Principal Component Analysis of Oxford Cognitive Screen in Patients With Stroke

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Administering the Oxford Cognitive Screen (OCS): A demonstration

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Principal Component Analysis of Oxford Cognitive Screen in Patients With Stroke

Marco Iosa^1,2, Nele Demeyere³, Laura Abbruzzese⁴, Pierluigi Zoccolotti^1,2 and Mauro Mancuso^4,5*

• ¹Department of Psychology, Sapienza University of Rome, Rome, Italy
• ²IRCCS Fondazione Santa Lucia, Rome, Italy
• ³Department of Experimental Psychology, University of Oxford, Oxford, United Kingdom
• ⁴Tuscany Rehabilitation Clinic, Arezzo, Italy
• ⁵Physical and Rehabilitative Medicine Unit, NHS-USL Tuscany South-Est, Grosseto, Italy

Cognitive deficits occur in most patients with stroke and are the important predictors of adverse long-term outcome. Early identification is fundamental to plan the most appropriate care, including rehabilitation and discharge decisions. The Oxford Cognitive Screen (OCS) is a simple, valid, and reliable tool for the assessment of cognitive deficits in patients with stroke. It contains 10 subtests, providing 14 scores referring to 5 theoretically derived cognitive domains: attention, language, number, praxis, and memory. However, an empirical verification of the domain composition of the OCS subtests in stroke data is still lacking in the literature. A principal component analysis (PCA) was performed on 1,973 patients with stroke who were enrolled in OCS studies in the UK and in Italy. A number of six main components were identified relating to the domains of language and arithmetic, memory, visuomotor ability, orientation, spatial exploration, and executive functions. Bootstrapped split-half reliability analysis on patients and comparison between patients and 498 healthy participants, as that between patients with left and right hemisphere damage, confirmed the results obtained by the principal component analysis. A clarification about the contribution of each score to the theoretical original domains and to the components identified by the PCA is provided with the aim to foster the usability of OCS for both clinicians and researchers.

Introduction

Cognitive deficits occur in 50–78% of patients with stroke (1), and their early identification is fundamental to plan the most appropriate neurorehabilitation program (2). The Oxford Cognitive Screen (OCS) is a screening tool providing a “snapshot” of the patient's cognitive profile helpful for designing the rehabilitation program according to the patient's needs (3). The OCS entails 10 subtests: picture naming, semantics, orientation, visual field, sentence reading, number writing and calculation, broken hearts, imitation, recall and recognition, and trails. It is easy to administer and score, takes a relatively short time, can be delivered at the bedside, and can be administered in the acute phase (3, 4).

The Oxford Cognitive Screen was initially tested on 140 neurologically healthy English participants and 208 acute patients with stroke demonstrating its reliability, convergent and divergent validity, and sensitivity in differentiating between patients with right vs. left brain damage (4). In a successive study (5) on 200 patients with stroke, the OCS was shown to be more sensitive than the Montreal Cognitive Assessment (MoCA) in highlighting cognitive impairments in this type of patients. In addition, OCS was found to be more inclusive for participants with aphasia and not dominated (as MoCA) by left hemisphere impairments, instead of giving differentiated profiles across the contrasting domains. Similar results on patients with stroke were obtained by the comparison of OCS with the Mini-Mental State Examination (6). Overall, the OCS detects important cognitive deficits after stroke not assessed in standard cognitive screening developed for dementia, it is inclusive for patients with aphasia and neglect, and it is less confounded by co-occurring difficulties in these domains.

The OCS has been validated and standardized in many other languages, including Italian (4), Spanish (7), Brazilian Portuguese (8), Chinese (9), Dutch (10), Russian (11), and Danish (12).

The original study classified the OCS subtests under five different theoretical domains: attention (divided into the subdomains of executive functions and visual attention), memory, language, praxis, and number (Figure 1) (4).

FIGURE 1

Figure 1. The visual snapshot of the OCS is a compact modality of OCS scoring, in which compromised domains are colored. It provides a quick but informative overview of the cognitive profile of the patient.

