Brain temperature: What do we know?

Measuring Brain Temperature in Youth Bipolar Disorder Using a Novel Magnetic Resonance Imaging Approach: A Proof-of-concept Study

BACKGROUND

There is evidence of alterations in mitochondrial energy metabolism and cerebral blood flow (CBF) in adults and youth with bipolar disorder (BD). Brain thermoregulation is based on the balance of heat-producing metabolism and heat-dissipating mechanisms, including CBF.

OBJECTIVE

To examine brain temperature, and its relation to CBF, in relation to BD and mood symptom severity in youth.

METHODS

This study included 25 youth participants (age 17.4 ± 1.7 years; 13 BD, 12 control group (CG)). Magnetic resonance spectroscopy data were acquired to obtain brain temperature in the left anterior cingulate cortex (ACC) and the left precuneus. Regional estimates of CBF were provided by arterial spin labeling imaging. Analyses used general linear regression models, covarying for age, sex, and psychiatric medications.

RESULTS

Brain temperature was significantly higher in BD compared to CG in the precuneus. A higher ratio of brain temperature to CBF was significantly associated with greater depression symptom severity in both the ACC and precuneus within BD. Analyses examining the relationship of brain temperature or CBF with depression severity score did not reveal any significant finding in the ACC or the precuneus.

CONCLUSION

The current study provides preliminary evidence of increased brain temperature in youth with BD, in whom reduced thermoregulatory capacity is putatively associated with depression symptom severity. Evaluation of brain temperature and CBF in conjunction may provide valuable insight beyond what can be gleaned by either metric alone. Larger prospective studies are warranted to further evaluate brain temperature and its association with CBF concerning BD.

Brain temperature regulation in poor-grade subarachnoid hemorrhage patients - A multimodal neuromonitoring study

Elevated body temperature (T_core) is associated with poor outcome after subarachnoid hemorrhage (SAH). Brain temperature (T_brain) is usually higher than T_core. However, the implication of this difference (T_delta) remains unclear. We aimed to study factors associated with higher T_delta and its association with outcome. We included 46 SAH patients undergoing multimodal neuromonitoring, for a total of 7879 h of averaged data of T_core, T_brain, cerebral blood flow, cerebral perfusion pressure, intracranial pressure and cerebral metabolism (CMD). Three-months good functional outcome was defined as modified Rankin Scale ≤2. T_brain was tightly correlated with T_core (r = 0.948, p < 0.01), and was higher in 73.7% of neuromonitoring time (T_delta +0.18°C, IQR -0.01 - 0.37°C). A higher T_delta was associated with better metabolic state, indicated by lower CMD-glutamate (p = 0.003) and CMD-lactate (p < 0.001), and lower risk of mitochondrial dysfunction (MD) (OR = 0.2, p < 0.001). During MD, T_delta was significantly lower (0°C, IQR -0.2 - 0.1; p < 0.001). A higher T_delta was associated with improved outcome (OR = 7.7, p = 0.002). Our study suggests that T_brain is associated with brain metabolic activity and exceeds T_core when mitochondrial function is preserved. Further studies are needed to understand how T_delta may serve as a surrogate marker for brain function and predict clinical course and outcome after SAH.

Magnetic Resonance Spectroscopy Thermometry at 3 Tesla: Importance of Calibration Measurements

