Functional magnetic resonance imaging in disorders of consciousness: preliminary results of an innovative analysis of brain connectivity

Diffusion tensor imaging magnetic resonance imaging (DTI-MRI) helps to tailor speech therapy: A case report with a short narrative review

BACKGROUND

Stroke has functional sequelae, including motor weakness, spasticity, dysphagia, and neurogenic bladder deteriorating activities of daily living. Speech therapy is more often an essential part of the rehabilitation program. Studies in aphasia have primarily focused on two major pathways: the arcuate fasciculus and the superior longitudinal fasciculus. The arcuate fasciculus is a major fiber bundle connecting Broca's area (associated with language production) and Wernicke's area (associated with language comprehension). Damage to this pathway can result in different types of aphasia, depending on the location and extent of the injury.

CASE DESCRIPTION

Tractography is a neuroimaging technique used to map the white matter tracts in the brain. Our patient had an occlusion of the external carotid and left carotid siphon. The patient exhibited early complex language deficits involving both motor expressive and comprehensive abilities. After three months the patient presented conduction aphasia caused by damage to the inferior parietal lobule, which extends into the subcortical white matter and damages the arcuate fascicle. This patient was re-assessed for spasticity and language treatments needs, few months after the stroke. Considering first two months language improvement followed by any further language expressive improvement from 3rd to 6th month, a diffusion tensor imaging (DTI) tractography was requested to study interconnections between cortical and subcortical matter. The brain magnetic resonance imaging (MRI) performed in our hospital showed an extensive malacic gliotic area in the left temporal-frontal parietal site.

CONCLUSION

Brain MRI is confirmed as a multimodal tool evaluating the damage, both from the point of structural and functional view. Tractography in aphasia allows focusing on major pathways. The involvement of the arcuate fascicle, whose lesion disconnects Broca's and Wernicke's areas, is related to clinical improvement, and represents a neural correlate of the brain injury recovery process that physicians and speech therapists might be aware of it, tailoring the plane of care of each patient.

Measuring the Complexity of Consciousness

The grand quest for a scientific understanding of consciousness has given rise to many new theoretical and empirical paradigms for investigating the phenomenology of consciousness as well as clinical disorders associated to it. A major challenge in this field is to formalize computational measures that can reliably quantify global brain states from data. In particular, information-theoretic complexity measures such as integrated information have been proposed as measures of conscious awareness. This suggests a new framework to quantitatively classify states of consciousness. However, it has proven increasingly difficult to apply these complexity measures to realistic brain networks. In part, this is due to high computational costs incurred when implementing these measures on realistically large network dimensions. Nonetheless, complexity measures for quantifying states of consciousness are important for assisting clinical diagnosis and therapy. This article is meant to serve as a lookup table of measures of consciousness, with particular emphasis on clinical applicability. We consider both, principle-based complexity measures as well as empirical measures tested on patients. We address challenges facing these measures with regard to realistic brain networks, and where necessary, suggest possible resolutions.

Functional Networks in Disorders of Consciousness

Severe brain injury may cause disruption of neural networks that sustain arousal and awareness, the two essential components of consciousness. Despite the potentially devastating immediate and long-term consequences, disorders of consciousness (DoC) are poorly understood in terms of their underlying neurobiology, the relationship between pathophysiology and recovery, and the predictors of treatment efficacy. Recent advances in neuroimaging techniques have enabled the study of network connectivity, providing great potential to improve the clinical care of patients with DoC. Initial discoveries in this field were made using positron emission tomography (PET). More recently, functional magnetic resonance (fMRI) techniques have added to our understanding of functional network dynamics in this population. Both methods have shown that whether at rest or performing a goal-oriented task, functional networks essential for processing intrinsic thoughts and extrinsic stimuli are disrupted in patients with DoC compared with healthy subjects. Atypical connectivity has been well established in the default mode network as well as in other cortical and subcortical networks that may be required for consciousness. Moreover, the degree of altered connectivity may be related to the severity of impaired consciousness, and recovery of consciousness has been shown to be associated with restoration of connectivity. In this review, we discuss PET and fMRI studies of functional and effective connectivity in patients with DoC and suggest how this field can move toward clinical application of functional network mapping in the future.

Functional Evaluation of Awareness in Vegetative and Minimally Conscious State

Objective:

The aim of this study was to assess differences in brain activation in a large sample of Vegetative State (VS) and Minimally Conscious State (MCS) patients, using functional magnetic resonance imaging (fMRI).

Methods:

We studied 50 patients four to seven months after brain injury. By using international clinical criteria and validated behavioural scales such as the Glasgow Coma Scale and the Clinical Unawareness Assessment Scale, the patients were grouped into VS (n=23) and MCS (n=27). All patients underwent to fMRI examination. After 6 months, the patients were reassessed using Glasgow Outcome Scale and Revised Coma Recovery Scale.

Results:

fMRI showed significant (p<0.01, cluster-corrected) brain activation in the primary auditory cortex bilaterally during the acoustic stimuli in patients with both VS and MCS. However, ten patients clinically classified as VS, showed a pattern of brain activation very similar to that of MCS patients. Six months later, these ten VS patients had significant clinical improvement, evolving into MCS, whereas the other VS patients and patients with MCS remained clinically stable.

Conclusion:

Brain activity could help in discerning whether the status of wakefulness in VS is also accompanied by partial awareness, as occurs in MCS. This may have very important prognostic implications.

Transient effects of tumor location on the functional architecture at rest in glioblastoma patients: three longitudinal case studies

Background

The cognitive function of brain tumor patients is affected during the treatment. There is evidence that gliomas and surgery alter the functional brain connectivity but studies on the longitudinal effects are lacking.

Methods

We acquired longitudinal (pre- and post-radiotherapy) resting-state functional magnetic resonance imaging on three selected glioblastoma patients. These cases were selected to study three models: a lesion involving a functional hub within a central system, a lesion involving a peripheral node within a central system and a lesion involving a peripheral node of a non-central system.

Results

We found that, as expected, the tumor lesion affects connections in close vicinity, but when the lesion relates to a functional hub, these changes involve long-range connections leading to diverse connectivity profiles pre- and post-radiotherapy. In particular, a global but temporary improvement in the post-radiotherapy connectivity was obtained when treating a lesion close to a network hub, such as the posterior Cingulate Cortex.

Conclusions

This suggests that this node re-establishes communication to nodes further away in the network. Eventually, these observed effects seem to be transient and on the long-term the tumor burden leads to an overall decline of connectivity following the course of the pathology. Furthermore, we obtained that the link between hubs, such as the Supplementary Motor Area and posterior Cingulate Cortex represents an important backbone by means of which within and across network communication is handled: the disruption of this connection seems to imply a strong decrease in the overall connectivity.