A Chinese study tested the reliability of OCS with 5 domains, but the first one was named as “attention and executive function,” and the others were language, memory, number processing, and praxis. The authors found a nearly acceptable level of data-to-model fit, with an improvement in the fitting model obtained when the two subtests related to numerical cognition and praxis were dropped from the model. This yielded an acceptable fit in a model including only three domains:(1) attention and executive function; (2) memory; and (3) language (9). The internal consistency of each of these three domains was tested using Cronbach's alpha coefficient, finding values of 0.3, 0.52, and 0.44 for attention, memory, and language, respectively. These values were lower than the Cronbach's alpha equal to 0.907 evaluated for assessing internal consistency among all the items in a Spanish study (7). This difference could be due to the fact that, in the Chinese study, the Cronbach's alpha was computed on each one of the identified three dimensions on the patients' sample, whereas in the Spanish study, it was computed on all the subtests and collapsing patients and healthy elderly. The Chinese study (9) investigated the structural validity of OCS, but it was done by a confirmatory (and not by an exploratory) factor analysis in which the hypothesis of five and three domains was a priori formulated and tested in a sample of 100 patients and 120 controls. Given the known heterogeneity in the cognitive consequences of stroke, it would be important to also carry out exploratory factorial analyses of the OCS on large samples of patients with stroke and healthy controls. Information on this is still limited in the literature.

A recent study conducted on 237 patients with stroke identified only three main components of cognitive functions impaired 1 week after stroke assessed by OCS and the National Institutes of Health Stroke Scale (13). Authors interpreted their results suggesting that neurological deficits following stroke are correlated in a low-dimensional structure of impairment, related neither to the damage of a specific area nor to a vascular territory, but rather reflecting widespread network impairments caused by focal lesions. The first component resulted linked to language, calculation, memory, praxis, and right-sided neglect and was found to be mainly related to left hemisphere damage. The second component was linked to left visuomotor deficits and spatial neglect and mainly related to damage of right cortico-subcortical regions. The third component was linked to right motor deficits and damage in the left subcortical regions. However, the proposed model explained only 50% of the variance, and it was dominated by left hemisphere impairments, similar to other cognitive assessment tools (5, 6). It would appear that while clinicians highlight a high clinical variability among patients with stroke, psychometric tests reveal a limited set of dimensions accounting for a large proportion of variance in performance of the patient with stroke. This could be due to the fact that the large-scale physiological abnormalities following a stroke reduce the variety of neural states visited during task processing and at rest, resulting in a limited repertoire of behavioral states (14).

Overall, a large variability of results and related interpretations emerges from the previous studies on OCS. Presumably, this is due to methodological differences such as whether healthy subjects have been included into the analyses with patients or not, and whether psychometric properties were measured on the OCS in general or on its specific domains.

Despite the general utility of OCS as a cognitive screening tool, the lack in the scientific literature of an exploratory psychometric analysis of OCS domains has led to some critical issues related to its use in clinical routine. A first issue is that in the original OCS under the umbrella domain of attention, executive functions and visuospatial attention are merged, putting together two conceptually different cognitive functions. Even if attention plays a central role in both these functions, neither executive functions nor visuospatial attention can be used to define the impairment of the attention function. This problem also implies that the original OCS does not allow the spatial inattention to emerge as a possible deficit distinct from the attentive component, despite three scores of original OCS could be used to assess unilateral spatial neglect (cancelation, space symmetry, and object asymmetry). Because of the role played by spatial inattention in affecting neurorehabilitative outcomes in patients with stroke (15–17), it would be fundamental to detect and hence to treat this syndrome in a very early phase of stroke. Another critical issue concerns the separation between the “number” and “language” domains in the original OCS; indeed, more recent literature has shown that number writing and calculation should be considered as associated with the language domain (18, 19), indicating the importance of checking the factorial composition of subtests related to linguistic and number processing. These problems may have contributed to the gap between clinicians claiming a high clinical variability among patients with stroke and scientific psychometric tests revealing a limited set of dimensions accounting for a large proportion of variance in the cognitive functions of patients with stroke.

Therefore, the aim of this study was to carry out a factor analysis on a large number of patients with stroke to identify the main OCS domains to solve some scientific and clinical issues related to this useful and valid screening tool.

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