Report on the Use of MR Spectroscopy to Assess Muscle Mitochondrial Dysfunction in Long COVID

Academic Background

The COVID-19 pandemic has had a profound impact on global health, with many patients experiencing prolonged symptoms even after recovery, commonly referred to as “Long COVID” or “Post–COVID-19 Condition” (PCC). These symptoms include fatigue, breathlessness, and brain fog, significantly affecting patients’ quality of life. Although previous studies have suggested that mitochondrial dysfunction may be related to fatigue following viral infections, in vivo research on mitochondrial function in Long COVID patients remains limited.

Mitochondria are the primary sites of cellular energy production, and their dysfunction can lead to insufficient energy supply, thereby causing symptoms such as fatigue. Additionally, mitochondria play a crucial role in antiviral immune responses. Early studies found that the SARS-CoV-2 virus can disrupt mitochondrial antiviral defense mechanisms and interfere with key mitochondrial processes such as oxidative phosphorylation, ultimately leading to cell death. Recent research further suggests that COVID-19 infection may inhibit the transcription of genes related to mitochondrial oxidative phosphorylation, activating glycolysis and immune stress responses, which could be a contributing factor to fatigue in Long COVID patients.

Based on this background, this study aimed to compare mitochondrial function between Long COVID patients and healthy controls using Magnetic Resonance Spectroscopy (MRS) and to explore the relationship between MRS parameters and fatigue symptoms.

Source of the Paper

This paper was authored by Dr. Lucy E. M. Finnigan, Dr. Mark Philip Cassar, and others from the Oxford Centre for Clinical Magnetic Resonance Research (OCMR) and published in the December 2024 issue of Radiology. The study was funded by AXCella Therapeutics, the National Institute for Health Research (NIHR), and other institutions.

Research Process

Study Participants and Inclusion Criteria

This prospective, observational, single-center study recruited 41 Long COVID patients and 29 healthy controls. Inclusion criteria for Long COVID patients included: age between 18 and 65 years, confirmed COVID-19 infection via PCR test, antibody test, or general practitioner diagnosis, and no reinfection for at least three months post-infection. Patients were required to report moderate to severe fatigue (assessed using the Chalder Fatigue Questionnaire) and were excluded if they had other conditions that could cause fatigue, such as severe anemia, hypothyroidism, or diabetes. Healthy controls had no history of COVID-19 infection and no fatigue symptoms.

Experimental Methods

All participants underwent 1H and 31P magnetic resonance spectroscopy to measure metabolic changes in the gastrocnemius muscle during exercise and recovery. The experimental steps were as follows:

1. 1H Magnetic Resonance Spectroscopy: A 3T MRI system was used to localize the region of interest in the medial gastrocnemius muscle using the Stimulated-Echo Acquisition Mode (STEAM) sequence, measuring metabolites such as intramyocellular lipids (IMCL), acetylcarnitine, and carnosine.
2. 31P Magnetic Resonance Spectroscopy: The Depth-Resolved Surface Coil Spectroscopy (DRESS) sequence was used to measure parameters such as phosphocreatine (PCr), inorganic phosphate (Pi), and pH in the gastrocnemius muscle during exercise and recovery. Participants rested for 1 minute, followed by 2–5 minutes of plantar flexion exercise, with exercise duration adjusted based on individual ability.

Data Analysis

The Oxford Spectroscopy Analysis Toolbox (OXSA) was used for spectral fitting, and the Advanced Method for Accurate, Robust, and Efficient Spectral Fitting (AMARES) was implemented in MATLAB. Linear mixed models were used to compare dynamic metabolic parameters between the two groups, and Pearson correlation analysis was used to assess the relationship between MRS parameters and fatigue scores.

Key Results

Participant Characteristics

The mean age of Long COVID patients was 44 years, while that of healthy controls was 34 years. The mean fatigue score of Long COVID patients was 29 (Likert scale), indicating moderate to severe fatigue.

Magnetic Resonance Spectroscopy Analysis

1. 1H Spectroscopy Results: No significant differences were found between Long COVID patients and controls in IMCL, creatine, or acetylcarnitine levels. However, carnosine levels were significantly higher in controls (mean difference: 1.15 mmol/L, p = 0.007).
2. 31P Spectroscopy Results: Long COVID patients had significantly higher resting phosphocreatine levels than controls (mean difference: 4.10 mmol/L, p = 0.03). Post-exercise, Long COVID patients had a significantly prolonged phosphocreatine recovery time constant (τPCr) (92.5 seconds vs. 51.9 seconds, p < 0.001) and a significantly lower maximum oxidative flux (Qmax) (mean difference: 0.16 mmol/L/s, p = 0.008).

Relationship Between Fatigue Scores and MRS Parameters

No significant correlations were found between MRS parameters and fatigue scores (r ≤ 0.25, p ≥ 0.10).

Conclusion

This study found significant differences in mitochondrial function-related parameters, such as phosphocreatine recovery time constant and maximum oxidative flux, between Long COVID patients and healthy controls, suggesting potential mitochondrial dysfunction. However, these changes were not significantly correlated with the severity of fatigue symptoms. These findings provide new insights into the pathophysiology of Long COVID and offer potential biomarkers for future therapeutic trials targeting mitochondrial function.

Research Highlights

1. Key Findings: Long COVID patients exhibited metabolic abnormalities related to mitochondrial dysfunction, such as prolonged phosphocreatine recovery time and reduced maximum oxidative flux.
2. Methodological Innovation: This study is the first to use multinuclear magnetic resonance spectroscopy (1H and 31P MRS) to assess mitochondrial function in Long COVID patients in vivo.
3. Clinical Significance: The results provide a new explanation for the pathological mechanisms of Long COVID and offer a basis for developing treatment strategies targeting mitochondrial dysfunction.

Additional Valuable Information

Although this study did not find a correlation between MRS parameters and fatigue scores, the researchers noted that the heterogeneity of Long COVID patients may be a contributing factor. Future studies should consider longitudinal assessments and incorporate other biomarkers and imaging techniques to provide a more comprehensive understanding of the pathological mechanisms of Long COVID.

---

Detailed data and analysis methods from this study can be found in the supplementary materials of the Radiology journal.