Cristina Santos
Physiotherapist specialising in vestibular rehabilitation
ENT – Otoneurology Unit
Cuf Descobertas Hospital, April 2022
Visual Dependence (VD) consists of placing excessive emphasis on visual information, to the detriment of vestibular and proprioceptive inputs. It necessarily reflects a sensory disorganisation. Following a brief introduction and description of VD and its functional impact, the issue will be contextualised within the framework of vestibular dysfunction, with brief considerations on strategies for rehabilitation and sensory reorganisation.
INTRODUCTION
The proper integration of concordant vestibular, visual and somatosensory inputs ensures an efferent response at all times, thereby maintaining visual and postural stability. This integration is fundamental to higher-order cognitive processes⁴,⁸,¹³, such as spatial perception and orientation, body representation, attention, concentration, memory and social cognition.
The importance or weighting given to each sensory input varies from person to person and depends on age (children up to the age of 2 rely on visual information to maintain their balance, a reliance that re-emerges in old age⁷,⁹,¹³), and is influenced by specific circumstances – this is what defines each individual’s sensory profile.
Sensory inputs are vital to the developmental process. They accompany and facilitate the process of integration, which reaches maturity at around 15 years of age13. It is the variety and richness of the sensory inputs with which we “feed” the system, together with the continuous adjustment and resolution of sensory conflicts, that enrich the individual’s repertoire and shape each person’s sensory experience.
Sensory disorganisation, such as in the context of visual dependence (VD), has a functional impact, resulting in a greater or lesser degree of disability. VD affects all sensory experiences, both present and future (if it persists), with implications directly related to the stage of individual development. In the presence of vestibular pathology, it may compromise the functional outcome of the compensatory process.
VISUAL DEPENDENCY
Since the 1950s, the concept of DV has been used to describe the excessive reliance on visual inputs, to the detriment of present vestibular and somatosensory inputs, or, in other words, the difficulty or inability to downplay the visual cues present10. The concept of visual dominance reflects the degree of reliance on visual inputs for spatial orientation¹¹.
Whilst this concept is not new, it is undeniable that the increasingly visual world in which we live has a decisive influence on the value we place on this sensory input. The high – and often excessive – exposure to visual stimuli, particularly through the use of smartphones, enhanced by their practicality and portability (undeniable characteristics), subjects us daily to a vast amount of stimuli, in an abundance of colour, light and movement that amplifies their impact, resulting in sensory overload12.
The consequences of placing excessive emphasis on visual inputs must be assessed and interpreted on a case-by-case basis, taking into account age and stage of development, individual characteristics and the surrounding circumstances; they must also take into account extremely important factors such as the presence of sensory pathology or dysfunction, particularly vestibular dysfunction (and, where applicable, determine the stage of central compensation at which the individual currently finds themselves).
The over-reliance on visual input, which characterises visual dyspraxia, leads to changes in spatial perception and orientation. Consequently, symptoms such as functional difficulty or inability may arise in situations with high visual demands (whether at rest or in motion), as a result of the conflict arising from this over-reliance on visual input.
This condition is also characterised by greater difficulty in low-light conditions, a constant feeling of instability, distortion of the visual field, discomfort or a “heavy feeling in the eyes”, leading to anxiety that results in the development of avoidance strategies and phobias.
The symptoms may lead to avoidance of visually stimulating activities or environments, which can result in social isolation and functional limitations. Furthermore, this avoidance shapes and progressively impoverishes the individual’s sensory experience.
Furthermore, if we consider that every head movement triggers a reflexive oculomotor response, this response – particularly in a visually rich environment – may not be perceived as appropriate. This may give rise, as a defensive or protective response, to a reduction in head movement, leading to increased vestibular sensitivity, the symptoms of which in turn fuel the avoidance process itself. Consequently, the individual becomes caught in a cycle of overlapping symptoms, which complicates and confuses the medical history, resulting in a functionally disabling condition characterised by an increasingly restricted sensory experience, to which they become a prisoner.
In 2017, the European Society for Clinical Evaluation of Balance Disorders published important considerations regarding visual input in balance control. They argue that, whilst not overlooking vestibular migraine, the psychiatric context and the presence of traumatic brain injury (TBI), DV is primarily facilitated by the presence of vestibular pathology, due to the increased contribution of visual input to the normal process of central compensation within the context of sensory integration.
They emphasise and recommend the need to take a comprehensive medical history, stressing that this is a genuine condition, the symptoms of which should not simply be interpreted as being of psychiatric origin9.
When analysing the context of visual deprivation following unilateral vestibular lesion (UVL), it is first necessary to consider the undeniable and essential contribution of vision during the acute phase – the onset of the compensatory process. The presence of symptoms consistent with visual impairment, following vestibular symmetry, indicates that the mechanisms of sensory integration have been unable to “down-regulate” the visual input as the central compensatory process progresses2,3. A visual profile, combined with habits of excessive visual input, contributes significantly to this difficulty or inability.
It is now accepted that the combination of psychological factors and the severity of DV are the best predictors of the outcome of functional rehabilitation following UVL3,6,11.
In 2014, M. Lacour, in his discussion of the 10 indications for functional vestibular rehabilitation (SR), highlighted not only the importance of early initiation of SR and the educational value of training and sensory input, but also the need to consider the patient’s sensory, motor and cognitive profile, in order to enable a prophylactic approach to sensory reorganisation during rehabilitation⁶. Knowledge of the patient’s sensory profile enables the rapid implementation of an appropriate sensory regimen, particularly a visual one, in which the progression of the compensation process will dictate how it is managed.
The approach to sensory reorganisation, in the context of visual impairment, necessarily involves changing habits and managing a visual diet that is tailored to the individual’s profile, habits and needs, and forms part of a balanced sensory diet. It is also essential to facilitate and promote coping strategies, guided by the functional impact of the symptoms, based on physiology and respect for the individual difficulties and responses exhibited.
In certain circumstances, even in the absence of vestibular dysfunction, VR intervention may be necessary, using vestibular desensitisation strategies, in order to facilitate head movement and stimulate reflex activity, or to inhibit the processing of visual information, using optokinetic stimulation (OKS)¹. The strategies employed will be determined by an individual assessment, with sensory organisation and hierarchy serving as the primary criteria for their selection.
Article on the exhibition on the theme of “Visual Dependence”, presented at the Otoneurology Meeting, which took place on 8 and 9 April 2022 at the Cuf Descobertas Hospital, Lisbon.
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