To demonstrate the importance of calibration measurements in 3 Tesla proton magnetic resonance (MR) spectroscopy (¹H-MRS) thermometry for human brain temperature estimation for routine clinical applications. In vitro proton MR spectroscopy to obtain calibration constants of the water-chemical shift was conducted at 3 Tesla with a temperature-controlled phantom, containing a pH-buffered aqueous solution of N-acetyl aspartate (NAA), creatine (Cr), methylene protons of Cr (Cr2), dimethyl silapentane sulfonic acid (DSS), and sodium formate (NaFor). Estimations of absolute human brain temperature were performed utilizing the correlation of temperature to the water-chemical shift for the resonances of NAA, Cr, and Cr2. Data for calibration of the metabolites' chemical shift differences and in vivo temperature estimations were acquired with single-voxel point-resolved spectroscopy (PRESS) sequences (repetition time/echo time = 2000/30 ms; voxel size 2 × 2 × 2 cm³). Spectroscopy data were quantified in the time-domain, and a Pearson correlation analysis was performed to estimate the correlation between the chemical shift of metabolites and measured temperatures. The correlation coefficients (r) of our calibration measurements were NAA 0.9975 (±0.0609), Cr -0.9979 (±0.0621), Cr2 - 0.9973 (±0.0577), DSS -0.9976 (±0.0615), and NaFor -0.8132 (±2.348). The mean calculated brain temperature was 37.78 ± 1.447°C, and the mean tympanic temperature was 36.83 ± 0.2456°C. Calculated temperatures derived from Cr and Cr2 provided significant (p = 0.0241 and p = 0.0210, respectively) correlations with measured temperatures (r = 0.4108 and r = -0.4194, respectively). Calibration measurements are vital for ¹H-MRS thermometry. Small numeric differences in measured signal and data preprocessing without any calibration measurements reduce accuracy of temperature calculations, which indicates that calculated temperatures should be interpreted with caution. Application of this method for clinical purposes warrants further investigation and a more practical approach.

Brain Temperature Is Increased During the First Days of Life in Asphyxiated Newborns: Developing Brain Injury Despite Hypothermia Treatment

BACKGROUND AND PURPOSE

Therapeutic hypothermia is the current treatment for neonates with hypoxic-ischemic encephalopathy. It is believed to work by decreasing the brain temperature and reducing the baseline metabolism and energy demand of the brain. This study aimed to noninvasively assess brain temperature during the first month of life in neonates with hypoxic-ischemic encephalopathy treated with hypothermia.

MATERIALS AND METHODS

Neonates with hypoxic-ischemic encephalopathy treated with hypothermia and healthy neonates were enrolled prospectively. MR imaging was used to identify the presence and extent of brain injury. MR imaging multivoxel spectroscopy was used to derive brain temperatures in the basal ganglia and white matter at different time points during the first month of life. Brain temperature measurements were compared between neonates with hypoxic-ischemic encephalopathy and healthy neonates.

RESULTS

Forty-three term neonates with hypoxic-ischemic encephalopathy treated with hypothermia had a total of 74 spectroscopy scans, and 3 healthy term neonates had a total of 9 spectroscopy scans during the first month of life. Brain temperatures were lower in neonates with hypoxic-ischemic encephalopathy during hypothermia, compared with the healthy neonates (respectively, on day 1 of life: basal ganglia, 38.81°C ± 2.08°C, and white matter, 39.11°C ± 1.99°C; and on days 2-3 of life: basal ganglia, 38.25°C ± 0.91°C, and white matter, 38.54°C ± 2.79°C). However, neonates with hypoxic-ischemic encephalopathy who developed brain injury had higher brain temperatures during hypothermia (respectively, on day 1 of life: basal ganglia, 35.55°C ± 1.31°C, and white matter, 37.35°C ± 2.55°C; and on days 2-3 of life: basal ganglia, 35.20°C ± 1.15°C, and white matter, 35.44°C ± 1.90°C) compared with neonates who did not develop brain injury (respectively, on day 1 of life: basal ganglia, 34.46°C ± 1.09°C, and white matter, 33.97°C ± 1.42°C; and on days 2-3 of life: basal ganglia, 33.90°C ± 1.34°C, and white matter, 33.07°C ± 1.71°C). Also, brain temperatures tended to remain slightly higher in the neonates who developed brain injury around day 10 of life and around 1 month of age.

CONCLUSIONS

Therapeutic hypothermia using current guidelines decreased the brain temperature of neonates with hypoxic-ischemic encephalopathy during the first days of life but did not prevent an early increase of brain temperature in neonates with hypoxic-ischemic encephalopathy who developed brain injury despite this treatment.

Nasopharynx is well-suited for core temperature measurement during hypothermia therapy

BACKGROUND

Rectal temperature is commonly used as the core temperature during therapeutic hypothermia therapy in neonates with hypoxic-ischemic encephalopathy (HIE). The purpose of this study was to examine whether nasopharyngeal temperature could serve as a substitute for rectal temperature.

METHODS

We prospectively investigated 40 neonates with HIE who underwent therapeutic hypothermia by selective head cooling, which involved cooling the body to 34°C for 72 h. During this period, nasopharyngeal temperature was measured and compared with rectal temperature every hour.

