WRITER Natalia Kelly

Visual fixation is fundamental to how children see, process, and respond to their surroundings, yet it is often overlooked in the assessment of cerebral visual impairment (CVI). In this article, Natalia Kelly examines the position, stability, and duration of fixation, as well as refixation behaviour, and explains how closer clinical evaluation of these features may improve diagnosis, guide targeted intervention, and support functional vision in children with CVI.

The visual system is composed of extensive networks that connect ocular structures with cortical and subcortical areas in the brain. This neural circuitry creates an integrated symbiotic flow between visual function, ocular movements, and visual processing to produce vision.1 The harmonious amalgamation of the whole visual system, coupled with exposure to visual experiences, is believed to play a vital and influential role to initiate global development.2 However, the vast neural resources dedicated to vision are vulnerable to genetic variations, neurodevelopmental anomalies, and acquired insult.

Cerebral visual impairment is asserted to be the most common cause of neurological-based paediatric vision impairment across the world.2,3 CVI is a heterogeneous and complex condition that is characterised by verifiable visual dysfunction resulting from insult or maldevelopment of the retrochiasmal visual pathway and associated cortical networks. The visual dysfunction observed in CVI cannot be attributed to a disorder affecting the eyes,3 although ophthalmic conditions may co-exist, such as uncorrected refractive error, strabismus, amblyopia, optic atrophy, oculomotor apraxia, and nystagmus.2,4

The aetiology of CVI is quite diverse, with the most common causes pertaining to hypoxicischaemic encephalopathy (HIE), genetic variants, and deviations in brain development.2 Complex underlying neurological comorbidities, including global developmental delay, cerebral palsy, and epilepsy, are also frequently associated with CVI. Collectively, these neurodevelopmental conditions may cause disruption to intrinsic global development, including interruption to visual development.1,2

Considering the genesis of CVI is founded in deviations of cortical architecture and/ or function, it is not surprising that this condition is associated with a broad range of visual dysfunction and considerable variability in both clinical presentation and severity of visual impairment. Children with CVI display varied deficits in basic visual functions, such as reduced acuity, decreased contrast sensitivity, visual field loss, and diverse oculomotor dysfunctions, in particular atypical fixation.2,5 Higher-order visual processing insufficiencies, such as visual response latency, intolerance to visual crowding, and short visual attention concomitantly contribute to vision loss in CVI.2,3,5 Despite the multifarious presentations of CVI, atypical fixation appears to be the most frequently reported visual dysfunction.2 Visual fixation involves the steady and maintained control of eye movements while looking. Fixation is the mechanism that allows visual information to be accurately positioned on the retina and then subsequently processed by the brain. Atypical fixation in CVI may include abnormalities in fixation position, stability, and duration. Additionally, repeated refixation behaviours are commonly observed in CVI.2,5

THE IMPORTANCE OF FIXATION

Fixation is dependent on the coordination of oculomotor networks. Consequently, children with CVI are susceptible to impaired fixation due to the underlying, interconnected neuroanatomical anomalies affecting the cortical, subcortical, and brainstem regions involved in oculomotor control. These cortical areas include, but are not limited to, the pre-frontal and parietal cortices, cerebellum, superior colliculus, basal ganglia, thalamus, brainstem nuclei, and reticular formation.6 Additionally, coexisting conditions such as nystagmus, strabismus, amblyopia, visual field loss, and paroxysmal ocular deviations frequently seen in CVI can contribute to, or exacerbate, impaired fixation.

Despite the broad visual functional impact of CVI, traditional clinical practice has prioritised visual acuity with less emphasis on fixation as a foundational component for visual function and visual processing.

The conventional models of vision focus on sensory input of visual information and perceptual interpretation. However, efficient visual performance also depends on the ability of the oculomotor system to accurately direct, stabilise, and maintain the retinal image. Although fixation anomalies are commonly observed in children with CVI, they are not routinely assessed as clinical indicators and seldom used as therapeutic targets within habilitation programs. Consequently, the underrepresentation of fixation presents as a critical gap in current clinical practice. Greater recognition of fixation as a core component of vision may help guide more comprehensive clinical assessment and habilitation strategies aimed at optimising visual development in children with CVI.

In CVI, impairment in core visual functions such as near and distance vision, contrast sensitivity, and visual fields have been firmly established.2,4 However, the impact of ocular and/or cortical pathology on oculomotor control is comparatively less understood.

Disruption to fixation may compromise the quality and stability of the retinal image and/or reduce efficient higher-order visual processing. There is emerging evidence to indicate that oculomotor control, particularly visual fixation, underpins our ability to optimise visual function and facilitate visual processing. Studies have shown that atypical fixation in those with visual impairment has a relationship with visual acuity and contrast sensitivity.7

Research has also shown that fixation is an important foundational factor in visual processing where stable fixation in infancy can increase directionality of white matter which, in turn, improves efficient transfer of information within the brain.8 The evidence of fixation in both visual function and processing underscores the gap in current clinical practice and emphasises the importance of investigating oculomotor control within CVI.

TYPES OF VISUAL FIXATION

Visual fixation is predominantly composed of three interrelated components including fixation position, fixation stability, and fixation duration. In CVI, refixation behaviour has also been observed.5 Various clinical and instrumental methods have been employed to measure the integrity of visual fixation including subjective observation, video recordings, or eye tracking technologies. The selection of the assessment method is commonly guided by the underlying pathology as well as the cognitive, motor, and attentional ability to engage with the assessment tool.

