Saving Sight in Glaucoma: Why the Brain May Hold the Key
Glaucoma is not well understood. It is a progressive disease of the optic nerve, located at the back of the eye. It is recognised that elevated eye pressure, or intraocular pressure (IOP), is a risk factor for the development of glaucoma. However, only 40-50% of glaucoma patients have elevated IOP. Furthermore, not all individuals with high IOP develop glaucoma. Finally, even when the IOP is reduced patients may continue to deteriorate and lose vision.
These factors suggest that IOP and glaucoma do not have a straight-forward relationship and that there are other variables that are at play. The optic nerve has an important relationship with the brain. The optic nerve is located at the back of the eye but also at the front of the brain.
The optic nerve connects the eyeball to the brain. It is bathed by the cerebrospinal fluid that surrounds the brain in a space called the subarachnoid space.
How is the optic nerve connected to the brain?
The optic nerve is comprised of approximately one million nerve fibres which take electrical impulses from the retina to the brain for processing. These fibres are surrounded by a sheath. The pressure that this fluid produces is called the intracranial pressure (ICP). There is a thin connective tissue barrier, known as the lamina cribrosa, that separates the eye (and the IOP) from the CSF.
The lamina cribrosa is a mesh-like structure in the back of the sclera (white of the eye) that allows optic nerve fibres to leave the eye to enter the orbit, and is a barrier between the interior of the eye anteriorly and the optic nerve fibres within the orbit surrounded by the CSF fluid, all within the optic sheath.
The optic nerve fibres are in orange and are contained within the optic nerve sheath. The lamina cribrosa separates these fibres from the inside of the ye (in green).
What is the relationship between IOP and ICP (eye versus brain pressure)?
Recently, the pressure dynamics across the lamina cribrosa has been raised as a possible factor in glaucoma. The IOP is opposed by the intracranial pressure (ICP). It is recognised that ICP influences the shape of the optic nerve head. There is a pressure gradient between the ICP and IOP known as the translaminar pressure gradient. The main factors that contribute to the gradient are the ICP, lamina cribrosa thickness, IOP and the pressure inside the orbit tissue.
The lamina cribrosa is considered to be the primary site of damage in glaucoma, associated with the distortion of the nerve fibres.
What is our novel approach?
Currently, ICP is only measured via lumbar puncture – an invasive procedure similar to an epidural. Efforts to measure ICP noninvasively or identify a non-invasive surrogate biomarker have not proven to be as reliable and accurate as lumbar puncture. Our approach involves using amplified MRI (aMRI), an innovative new MRI modality that has been pioneered by Dr Samantha Holdsworth, founder of the Mātai Research Institute in Gisborne. aMRI will allow non-invasive assessment of the lamina cribrosa. It can identify small movements of the fluid in the subarachnoid space.
This project will be led by Associate Professor Samantha Holdsworth, Medical Physicist and founder of Mātai and Professor Helen Danesh-Meyer, Scientific Director at Vision Research Foundation.
“Our study will be conducted at Mātai Medical Research Institute in Gisborne, NZ. This facility has world class equipment and staff conducting leading edge MRI research. With exclusive focus on research and innovation, the Mātai facility is specialised for developing new signals and techniques in neuro-imaging.” Holdsworth said.
Mātai uses a fully upgraded GE SIGNA™ Premiere 3T MRI in their research. The device is supported by internationally renowned specialist radiographers, radiologists, signal processors and engineers, and is directed by specialist MRI physicist Dr Samantha Holdsworth.
“Our team will be the very first in the world to investigate glaucoma with aMRI.”
-Associate Professor Samantha Holdsworth
Associate Professor Samantha Holdsworth
Samantha is a medical physicist, Associate Professor and principal investigator at the Centre for Brain Research, PhD Radiology. Dr Samantha Holdsworth is a medical physicist with 19 years of experience in MRI acquisition, post-processing, and analysis. After 11 years at Stanford as a senior scientist, she returned home to Gisborne, with the goal of making a difference to lives in her community. Samantha has successfully translated a variety of her MRI methodologies to clinical practice through her streamlined image reconstruction methods, leading to better detection of brain disorders and disease. She is a pioneer of fast, high resolution MRI methods and amplified MRI (a new method of visualising brain motion). Her key research interest is in the application of novel imaging technologies for the early detection of concussion/mild traumatic brain injury (mTBI) and obstructive disorders of the brain.