RESULTS

For 40 neonates included in this study, the mean rectal and nasopharyngeal temperatures were 34.3 ± 0.4°C (n = 2920) and 34.3 ± 0.4°C (n = 2920), respectively. Nasopharyngeal temperature strongly correlated with rectal temperature (R² = 0.623, P < 0.0001) and magnitude of the mean difference between nasopharyngeal and rectal temperature varied little during the 72 h of therapeutic hypothermia.

CONCLUSIONS

Nasopharyngeal temperature in neonates with perinatal HIE undergoing therapeutic hypothermia may be a suitable substitute for rectal temperature.

Post-Activation Brain Warming: A 1-H MRS Thermometry Study

Purpose

Temperature plays a fundamental role for the proper functioning of the brain. However, there are only fragmentary data on brain temperature (T_br) and its regulation under different physiological conditions.

Methods

We studied T_br in the visual cortex of 20 normal subjects serially with a wide temporal window under different states including rest, activation and recovery by a visual stimulation-Magnetic Resonance Spectroscopy Thermometry combined approach. We also studied T_br in a control region, the centrum semiovale, under the same conditions.

Results

Visual cortex mean baseline T_br was higher than mean body temperature (37.38 vs 36.60, P<0.001). During activation T_br remained unchanged at first and then showed a small decrease (-0.20 C°) around the baseline value. After the end of activation T_br increased consistently (+0.60 C°) and then returned to baseline values after some minutes. Centrum semiovale T_br remained unchanged through rest, visual stimulation and recovery.

Conclusion

These findings have several implications, among them that neuronal firing itself is not a major source of heat release in the brain and that there is an aftermath of brain activation that lasts minutes before returning to baseline conditions.

Brain temperature in neonates with hypoxic-ischemic encephalopathy during therapeutic hypothermia

OBJECTIVE

To noninvasively determine brain temperature of neonates with hypoxic-ischemic encephalopathy (HIE) during and after therapeutic hypothermia.

STUDY DESIGN

Using a phantom, we derived a calibration curve to calculate brain temperature based on chemical shift differences in magnetic resonance spectroscopy. We enrolled infants admitted for therapeutic hypothermia and assigned them to a moderate HIE (M-HIE) or severe HIE (S-HIE) group based on Sarnat staging. Rectal (core) temperature and magnetic resonance spectroscopy data used to derive regional brain temperatures (basal ganglia, thalamus, and cortical gray matter) were acquired concomitantly during and after therapeutic hypothermia. We compared brain and rectal temperature in the M-HIE and S-HIE groups during and after therapeutic hypothermia using 2-tailed t-tests.

RESULTS

Eighteen patients (14 with M-HIE and 4 with S-HIE) were enrolled. As expected, both brain and rectal temperatures were lower during therapeutic hypothermia than after therapeutic hypothermia. Brain temperature in patients with S-HIE was higher than in those with M-HIE both during (35.1 ± 1.3°C vs 33.7 ± 1.2°C; P < .01) and after therapeutic hypothermia (38.1 ± 1.5°C vs 36.8 ± 1.3°C; P < .01). The brain-rectal temperature gradient was also greater in the S-HIE group both during and after therapeutic hypothermia.

CONCLUSION

For this analysis of a small number of patients, brain temperature and brain-rectal temperature gradient were higher in neonates with S-HIE than in those with M-HIE during and after therapeutic hypothermia. Further studies are needed to determine whether further decreasing brain temperature in neonates with S-HIE is safe and effective in improving outcome.

The brain is hypothermic in patients with mitochondrial diseases

We sought to study brain temperature in patients with mitochondrial diseases in different functional states compared with healthy participants. Brain temperature and mitochondrial function were monitored in the visual cortex and the centrum semiovale at rest and during and after visual stimulation in seven individuals with mitochondrial diseases (n=5 with mitochondrial DNA mutations and n=2 with nuclear DNA mutations) and in 14 age- and sex-matched healthy control participants using a combined approach of visual stimulation, proton magnetic resonance spectroscopy (MRS), and phosphorus MRS. Brain temperature in control participants exhibited small changes during visual stimulation and a consistent increase, together with an increase in high-energy phosphate content, after visual stimulation. Brain temperature was persistently lower in individuals with mitochondrial diseases than in healthy participants at rest, during activation, and during recovery, without significant changes from one state to another and with a decrease in the high-energy phosphate content. The lowest brain temperature was observed in the patient with the most deranged mitochondrial function. In patients with mitochondrial diseases, the brain is hypothermic because of malfunctioning oxidative phosphorylation. Neuronal activity is reduced at rest, during physiologic brain stimulation, and after stimulation.