In neurologically complex paediatric populations such as CVI, fixation assessment is often limited to observed qualitative descriptors such as central, steady, maintained. This approach is often the most clinically appropriate and feasible method. The heterogeneous ophthalmic, neurological, and motor profiles seen in CVI, including reduced sustained fixation, attentional variability, and unstable head and postural control,2 limit the child’s ability to engage reliably with higher technological instruments such as eye trackers in a clinical setting. So, while qualitative assessments lack the precision of instrumental measures, they remain clinically meaningful because they capture fixation behaviour in children who are unable to participate reliably in objective testing.

Children with CVI commonly demonstrate reduced qualitative elements of fixation including eccentric, unstable or brief behaviours, thus it is warranted for fixation impairment to be specifically investigated in clinical assessments and CVI-intervention. It is proposed that objective fixation metrics, including fixation position, stability, duration, and refixation patterns, could be considered for inclusion in routine clinical vision assessments.

To understand how fixation contributes to functional vision, it is important to consider individual components, each of which provides distinct information regarding oculomotor performance.

Fixation position refers to the specific location on the retina where the visual image is directed during looking. Central fixation refers to the image projected on the fovea to produce optimum vision, while eccentric viewing occurs when the image is cast over a retinal area other than the fovea, which likely produces an inferior visual image. The visual implication, for a child who is eccentrically viewing, is that the image falls on an anatomical area that would naturally produce reduced acuity. Furthermore, variable eccentric points and fixation instability are commonly noted in those who eccentrically view, and this may exacerbate blurred vision.

Fixation stability is the capacity to minimise involuntary eye movements to allow for a target to be precisely positioned and steady so the image can be focused on the retina. Unstable fixation can present as inefficient eye movement patterns characterised by slower and more frequent corrective saccades. Unstable fixation can result in an inconsistent oculomotor reference point required to focus an image, resulting in blurred vision.

Fixation duration enables the image to be maintained on a retinal area so the information can be relayed to the cortical centres for interpretation and perception. Therefore, reduced fixation duration can interfere with visual perception.

RECOGNISING REFIXATION

A subtle yet clinically observed fixation anomaly seen in children with CVI is repeated refixation behaviour, which is characterised by recurring diversion of the eyes away from a visual stimulus and their subsequent return to the same target. Refixation behaviour has the potential to have important implications for clinical assessment, interpretation, and management of visual function. Clinically, refixation may be identified as a fixation anomaly or present as intermittent loss of gaze, repeated gaze shift, or multiple attempts to visually re-engage in looking at the same target. These behaviours may be interpreted as a manifestation of unstable or poorly maintained fixation. However, it is proposed that refixation should be considered as a discernible component of fixation behaviour.

Recognising refixation as a distinct and meaningful feature of visual behaviour may improve both the accuracy of fixation assessment and the interpretation of visual performance in children with CVI. During clinical assessment and intervention, allowing sufficient time for the child to re-engage with a target at its original location, rather than immediately relocating the stimulus to the child’s shifted gaze, may provide a more accurate representation of their visual capabilities.

Without understanding refixation behaviour, the clinician may interpret fixation ability inaccurately and/or underestimate fixation ability. Consequently, the inability to recognise refixation behaviour may inadvertently modify assessment procedures in a manner that masks the child’s natural visual strategy.

Refixation behaviour is not well documented in the literature.5 However, given the close interaction between fixation and visual processing, refixation may reflect an adaptive compensatory response to inefficient perceptual processing whereby repeated fixation to the same stimuli is required to acquire or confirm the visual information.

Other than CVI research, studies in stroke and visual search paradigms have associated refixation with inefficient information processing. Empirical evidence proposes that repeated refixation serves as an adaptive strategy to recover missed information or to compensate for visual field deficits.

Collectively, the literature suggests that refixation may reflect both underlying oculomotor dysfunction and adaptive compensatory behaviour secondary to inefficient visual processing. Consistent evaluation of refixation, alongside fixation position, stability, and duration, may provide valuable clinical insight into fixation performance in children with CVI. Further research regarding refixation behaviours in children with CVI is warranted to determine the prevalence, mechanisms, and functional significance of refixation.

Clinicians should be encouraged to incorporate fixation as a routine component of clinical assessment in children with CVI. Methodical evaluation of fixation behaviour, including position, stability, duration, and refixation may improve the identification and characterisation of visual dysfunction and assist in guiding targeted intervention. Table 1 provides a practical clinical guide for assessing fixation in children with CVI.

CONCLUSION

Visual fixation is a fundamental component of vision, which provides a stable retinal image required for effective visual function and higher-order visual processing. As one of the most commonly reported visual dysfunctions in children with CVI, fixation warrants greater emphasis in both clinical assessment and vision habilitation. Systematic evaluation of fixation position, fixation stability, fixation duration, and refixation behaviour may provide valuable insight into a child’s visual profile. This may support more accurate identification of visual dysfunction and inform targeted intervention. Recognising fixation as a fundamental component of routine clinical vision assessment has the potential to enhance clinical decision making, inform habilitation strategies, and optimise visual development and functional vision in children with CVI.

Natalia Kelly BOrth&OphthSc PGDiplHltResMthds is an orthoptist who established Vision Matters, a private orthoptic practice in Melbourne. Her practice specialises in innovative vision rehabilitation with a focus on paediatric and adult low vision, biofeedback training, and neurological vision impairment assessment and intervention.

References
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