New technologies for diagnostics and therapy significantly extended the average lifespan of worldwide population, with remarkable improvement in quality of life. Along with the reduction of morality rates and consequent increase of life expectancy, geriatric individuals represent the fastest growing part of the population [1]. In 2019, approximately 7 million people over the age of 65 presented at least three chronic diseases, with almost half of them suffering from one severe pathology [2], such as diseases of the heart, malignant neoplasms, cerebrovascular diseases, chronic obstructive pulmonary disease, pneumonia, diabetes mellitus, Alzheimer’s disease, renal diseases, septicemia, liver diseases, and Parkinson’s disease (PD).

When the risk for developing these conditions increases with longevity, a cascade effect of morbidity is established and even treatments impair other organ system, causing susceptibility to further problems [2]. The presence of multi-morbidity negatively affects the autonomy of elderly patients in daily life activities. While physical implications related to normal aging are generally perceived, cognitive impairment is usually slow and insidious: one of the most frequent and disabling neurodegenerative pathologies, second to Alzheimer’s disease, is PD. From 1990 to date, the overall number of people affected by PD reached 9.4 million: this estimation is expected to double [3] within 2040, with a prevalence of 1% for patients aged over 60 and of 4% for older age-groups [4].

PD is characterized by a slow and progressive evolution, which causes the deterioration of dopaminergic neurons of the substantia nigra, leading to severe impairment at both motor (bradykinesia, resting tremor, rigidity, postural instability, and difficulty in walking) and non-motor (neuropsychiatric dysfunctions, such as depression, dementia, and apathy) level. As phenotypic differences make PD complex to manage [5], its onset and progression have an impact on daily activities, leading to scarce compliance with oral hygiene and increasing the risk for the main oral pathologies, that are caries and periodontitis. Furthermore, missing regular dental appointments results in decline of oral health and motivation [6], for the interference of individual changes (such as psychiatric conditions) with proper dental care.

Current therapies for PD are symptomatic treatments which slow down or restore the affected dopaminergic neurons [7]: specific drugs, taken alone or in combination with other, are however linked to oral side effects, e.g., xerostomia, bruxism, dry throat, gingivitis, lingual edema, dysgeusia and glossitis [8–10]. On the other hand, as gradual disability affects ordinary gestures, including patients’ home care procedures, poor oral hygiene is usually associated with a greater number of decayed and extracted teeth, halitosis, and periodontal inflammation.

Recent systematic reviews focus on: (i) factors associated with oral health problems [11], mainly caries, periodontal inflammation and prosthetic needs, and less frequently orofacial pain and dysfunction; (ii) on the role of health care providers [12]. As interdisciplinary approach is recommended to overcome difficulties of dental treatments, a complete overall assessment of oral conditions in PD patients can be relevant in carrying out procedures safely [13].

In the light of these considerations, the aim of this review was to investigate overall oral conditions in patients with PD, underlying that it is fundamental to focus on their limited manual skills in daily life, providing tailored supportive oral care instructions.

A number of 226 articles were obtained from the initial search, 84 in PubMed, 69 in Cochrane Library and 73 in Scopus. After removing duplicates, 200 were evaluated. After following the study selection procedure, 16 were included (see Figure 1).

Patient with PD generally present a lower oral health related quality of life (OHRQoL) compared to healthy individuals [25], due to xerostomia, dysphagia and sialorrhea, which alter the salivary clearance, negatively impact the microflora and cause halitosis [37]. Xerostomia is one of the most common oral manifestations in PD patients, found in 55% of cases [10], implying a decrease of saliva quantity and quality, linked to the disease itself or the drugs taken. Capacity of remineralising/buffering and antimicrobial power therefore determine carious lesions, periodontal pockets, as well as oral ulcers and problems with retention of prostheses [8, 10]. Furthermore, a reduced mobility and rigidity of the muscles, which promote the self-cleansing function, leads to infections and inflammation [34], with proliferation of a distinct microbiota in both saliva and subgingival plaque compared to healthy controls [26]. While dysphagia causes weight loss, social distancing (the patient prefers to isolate himself while eating), problems in taking medications, and acid reflux, with enamel erosion and dentin hypersensitivity [9], sialorrhea is connected to impaired swallowing and reduced lip closure.

Concerning oral habits, outcomes examined in this review revealed for all studies an average frequency of brushing of at least twice a day for PD patients, while non homogenous data are reported for brushing technique used, with authors declaring a prevalent use of the electric toothbrush [36], vs. authors highlighting a prevalent use of the manual one [27]. Discrepancies between studies regarding brushing methods could be related to different samples considered, as PD patients were not always compared with healthy controls, and to different duration and severity of disease assessed. However, authors suggest that clinical recommendations should be driven by the necessity of simplify manoeuvres: electric device can represent a valid option in this sense, especially in case of severe phase of disease.

Use of interdental devices in patients with PD seems to be lower compared to healthy patients in all articles examined, with a mean difference of approximately 15% between groups [27]. Due to evident difficulties at home beside they are aware of the need for good oral conditions [38], lack of adequate interdental cleaning easily leads to accumulation of biofilm, which was reported in greater quantities for PD patients by several studies [25–27, 29, 34, 35]. Although frequency and modality of cleaning teeth can be superimposed with the comparison group, results did not show a sufficient plaque control, also for the presence of apathy, depression, and forgetfulness.

