Introduction

Magnetic resonance imaging (MRI) offers advantages such as accessibility, contrast adaptability, pathophysiologic specificity, and the potential for repeated studies without adverse health effects, and has thus become a method of choice in clinical practice, especially when a powerful imaging method for the brain is needed. Magnetic resonance spectroscopy (MRS), a newer modality, is the most widely available noninvasive method to evaluate cellular metabolism and monitor neurometabolic disorders [ 1 ].

However, MRS is limited to the analysis of specificregions of interest in areas much larger compared to those amenable to the level of resolution provided by MRI (typically 1–10 cm³ for MRS vs. 1–10 mm³ for MRI). An MRS spectrum is a group of peaks at different radiofrequencies, which show proton nuclei in various chemical environments. In these spectra, the area of resonance is considered relative to the chemical concentration [ 1 ].

Because it is able to provide information about the chemical composition of the brain, MRS can be used to evaluate neural systems [ 2 ]. This technique can usually detect small molecules at concentrations of 0.5–10 mM within cells or extracellular spaces. Magnetic resonance spectra thus provide information about metabolite concentrations, making MRS a useful procedure to monitor metabolic fluctuations due to disease, and track treatment efficacy. Several metabolites can be detected with various nuclei for spectroscopy, such as ¹H, ³¹P, ¹⁹F, ¹³C and ²³N. Hydrogen (¹H) MRS(¹HMRS) is the main method in biomedicine because of its sensitivity, availability, and the presence of readily detectable¹H nuclei in most metabolites [ 3 ]. Accordingly, ¹HMRS has currently been a well-known noninvasive method to quantify brain metabolite concentrations in the living individuals, and become a powerful assessment tool for many pathologic conditions [ 4 ].

Spectra obtained with ¹HMRS can reveal biochemical variations associated with pain states. Because the pain experience in patients is too complex to be readily quantified, both research and the management of pain are challenging. Currently this gap can be partially filled with neuroimaging techniques. For example, ¹HMRS studies have shown that treatment can decrease glutamate levels in the insula of patients with fibromyalgia. This alteration in glutamate levels was related with clinical changes and functional MRI findings in these patients [ 5 ].

In painful conditions such as low back pain [ 6 , 7 ], complex regional pain syndrome [ 8 ] and neuropathic spinal cord injury pain [ 9 ], biochemical changes take place in the brain. Siddall et al., demonstrated that MRS spectra obtained in the anterior cingulate cortex, thalamus and prefrontal cortex may distinguish patients with low back pain from healthy people with accuracies of 100%, 99%, and 97%, respectively [ 10 ].

In chronic headaches, electrophysiological and neuroimaging studies have revealed changes in brain excitability, biochemistry, function, and structures. Nevertheless, according to Lai et al., “it remains undetermined whether these common features of neural plasticity can be regarded as neurologic signatures for chronic headaches” [ 11 ].

In the present study, we aimed to identify correlations between clinical symptoms, including trigger point count, headache frequency and intensity, anxiety and depression in patients with tension-type headaches (TTH), i.e. the most prevalent type of headache [ 12 ], and the concentration of brain metabolites in the anterior cingulate cortex, thalamus and primary somatosensory cortex.

Material and Methods

Participants

In this observational study, the participants were 24 patients (4 men and 20women) with chronic TTH, recruited among patients referred to the Imam Reza neurology clinic for their headaches. Patients with a diagnosis of chronic TTH and any trigger points in their posterior cervical muscles were included in the study; patients were excluded if they had cervical disk herniation or any neurological or rheumatoid disorder, and if they were using opioid prophylaxis, antidepressant oranti-anxiety drugs, or if they were pregnant or breastfeeding. All participants signed a consent form before entering the study.

Procedure

To evaluate clinical symptoms, the number of trigger points was recorded for each participant, along with self-reported intensity and frequency of headaches during the previous month. Intensity was recorded as a number between 0 and 10 on a numeric pain scale that indicated average intensity during the previous month; frequency was recorded as the number of days per month when headaches occurred. The levels of anxiety and depression were recorded with reliable, validated instruments, i.e. the Beck Anxiety Inventory (BAI) and Beck Depression Inventory II (BDI-II) [ 13 , 14 ]. The BAI and BDI- II instruments both consist of 21 items evaluating the level of anxiety and depression. The total score ranges from 0 to 66, with higher scores indicating greater anxiety and depression.

