This is the first study investigating whether cfDNA fragment size and level in LDD patients are associated with various aspects of pain, such as LBP. Elevated cfDNA levels have been reported to be associated with a variety of pathological processes [8–10]. For example, an association between cfDNA and pain has been reported in patients with sickle-cell disease [11]. That study reported that cfDNA levels in patients' plasma were higher during acute painful episodes with steady-state levels.
Against this background, we speculated that the cfDNA fragment size and levels in LDD patients might also vary depending on the degree of pain. We first compared cfDNA fragment size and levels in LDD patients and healthy controls. cfDNA fragment size tended to be shorter in patients than in controls, but this difference was not significant. Previous reports on cfDNA in cancer patients showed that cfDNA size is shorter in fragments originating in tumor cells than in those from nonmalignant cells [12–14]. An increased number of short cfDNA fragments may reflect an increase in tumor DNA level relative to nontumor DNA in the blood of cancer patients. It is thought that DNA methylation may affect cfDNA size [15, 16]. However, the mechanism responsible for the shorter cfDNA fragment size is not well understood.
We also found that cfDNA level was higher in patients with LDD than in healthy controls. Interestingly, the cfDNA level correlated positively with some NRS scores. It is difficult to treat LN in patients with LDD, and postoperative symptoms may remain, and the lack of an index exacerbates this problem to evaluate pain objectively. Our results suggest that the cfDNA level may help in quantifying LBP and LP, and LN.
Subsequently, we investigated whether spinal surgery's invasiveness affects cfDNA fragment size and level by measuring these before and after surgery in LDD patients. We found no significant change in cfDNA fragment size from before to after surgery, although cfDNA level increased postoperatively in all patients. A previous study reported that lactate dehydrogenase (LDH), an enzyme found in various living cells, functions as a biomarker of injury and disease [17]. That study found that elevated cfDNA levels and elevated LDH levels were associated with tissue damage, directly associated with pain [18]. Patients were receiving chemotherapy exhibit significant increases in cfDNA levels 24 h and eight days after chemotherapy [19] and several cycles after chemotherapy [20]. These data led us to investigate whether the invasiveness of spinal surgery would be related to an increase in cfDNA level. In recent years, minimally invasive spinal surgery has attracted much attention. However, surgical time, bleeding loss, length of hospital stay, and postoperative CRP levels are often used to evaluate surgical invasiveness [21]. Surgical procedures induce a complex stress response proportional to the magnitude of the injury, operating time, and intraoperative blood loss. The adverse metabolic and hemodynamic effects of this stress response can cause many problems during the perioperative period. Thus, decreasing the stress response to surgery is a key factor for improving clinical outcomes.
Circulating cfDNA in the blood may be helpful for objectively assessing the body's response to an invasive procedure such as surgery, and its measurement may have potential value for diagnosis and prognosis. Our results suggest that cfDNA level may be a new biomarker for assessing the invasiveness in spinal surgery. A previous report noted that the extent to which postoperative CRP level is elevated seems to depend upon the severity of the procedure [22]. However, there was no correlation between cfDNA level and postoperative CRP level. Further research is needed to clarify whether cfDNA level is related to CRP level and postoperative pain.
A limitation of this study is the small sample size in both the patient and control groups. However, the post hoc power analysis at the cfDNA level was 0.8 or higher between LDD patients and the healthy controls. In addition, the ages of the two groups differed significantly. However, it is difficult to obtain samples from patients with an average age of 70 who do not experience pain because many older people have some form of locomotor disorder. Another limitation is that we did not compare cfDNA fragment size and levels for different surgical procedures. To evaluate surgical invasiveness more concretely, we think it is necessary to compare cfDNA size and level between decompression surgery and fusion surgery or traditional open surgery and minimally invasive spinal surgery. Additionally, comorbidities can affect the cfDNA fragment size and level; to reduce the possible bias. It would help control for comorbidities in future studies. Finally, we have not investigated the role of cfDNA in predicting symptom response to treatment and sustainability in the long term.