Detection of higher visual function deficits and validation of multifocal pupillography in stroke, chiasmal compression and anterior ischemic optic neuropathy.

Abstract

It is well established that neural damage can result in visual dysfunction, both visual field loss and in higher visual function (HVF) loss such as perceptions of depth, colour, motion and faces. This thesis examines these visual deficits in the common neurological diseases of stroke, chiasmal compression, and anterior ischemic optic neuropathy (AION).

While it is established that isolated complete HVF deficits do occur in stroke, they are also known to be rare. However, as HVFs are not routinely tested in clinical practice, it is unknown how common more subtle defects are, and what tools are effective in detecting these. Chapter 3 explores these questions, outlining that colour and depth perceptions are the most commonly affected, that Ishihara (colour) and stereofly and randot (depth), are the most useful tests, and outlines recommendations for improvement in some of these tools.

The relatively new invention of multifocal pupillographic objective perimetry (mfPOP) provides a number of benefits from other forms of perimetry. It measures both eyes at once, allowing measures of direct and consensual responses, it is objective, and it allows repeat measures of each region giving a measure of error. This advancement opens up new opportunities to investigate pupillary physiology in neurological disorders and adds new challenges in how to combine these signals into a single meaningful measure. Chapter 4 investigates the physiology of the pupil in stroke, chiasmal compression, and AION, and investigates how these components can be appropriately combined into a single measure. Results show naso-temporal differences are consistent with known physiology in control subjects and provides evidence that denser nasal retinal input may underpin the greater contraction anisocoria seen in temporal fields than in nasal fields.

With the intention that mfPOP be used in clinical practice, it must demonstrate it can perform as well as traditional perimetry, such as Humphrey and Matrix devices, in a wide range of disorders. Currently mfPOP testing neurological disorders has been very limited, and this represents a large gap in the literature. Chapter 5 compares mfPOP to Humphrey and Matrix perimeters, showing they mfPOP does not correlate well with these devices, and compares their utility in neurological disease. It shows that Humphrey appears the most useful device overall, with Matrix being exceptionally good in chiasmal compression, while mfPOP does not appear effective in these disorders.

With the first mfPOP approach having limitations in its diagnostic ability, a second stimulus protocol was designed using colour opponency with the measure of response latency (rather than amplitude), thought to preferentially stimulate cortical input to the pupil response, and may allow detection of cortical lesions. Chapter 6 investigates this new colour exchange protocol and latency measure, contrasting with the more common luminance approach used in chapter 5. It shows that the colour protocol shows a number of subtle differences compared to the luminance protocol, but does not show any greater utility in neurological disease. It reveals that latency and amplitude appear to have a weak positive relationship, and that mfPOP repeats appear to correlate well, but all measures have substantial variation.

These finding open up a number of future directions, from a larger and more focused HVF study into colour and depth perception, to considering retinal density as contributing towards biases in pupillary components, exploring hemifield ratios as a measure of early detection of chiasmal compression, and trialling other mfPOP methods to determine whether neurological disorders can be detected through pupillometry. Show more