalities on TEM: irregular arrangement (all cases), collagen disruption and increased extracellular fluid (5/6 cases) while control ONS showed regular, compact, normal collagen. PM showed marked decrease in green yellow birefringence (<20% in all cases with <5% in 4 cases) compared to control ONS (75% birefringence) which confirmed abnormal collagen content and arrangement in cases. All 6 cases showed increased cellularity on LM and/or TEM with active fibroblasts (3/6 cases) and chronic inflammatory cells (lymphocytes) in all 6 cases.

Conclusions: Disruption and disorganization of dural collagen in the ONS from IIH points to significant shear forces on distal ONS from raised ICP while increased cellularity indicates tissue repair. These mechanical and inflammatory components could contribute to visual loss in IIH. Early aggressive medical and/or surgical treatment may be beneficial in IIH.