Bladder calculi are a common and frequently occurring disease in urology, and their incidence rate exhibits regional and dietary cultural differences(5, 6). Bladder calculi can be divided into primary and secondary; the latter often appears secondary to upper urinary tract stones, obstructive disease, and urinary tract infection, and the former is defined as children’s bladder calculi without the above conditions(7). Primary bladder calculi are often accompanied by malnutrition and are related to socio-economic conditions(7). At present, the living standards and environment of Chinese people have improved, and the incidence of primary bladder calculi has declined. However, secondary bladder calculi have become common. This study reported that the proportion of primary and secondary bladder calculi was high at 1:10. We believe that this high proportion may be related to the poor economic level and population structure in Southern Hunan, that is, the region is still dominated by economically backward rural areas. In this study, the urban population accounted for 38.6%, and the rural population accounted for 61.4%. Therefore, understanding the role of socio-economic factors in each region in primary bladder calculi is crucial to establish a comprehensive prevention and health care strategy.
In this study, the incidence of bladder calculi in women in Southern Hunan was 4.0%, which was similar to that reported in literature(8). Rare female bladder calculi generally present other abnormal factors, such as common iatrogenic factors including residual fragments of bladder catheter balloon rupture, ectopic bladder with contraceptive rings, radical resection of rectal cancer, or total hysterectomy(9). All of the above factors were present in the female patients with bladder calculi in this study. Therefore, for female patients with bladder calculi, the medical staff should understand other latent or hidden iatrogenic or non-iatrogenic factors besides the treatment of the stones.
With the development of endourology, transurethral cystoscopic lithotripsy has become the first choice for the treatment of bladder calculi. In this study, almost all cystolithiasis were treated by intracavitary minimally invasive surgery (97.7%). However, surgery may lead to collateral damage or increased cost. Preventing bladder calculi should be the priority of clinicians. Stone composition is the basis for stone prevention and drug dissolution treatment and can be analyzed using two methods: physical and chemical. Among which, infrared spectroscopy is the most definite and widely used technique. We analyzed the composition of bladder calculi by infrared spectroscopy and observed five categories from 352 cases, namely, CaOx, Apa, UA, Str, and Cys. At present, CaOx stones are the most common among bladder calculi in Southern Hunan, China. This finding is consistent with previous studies. We also found that CaOx + Apa stones were the most common mixed stones, UA stone was the most common single stone, and the number of pure UA stone was significantly higher than that of pure COM stone. In addition, the incidence of UA stones increased with age. Therefore, the increasing incidence of bladder calculi in recent years may be related to the aging population. For the compositions of bladder calculi, the pure UA stones were significantly larger than the pure COM stones. UA stones are X-ray negative stones and are often accompanied by hyperuricemia(10). When the large and single stone found in the bladder is not transparent to X-ray or accompanied by hyperuricemia, it can be preliminarily diagnosed as UA stone to serve as a reference for the medical treatment.
Bladder calculi are often accompanied by prostate hyperplasia or secondary to upper urinary tract stones. The probable reason is the poor excretion of urine caused by stones, leading to urinary tract infection and bladder inflammation. Infection further obstructs the bladder outlet, and benign prostatic hyperplasia can cause bladder outlet obstruction, which will undoubtedly aggravate urinary tract infection and bladder inflammation. Therefore, infection, obstruction, and bladder stones form a vicious circle(11). In this study, bladder calculi with prostatic hyperplasia accounted for 78.4%, and oligolithiasis accounted for 38.1%. Most of the patients were older than 60 years old, which may be caused by the different degrees of benign prostatic hyperplasia in the male members of this age group of secondary bladder stones.
Etiological tracing is important in the context of high bladder calculus incidence. The potential relationship between kidney stones and metabolic syndrome has been extensively reported(12, 13). Metabolic syndrome includes obesity, diabetes, hypertension, and dyslipidemia. Kohjimoto et al.(12) reported a correlation between the amount of metabolic syndrome and the severity of kidney stones and found a 1.8-fold higher probability of recurrence or multiple stones compared with that in non-metabolic syndrome patients. Arias et al.(3) came to a similar conclusion. Possible causes were the increased urinary calcium, uric acid, and oxalic acid excretion and decreased urinary citrate excretion in patients with metabolic syndrome(12, 14). Hypertension, one of the “metabolic syndromes”, has been studied in relation to kidney stone formation. Lee et al.(13) believes that hypertension is the most significant risk factor for metabolic syndrome in patients with kidney stones. In their prospective study of hypertension and upper urinary tract stones, Cappuccio et al.(15) conducted 8 years of follow-up in hypertensive patients without upper urinary tract stones and found the relationship between hypertension and kidney stones. After the 8 years of follow-up, 10.3% of the patients developed kidney stones. However, some scholars hold the opposite opinion. Madore et al.(16) reported that the onset of nephrolithiasis appears to be independent of hypertension. In the present work, up to 26.1% of the patients had hypertension. Further binary logistic regression analysis showed that hypertension may be a risk factor for bladder stones. This conclusion is plausible, but our interpretation is speculative because of data unavailability for 24 h urinary calcium, sodium, and citric acid levels. However, our results suggested a potential relationship between hypertension and bladder calculus pathogenesis. The present work provides a theoretical basis for further bladder calculi prevention and recurrence and emphasizes regular health checks and interventions such as active blood pressure control. Most previous studies focused on the relationship between hypertension and kidney stones. Given that bladder calculi are a type of urolithiasis, their possible relation to hypertension similar to kidney stones must be further investigated. Despite its potential limitations, this study provides a way of understanding the pathogenesis of bladder calculi and suggests that hypertension, one of the metabolic syndromes, likely plays an important role in bladder stones.
This study has many limitations and deficiencies, including the small sample size, single center retrospective design, and inadequate data of observation and follow-up. Future investigation with a large sample size, multicenter design, and sufficient observation index of the cohort is needed to further verify hypertension as an independent risk factor for bladder calculi.