To evaluate neural plasticity, the concentration of N-acetyl aspartate (NAA), total choline (tCho), creatine (Cr), myo-inositol (M-Ino), and glutamate and glutamine (GLX) were determined in the anterior cingulate cortex, thalamus and primary somatosensory cortex of the left hemisphere with ¹HMRS. Single voxel spectroscopy was done at 1.5 tesla (Magnetom Avanto version B19, Siemens, Germany) with a standard 12-channel circular head coil and a conventional PRESS sequence (TR/TE= 1500/30; NSA= 128). Metabolite concentrations were expressed as ratios to the concentration of Cr.

Statistical analysis

The sample size for this study was calculated as 24 patients based on a previous study [ 15 ] (α= 0.05, β= 0.2). All analyses were performed using SPSS v.21 software. A significance level of p < 0.05 was used for all analyses. Normal distribution of data was verified by the Kolmogorov–Smirnov test. Pearson product–moment correlation coefficients were calculated to evaluate the relationship between clinical symptoms of TTH and concentrations of brain metabolites.

Results

Descriptive data for the participants are presented in Table 1. The mean values indicate that headache intensity was moderate and the levels of depression and anxiety were mild.

The findings of the study are summarized in Table 2. Overall, there was a negative correlation between trigger point count and Cho/Cr ratio in the primary somatosensory cortex [r= −0.509, n= 24, p= 0.01]. Higher numbers of trigger points correlated with lower Cho/Cr ratios. There were no correlations between the other clinical symptoms of TTH and brain metabolite concentrations.

Discussion

Our results showed a negative correlation between trigger point count and Cho/Cr ratio in the primary somatosensory cortex. Other clinical symptoms of TTH, including headache frequency and intensity, anxiety and depression were not associated with metabolite levels. These results are consistent with findings published by Petrou et al., in 2008. These authors showed that there were no correlations between anxiety, depression or pain catastrophizing scale score and metabolite concentrations in patients with fibromyalgia; however, they found that Cho/Cr ratio was positively correlated with the patients’ pain level [ 16 ].

Choline concentration may reflect cellular attenuation, membrane synthesis, and/or cell numbers in brain tissue. Changes in Cho/Cr ratio may indicate altered brain metabolism. Hyper-osmotic or hypo-osmotic conditions may also be reflected by choline alterations. Furthermore, choline is the precursor of acetylcholine [ 1 , 16 , 17 ]. Nociceptive inputs from trigger points may affect brain metabolism, osmotic condition and/or the amount of acetylcholine in the brain. In the present study, lower Cho/Cr ratios were associated with higher numbers of trigger points. This association suggests that the incidence or existence of trigger points is related to changes in brain metabolic activity. It can be hypothesized that the transformation of choline to acetylcholine influences the development of trigger points. Considering that acetylcholine modulates synaptic transmission and increases the signal to noise ratio in the cortex by suppressing inputs from environmental stimuli that do not need an immediate reaction, it is plausible that choline conversion to acetylcholine is a mechanism over riding the prolonged nociceptive inputs from trigger points to the central nervous system [ 18 ]. Besides, confirmation of this hypothesis needs further study.

It has been suggested that alterations in brain metabolites are involved in the pathogenesis of mood disorders such as anxiety and depression [ 19 - 22 ]. In patients with depression, blood flow and neuronal energy consumption are reduced, and neurotransmitter systems in the brain altered [ 19 ]. The role of altered glutamate activity in the pathophysiology of depression was recently confirmed [ 23 ]. In patients with anxiety, changes in brain function and structure as well as metabolic abnormalities are known to occur [ 24 , 25 ].In this connection, Harper et al. found a correlation between choline concentration and negative mood in patients with chronic pelvic pain syndrome [ 26 ]. Unique advantages of ¹HMRS are the ability to provide important quantitative biochemical information in localized brain areas, and to document brain metabolic activity efficiently in people with mood disorders [ 19 , 25 ]. In addition, an association has been reported between chronic headaches and central nervous system alterations [ 11 ]. The lack of correlation between headache characteristics and patients’ mood in the present study may be ascribed to the mild level of mood disorders in our participants. Moreover, investigating larger samples and populations may yield different results.

In this study, we investigated only the left hemisphere of the brain because of the high cost of MRS. Assessments in both hemispheres and other brain centers hold the potential to provide more information in future study.

Conclusion

In patients with TTH, the number of trigger points in cervical muscles correlated negatively with Cho/Cr ratio in the primary somatosensory cortex and patients with more trigger points had lower Cho/Cr ratios. However, there were no correlations between other brain metabolite concentrations and headache frequency, headache intensity, anxiety or depression.

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