Microdialysis enables the chemistry of the extracellular interstitial space to be monitored. Use of this technique in patients with acute brain injury has increased our understanding of the pathophysiology of several acute neurological disorders. In 2004, a consensus document on the clinical application of cerebral microdialysis was published. Since then, there have been significant advances in the clinical use of microdialysis in neurocritical care. The objective of this review is to report on the International Microdialysis Forum held in Cambridge, UK, in April 2014 and to produce a revised and updated consensus statement about its clinical use including technique, data interpretation, relationship with outcome, role in guiding therapy in neurocritical care and research applications.Electronic supplementary materialThe online version of this article (doi:10.1007/s00134-015-3930-y) contains supplementary material, which is available to authorized users.

Graphical abstract

Interleukin-1 receptor antagonist (IL1ra) has demonstrated efficacy in a wide range of animal models of neuronal injury. We have previously published a randomised controlled study of IL1ra in human severe TBI, with concomitant microdialysis and plasma sampling of 42 cytokines and chemokines. In this study, we have used partial least squares discriminant analysis to model the effects of drug administration and time following injury on the cytokine milieu within the injured brain. We demonstrate that treatment with rhIL1ra causes a brain-specific modification of the cytokine and chemokine response to injury, particularly in samples from the first 48 h following injury. The magnitude of this response is dependent on the concentration of IL1ra achieved in the brain extracellular space. Chemokines related to recruitment of macrophages from the plasma compartment (MCP-1) and biasing towards a M1 microglial phenotype (GM-CSF, IL1) are increased in patient samples in the rhIL1ra-treated patients. In control patients, cytokines and chemokines biased to a M2 microglia phenotype (IL4, IL10, MDC) are relatively increased. This pattern of response suggests that a simple classification of IL1ra as an 'anti-inflammatory' cytokine may not be appropriate and highlights the importance of the microglial response to injury.

A proof-of-concept aptamer-based optical assay is described for the determination of the immuno signalling molecule interleukin-6 (IL-6), a key marker of acute inflammation. The optical assay is based on the aggregation of gold nanoparticles (AuNP) coated in two complimentary “sandwich-style” aptamers, each with different IL-6 target moieties. IL-6 will recognise the complimentary aptamer pair and bind to it, thereby causing the aggregation of the corresponding functionalised nanoparticles. The aggregation of the AuNPs after exposure to IL-6 induces a visible colour change from red to pink, with a corresponding change in the absorption maximum from 520 to 540 nm. The change in the absorption maximum can be monitored visually, or by using a spectrophotometer or a plate reader. The optimal size and functionalisation of aptamer-coated AuNPs, and the potential assay formats were investigated using UV-vis spectrophotometry, transmission electron microscopy, and dynamic light scattering. The optical assay was applied for detecting mouse IL-6 in a mixed protein solution as a representative biological sample. The assay works in the 3.3 to 125 μg·mL−1 IL-6 concentration range, and the detection limit (at S/N = 3) is 1.95 μg·mL−1. This study was performed as a proof-of-concept demonstration of this versatile assay design, with a view to developing a similar assay for use in clinical samples in future. Graphical abstractSchematic representation of the aggregation of aptamer-functionalised nanoparticles in the presence of interleukin-6 (IL-6). The presence of mouse IL-6 in a mixed protein solution leads to a visible colour change, and a change in the absorption spectrum of the nanoparticles. Electronic supplementary materialThe online version of this article (10.1007/s00604-019-3975-7) contains supplementary material, which is available to authorized users.

Metabolic derangements following traumatic brain injury are poorly characterised. In this single-centre observational cohort study we combined 18F-FDG and multi-tracer oxygen-15 PET to comprehensively characterise the extent and spatial pattern of metabolic derangements. Twenty-six patients requiring sedation and ventilation with intracranial pressure monitoring following head injury within a Neurosciences Critical Care Unit, and 47 healthy volunteers were recruited. Eighteen volunteers were excluded for age over 60 years (11), movement related artefact (three) or physiological instability during imaging (four). We measured cerebral blood flow, blood volume, oxygen extraction fraction, and 18F-FDG transport into the brain (K1) and its phosphorylation (k3). We calculated oxygen metabolism, 18F-FDG influx rate constant (Ki), glucose metabolism and the oxygen/glucose metabolic ratio. Lesion core, penumbra and peri-penumbra, and normal-appearing brain, ischaemic brain volume and k3 hot-spot regions were compared with plasma and microdialysis glucose in patients. Twenty-six head injury patients, median age 40 years (22 male, 4 female) underwent 34 combined 18F-FDG and oxygen-15 PET at early, intermediate, and late time-points (within 24 hours, days 2–5, and days 6–12 post injury; n = 12, 8, and 14, respectively), and were compared with 20 volunteers, median age 43 years (15 male, 5 female) who underwent oxygen-15, and 9 volunteers, median age 56 years (3 male, 6 female) who underwent 18F-FDG PET. Higher plasma glucose was associated with higher microdialysate glucose. Blood flow and K1 were decreased in the vicinity of lesions, and closely related when blood flow was less than 25 ml/100 ml/min. Within normal-appearing brain, K1 was maintained despite lower blood flow than volunteers. Glucose utilisation was globally reduced in comparison with volunteers (p < 0.001). k3 was variable; highest within lesions with some patients showing increases with blood flow less than 25 ml/100 ml/min, but falling steeply with blood flow lower than 12 ml/100 ml/min. k3 hot-spots were found distant from lesions, with k3 increases associated with lower plasma glucose (Rho -0.33, p < 0.001) and microdialysis glucose (Rho -0.73, p = 0.02). k3 hot-spots showed similar K1 and glucose metabolism to volunteers despite lower blood flow and oxygen metabolism (p < 0.001, both comparisons); oxygen extraction fraction increases consistent with ischaemia were uncommon. We show that glucose delivery was dependent on plasma glucose and cerebral blood flow. Overall glucose utilisation was low, but regional increases were associated with reductions in glucose availability, blood flow and oxygen metabolism in the absence of ischaemia. Clinical management should optimise blood flow and glucose delivery and could explore the use of alternative energy substrates.