Functional craniology and brain evolution: from paleontology to biomedicine

Anatomical systems are organized through a network of structural and functional relationships among their elements. This network of relationships is the result of evolution, it represents the actual target of selection, and it generates the set of rules orienting and constraining the morphogenetic processes. Understanding the relationship among cranial and cerebral components is necessary to investigate the factors that have influenced and characterized our neuroanatomy, and possible drawbacks associated with the evolution of large brains. The study of the spatial relationships between skull and brain in the human genus has direct relevance in cranial surgery. Geometrical modeling can provide functional perspectives in evolution and brain physiology, like in simulations to investigate metabolic heat production and dissipation in the endocranial form. Analysis of the evolutionary constraints between facial and neural blocks can provide new information on visual impairment. The study of brain form variation in fossil humans can supply a different perspective for interpreting the processes behind neurodegeneration and Alzheimer’s disease. Following these examples, it is apparent that paleontology and biomedicine can exchange relevant information and contribute at the same time to the development of robust evolutionary hypotheses on brain evolution, while offering more comprehensive biological perspectives with regard to the interpretation of pathological processes.

Dynamic and permissive roles of TRPV1 and TRPV4 channels for thermosensation in mouse supraoptic magnocellular neurosecretory neurons

The transient receptor potential vanilloid 1 and 4 genes (trpv1, trpv4) encode temperature-sensitive cation channels hypothesized to mediate thermoresponses in mammalian cells. Although such channels were shown to participate in the peripheral detection of ambient temperature, the specific roles of these channels in central thermosensory neurons remain unclear. Here we report that the membrane potential and excitability of mouse magnocellular neurosecretory cells (MNCs) maintained at physiological temperature were lowered in an additive manner upon pharmacological blockade, or genetic deletion, of trpv1 and trpv4. However extracellular recordings from spontaneously active MNCs in situ showed that blockade or genetic deletion of trpv4 does not interfere with thermally induced changes in action potential firing, whereas loss of trpv1 abolished this phenotype. These findings indicate that channels encoded by trpv4 play a permissive role that contributes to basal electrical activity, but that trpv1 plays a dynamic role that is required for physiological thermosensation by MNCs.

Cortical energy demands of signaling and nonsignaling components in brain are conserved across mammalian species and activity levels

The continuous need for ion gradient restoration across the cell membrane, a prerequisite for synaptic transmission and conduction, is believed to be a major factor for brain's high oxidative demand. However, do energy requirements of signaling and nonsignaling components of cortical neurons and astrocytes vary with activity levels and across species? We derived oxidative ATP demand associated with signaling (P(s)) and nonsignaling (P(ns)) components in the cerebral cortex using species-specific physiologic and anatomic data. In rat, we calculated glucose oxidation rates from layer-specific neuronal activity measured across different states, spanning from isoelectricity to awake and sensory stimulation. We then compared these calculated glucose oxidation rates with measured glucose metabolic data for the same states as reported by 2-deoxy-glucose autoradiography. Fixed values for P(s) and P(ns) were able to predict the entire range of states in the rat. We then calculated glucose oxidation rates from human EEG data acquired under various conditions using fixed P(s) and P(ns) values derived for the rat. These calculated metabolic data in human cerebral cortex compared well with glucose metabolism measured by PET. Independent of species, linear relationship was established between neuronal activity and neuronal oxidative demand beyond isoelectricity. Cortical signaling requirements dominated energy demand in the awake state, whereas nonsignaling requirements were ∼20% of awake value. These predictions are supported by (13)C magnetic resonance spectroscopy results. We conclude that mitochondrial energy support for signaling and nonsignaling components in cerebral cortex are conserved across activity levels in mammalian species.