The swollen and sunken optic disc

A problem presented at the UK MMSG Nottingham 2006.

Presented by:
Mr Alexander Foss (Queen's Medical Centre, University of Nottingham)
Participants:
L Band, M Blyth, A Foss, C Grills, C Hall, O Jensen, M Johnston, J King, J Moles, C Please, G Richardson, J Siggers

Problem Description

The eye is firm to the touch as it is pumped up, like a football, by the ciliary body that secretes a fluid (the aqueous) from behind the iris. If the pressure rises too high then this leads to selective death of the axons that exit the eye. These axons exit as the optic nerve and leave the eye at a particular spot called the optic nerve head. At this point, the firm outer layer of the eye, the sclera, is perforated to let the axons pass through, but as a consequence of these perforations, the sclera is weak and will accordingly deform (the scleral sieve through which the axons pass is called the lamina cribrosa). Not only do the axons go from an area of one pressure to another, but they also undertake a sharp right angle change in direction. In glaucoma, the lamina cribrosa deforms backwards. Haemorrhages can occur at the optic nerve head and, as a consequence of both cell death and this backward deformation, the nerve head will appear "cupped".

This problem is to assemble a model of the back of the eye that has four compartments: the intraocular space; the optic nerve; the vascular space; and the outside. The Study Group is asked to look at flow of blood and of axoplasm as they traverse these compartments and to look for potential blocks in the flow. The aim is to see if there are spots where either the blood vessels or the axons experience high internal pressure that could, in turn, lead to rupture. Rupture of capillaries would be a convincing explanation for the observed haemorrhages at the optic nerve head and would be a novel and plausible explanation for the observed optic nerve fibre death observed in these conditions.

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Study Group Report

We have constructed a model of how the axons of retinal ganglial cells respond to the difference between the intraocular pressure and the CSF pressure that is characteristic of glaucoma and papilloedema. The model uses Poiseuille's law to describe flow along the axon, taking into account water driven through the membrane surrounding the axon by a difference in hydrostatic and osmotic pressures across the membrane. In addition to our model of fluid flow in the axon, we developed a model of the mechanical deformation of the axon in response to hydrostatic pressure differences, and shear forces induced by flow.

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Follow-Up Activities

The following publications have been written as a result of this problem:

Intracellular flow in optic-nerve axons: A mechanism for cell death in glaucoma
LR Band, CL Hall, G Richardson, OE Jensen, JH Siggers, & AJE Foss (2009)
Investigative Ophthalmology and Visual Science 50, 3750–3758.