Maia da Silva, Mari-Nilva; (2022) The Significance of Homonymous Hemianopia in Posterior Cortical Atrophy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Posterior cortical atrophy (PCA) is defined by the concurrence of posterior cortical dysfunction and imaging evidence of neurodegeneration in the occipital, parietal and/or posterior temporal lobes. Because Alzheimer disease (AD) is the commonest cause of PCA by far and a progressive nonophthalmic visual disorder is considered necessary for the diagnosis, the syndrome has often been called “the visual variant of AD”. However, the PCA visual syndrome has some caveats. One such is how a primary deficit such as homonymous hemianopia (HH) – historically excluded from PCA clinical characterisation – relates to the classical, higher-order elements of the condition, such as object recognition and spatial deficits. In fact, the very mechanism of HH in PCA has not been established. In this thesis, the visual fields of 24 patients with HH due to PCA were characterised. Only one patient had a bilateral proportional HH at baseline. The more affected hemifield was the left in two thirds of the patients. Statokinetic dissociation was demonstrated in all of them. Nineteen of these patients were prospectively studied. Over time, visual fields to both static and kinetic stimulations deteriorated, however more markedly to the former. A subgroup of patients had further visual fields acquired on the Octopus using different target-velocities (10deg/s, 5deg/s, and 3deg/s). A gradient of the effect of velocity was observed, with greater and earlier visual field loss occurring to slower than faster velocities. In addition, the lateralisation of the HH was compared to the performance in tests of lateralised cognitive functions. Right HH correlated with greater deterioration in left parietal functions (Spelling, Calculation), whereas left HH correlated with one (Spatial Perception), but not the other (Object Perception) of the two right hemisphere functions studied. The more proportional between the two hemifields was the HH – a pattern that tended to occur with time -, the worse the performance in basic visual skills, i.e., deficits that suggest bilateral occipital damage. Therefore, rather than signalling an extreme caudal (primary visual) variant of PCA, HH in these patients occurred in the context of a lateralised higher-order syndrome, highlighting the primacy of the hemisphere in disease progression. The predominance of left HH can then be interpreted as reflecting biases towards the right hemisphere by the syndrome definition. The relative resistance of visual fields to kinetic testing in these patients may represent a special case of the Riddoch phenomenon, as it seems to reflect a stage of neurodegeneration rather than the preservation of parallel pathways in the visual system as is likely to be the case with focal occipital damage.

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