Salivation, also known as drooling, occurs when there is excess saliva in the mouth over the edge of the lips[1, 2]. This is normal in infants, but drooling gradually decreases as development matures and usually disappears at approximately 15 to 18 months [3]. Sialorrhea can cause embarrassment, isolation, depression, and skin infection and can lead to pneumonia because saliva is pooled in the mouth. Many approaches, such as physiotherapy, aspiration, medication, radiotherapy and surgical intervention, have been used to manage sialorrhea [4]. However, these are symptomatic treatments, and there is no universally accepted treatment.
The advent of Botulinum toxin (BoNT) injections seems very promising. BoNT was initially thought to be the cause of food poisoning [5]. At the end of the 18th century, a number of disease outbreaks were linked to sausages in southwestern Germany [6]. In 1977, Allan Scott first injected a patient with strabismus [7]. Since then, BoNT has been increasingly used in clinical practice. Later, BoNT was also considered the treatment of choice for sialorrhea. Studies have shown that Rimabotulinum toxin B treatment of sialorrhea improves the unstimulated salivary flow rate and the clinical global impression of change [8]. A meta-analysis showed that BoNT types A and B significantly reduce salivation in patients with neurological disorders [9]. Growing research shows the potential of BoNT treatment for sialorrhea.
The effects of BoNT injections with sialorrhea have also been known for years, but their use was limited because of a lack of approval studies [10]. There are differences in the types and doses of Botulinum toxin used in different countries, such as the USA and Europe. OnabotulinumtoxinA and AbobotulinumtoxinA are off-label treatments for sialorrhea. A meta-analysis showed that the recommendation level of BoNT treatment for sialorrhea in adults and children is different [11]. BoNT remains a controversial and attractive drug. This may be related to do with the etiology and mechanism of sialorrhea. Sialorrhea is more common in Parkinson's disease (PD), cerebral palsy, stroke, a side effect of medications, amyotrophic lateral sclerosis, schizophrenic and intellectual disease. Sialorrhea may be due to increased salivary secretion, failure to clear saliva in the mouth, and miscoordination of the oral-facial and palatoglossus muscles [2]. Therefore, the sialorrhea mechanism caused by each disease may be different, so the efficacy and adverse effects of BoNT treatment may be different.
As the population ages, the burden of PD is increasing, seriously affecting people's activities of daily living [12]. PD is the second most common neurodegenerative disorder, affecting approximately 1% of adults older than 60 years [13–16]. An estimated 6.2 million people worldwide suffer from PD [12, 17]. As research continues, our understanding of PD continues to evolve. Initially, we recognized that it was typically characterized by motor symptoms, such as rigidity, bradykinesia, and resting tremor [18]. Motor symptoms are key to diagnosing PD [19], but non-motor symptoms (NMS) of PD are common and often overlooked, such as sialorrhea, dementia, depression, and sleep disorders [20]. NMSs are prevalent in over 90% of PD patients [21], and there are more women than men [22]. NMSs are common in early PD, reflecting that the disease damages multiple systems [23]. Some NMSs have a greater impact on the quality of life of people with PD [24]. The most common in the early stages of PD is excessive saliva [23]. A large number of patients with PD undergo sialorrhea, ranging from 32–74% [25]. Studies show that more than 80% of patients with PD experience sialorrhea [26, 27]. We initially thought that Parkinson's patients had excessive salivary production, but studies have found that Parkinson's patients have less salivary production than normal people [28–30]. Sialorrhea can be caused by excessive salivation, difficulty swallowing, or both [29, 31]. Some studies have shown that sialorrhea is thought to be due to dysphagia that reduces the effective removal of saliva rather than excessive drooling [32]. Some researchers have classified sialorrhea as gastrointestinal dysfunction or autonomic dysfunction [33, 34]. The pathogenesis of sialorrhea in PD remains controversial. Is BoNT injection safe and effective in the treatment of Parkinsonism
Currently, injections of BoNT reduce salivary production by blocking acetylcholine in the corresponding glands. A systematic review showed that BoNT is an effective method for treating sialorrhea with PD [35]. There was also study that reported no statistical significance of BoNT in the treatment of sialorrhea caused by PD [36]. Safety reports of BoNT in the treatment of sialorrhea in PD vary widely. Most randomized controlled trials of botulinum toxin for the treatment of salivation inPD have had small samples and low levels of evidence. There has also been no meta-analysis of the safety of BoNT in the treatment of sialorrhea in PD. To compare the results of prior studies of BoNT injection, we included only randomized controlled trials and crossover trials to meta-analyze the efficacy and safety of BoNT in the treatment of sialorrhea in PD. This meta-analysis was conducted according to the 2020 PRISMA statement [37]. Our review protocol was registered on PROSPERO (42021288334).