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Visual Fields in Neuro-Ophthalmology
Published in Vivek Lal, A Clinical Approach to Neuro-Ophthalmic Disorders, 2023
This chapter will discuss the techniques available to map out the visual field and then the types of visual field defect that may be produced in non-organic (functional) illness and in neuro-ophthalmological disease. Figure 2.1 illustrates the usual visual field defects that can occur with damage to the different sites along the visual pathway. Absolute defects are not always seen. The depth of the defect depends on the degree of damage and the disease process involved.2,3
Sensory System
Published in Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal, Principles of Physiology for the Anaesthetist, 2020
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal
The visual pathway is from the retina to the cerebral cortex. Rods and cones are the receptors in the retina. The rods are found throughout the retina (except for the fovea) and contain the photopigment rhodopsin. They are used in night vision and have high sensitivity but do not transmit colour vision. The cones contain three pigments: erythrolabe (red sensitive), chlorolabe (green sensitive) and cyanolabe (blue sensitive). They are concentrated in the fovea and are used in day and colour vision. The rods and cones are connected to ganglion cells, the axons of which are carried in the optic nerve and in the lateral geniculate body to the primary visual cortex. Fibres from the nasal halves of the retinas cross, whereas those of the temporal sides remain ipsilateral. Optic nerve fibres also pass to the midbrain pretectal areas (pupillary reflexes) and to the superior colliculus (eye movements).
The Consciousness of Muscular Effort and Movement
Published in Max R. Bennett, The Idea of Consciousness, 2020
At a later stage of evolution, mechanisms were put in place that allowed the nervous system to remove sensory information, using projections from the brain to the sensory gateway to the cortex, namely the thalamus, as shown in Figures 4.8B and 4.8C. We have already seen how information gathered by primary sensory neurons concerned with muscle receptors can be gated out before it reaches the somatosensory cortex by means of a collateral feedback from the motor cortex at the level of the thalamus. Such a feedback could occur via the well-known pathway from the motor cortex to the basal ganglia and from there to the reticular nucleus (that lies just outside the thalamus) which then projects to the somatosensory cortex (Figure 4.8B). Sensory information that is gathered by the retina is also ‘gated’ as it passes through the thalamus on the way to the visual cortex, as shown in Figure 4.8C. The primary visual pathway is from the retina to the thalamus and from there to the visual cortex; neurons exist in the cortex that project back to the thalamus where they can gate the incoming visual information. There is then evidence for the modulation of both signals arising from the primary sensory neurons, and signals arising from the visual sensory neurons at the level of the thalamus. In this way the brain itself can determine the sensory information which reaches it and so modulate the perceptions of the world which it might allow to reach consciousness.
Visual perceptual deficit screening in stroke survivors: evaluation of current practice in the United Kingdom and Republic of Ireland
Published in Disability and Rehabilitation, 2022
Michael J. Colwell, Nele Demeyere, Kathleen Vancleef
Stroke is one of the most commonly occurring neurological diseases in the world, resulting in deleterious consequences for its survivors [1–3]. Among the most significant of these consequences is impairment in sensory vision and visual perception [4,5]. Sensory vision refers to the initial processes within the visual pathway (known also as the afferent and efferent visual pathway), underpinned by light refraction by the cornea and lens followed by light transduction through the optic nerve to produce visual images [6,7]. Visual perception occurs further along the optic pathway, entailing cognitive processes which interpret and assign meaning to what is visually available [8]. Both sensory vision and visual perception are cardinal to daily living and can be adversely impacted by stroke [9].
Dysfunction in macula, retinal pigment epithelium and post retinal pathway in acute organophosphorus poisoning
Published in Clinical Toxicology, 2021
Padmini Dahanayake, Tharaka L. Dassanayake, Manoji Pathirage, Anuradha Colombage, Indika B. Gawarammana, Saman Senanayake, Michael Sedgwick, Vajira S. Weerasinghe
Our findings to a certain extent can be explained by structural and physiological characteristics of the retina and the visual pathways. There are nicotinic ACh receptors on the RPE cells and hence ACh is likely to be present in the outer photoreceptor segment [21]. OP poisoning would have caused an increase in ACh activity from the outer segments of the photoreceptors which stimulated the ACh receptors on the apical surfaces of RPE cells. This interaction probably has caused changes in ionic conductance in RPE leading to changes in EOG parameters. Some evidence from animals exposed to chlorpyrifos shows oxidative stress in RPE [22] and cell apoptosis, lipid peroxidation and DNA damage in retinal and RPE cells [23]. However, our cross-sectional data do not discriminate whether the RPE dysfunction is caused by excitotoxic damage to the retina or just a transient cholinergic disturbance in the RPE that outlasted the clinical recovery, but could eventually return to the normal state.
Non-Arteritic Anterior Ischaemic Optic Neuropathy Associated with Optic Nerve Hypoplasia and Elevated Intraocular Pressure
Published in Neuro-Ophthalmology, 2020
Nithya Rathinam, Nirupama Kasturi, Amit Kumar Deb, Subashini Kaliaperumal
Optic nerve hypoplasia is characterised by a small optic nerve head and a significantly reduced number of axons occurring due to irreversible axonal damage to the visual pathway at some time before the full development of the eye. It may be unilateral but often occurs bilaterally (in 65% to 75% of cases). An ophthalmoscopically detectable sign of optic nerve head hypoplasia is the “double-ring sign”, in which a peripapillary ring surrounds the small optic nerve head. Further, optic nerve hypoplasia is confirmed if the distance from the disc to the macula is equal to or greater than 3 disc diameters.3 Visual acuity depends on the extent of axonal loss and visual field defects can range from normal to localised defects in the nasal and inferior quadrants or diffuse constriction. The spectrum of optic nerve hypoplasia appears to be very wide also with respect to the extent of damage.4