In addition, gingival inflammation, registered through the presence of bleeding and gingivitis, is a consequence of deficient oral hygiene. Most studies reported a predominant presence of gingivitis in PD patients compared to healthy ones [25, 29], with presence of bleeding in 57.9% of cases vs. 51.50% in the control group [26]. Incorrect oral hygiene, in terms of lack of quality (not frequency) of manoeuvres [11], usually promotes gingivitis, which, if prolonged, can evolve into periodontitis or worsen an already compromised periodontal status.

Being chronic periodontitis an infectious disease linked to a variety of systemic disorders [39], several studies suggest a potential connection between chronic periodontitis and neurodegenerative diseases, such as PD [40]. Individuals with PD indeed present a bacterial composition different from healthy people; at the same time, patients with chronic periodontitis seem to have a higher risk of developing multi-organ systemic complications and PD, which can be reduced through improvement of oral health and regular periodontal treatment [39]. Understanding the mechanisms of biofilm alteration, constant inflammation, or even the bacterial products blood-circulation, would be advisable for periodontitis prevention: it was reported that microorganisms typical of red complex, such as P. gingivalis, are involved in neuroinflammation. Oral manifestations can be thus useful for an earlier diagnosis of systemic pathologies, or can represent the first steps of eventual chronic conditions, especially considering the slow and insidious progression of PD [41, 42]. As the microbial profile is associated with cognitive status [26], decrease in quantity of saliva, specific bacteria found in increased proportions (e.g., S. Mutans, Veillonella, Lactobacillaceae, T. Forsythia, P. Intermedia, Fusobacterium, Capnocytophaga, C. Rectus) and worsened periodontal conditions all suggest an increased risk for periodontal disease. Besides that, periodontal bacteria produce endotoxins, which increase the inflammatory impact and induce cells to generate and release pro-inflammatory cytokines, hypothesized to be associated with the damage or apoptosis of dopaminergic cells [43]: in this way, chronic inflammatory response due to periodontal disease seems to be one of the etiological causes of PD.

All studies included in this review found a greater number of missing teeth, plaque accumulation, pronounced bone loss and increased periodontal pockets in PD patients. Specific investigations also demonstrated a greater number of pockets larger than 4 mm and of mobile elements in PD patients compared to control group [29, 35, 44]. The number of elements present in the oral cavity does not seem to differ between PD patients and healthy patients: nevertheless, a greater number of root residues was found in the test group, probably because dental treatments are more difficult to perform in PD patients [35]. Neglection of pathological clinical signs leads to mobility and final loss of the dental elements [45], with consequent difficulty in chewing and swallowing and a further worsening of the quality of life [46, 47]. In this terms, rehabilitation through the use of removable partial dentures can significantly decrease the scores of Oral Health Impact Profile-49 (OHIP-49) [32], a tool employed for measuring the effect of oral health on quality of life. PD patients, however, declared their prostheses as less comfortable than healthy patients [48], reporting a reduced motor control of the facial muscles. The presence of removable prostheses requires a strong effort to be kept in position or to be active during chewing and phonation even in healthy subjects: for PD patients orofacial muscles do no longer properly operate and react in self-control during oral functions, as the device is probably perceived by the patient as foreign compared with natural dentition. Plus, despite patients generally show greater satisfaction in terms of food intake and oral well-being, difficulty in cleaning prostheses, due to cognitive deterioration and motor disorders, can lead to the formation of a reservoir of pathogenic microorganisms [49]. In this regard, use of mouthwash is recommended to prevent opportunistic infections, and it was registered as similar in test and control group: however, it can be noticed that this supplement can generate discomfort in PD for dysphagia or fear of choking.

Patients’ self-assessment of their oral hygiene was reported as moderate in the PD group and good in the control group: this is indicative of awareness, by PD patients, of being not sufficiently efficient and effective in oral hygiene manoeuvres. Despite the frequency of dental visits appears to be homogeneous among all authors, with regular visits at least twice per year, overall oral health in PD patients was finally found as deficient and inadequate [11]. Proper use of toothbrush and interdental aids require complex coordination and manual skills, with increasing difficulties as the disease advances. Even if appointments in dental practices could be frequent, cognitive decline and dementia prevent the patient from reporting symptoms/dental pain to professionals/caregivers and being truly compliant: that’s probably the reason why patients’ perception of oral health results worse than healthy people.

A compromised status with multiple missing teeth can modify eating habits in time [11], as the patient present greater difficulty in chewing and swallowing, finally choosing softer and sweeter foods, which are rich of sugars, and limited self-cleansing of the oral cavity, with an increase in carious receptivity. Despite some authors [38] stated that factors predisposing oral diseases in PD patients include propensity for cariogenic foods, use of drugs reducing salivation, physical and motor problems, and cognitive change, only one paper [35] among five found a higher prevalence of caries in PD patients. Concomitant factors potentially leading to a greater predisposition to carious lesions [50, 51] are poor oral hygiene, plaque accumulation, and change in oral flora with a prevalence of S. Mutans and Lactobacillaceae.

Regarding relevant factors associated with the disease, its duration and severity both lead to general worsening of oral conditions. As the severity increases, functional capacity decreases in terms of motor difficulties and cognitive deficiencies, along with depression and dementia [12]. An evident link between severity/duration of PD and prevalence of untreated cavities and gingival inflammation clearly pointed out that compromised abilities in PD patients have an impact on the quality of self-care in a severe manner [11].

Considering the exacerbation of clinical manifestations proceeding with the disease, the relevance of caregiver becomes extremely important in giving appropriate instructions, to determine an effective support in everyday difficulties. In practical terms, it is necessary for dental staff to focus on tailored oral hygiene protocols, paying attention to the real needs of patients and directly communicating with caregivers, to explain them how to improve oral hygiene with appropriate tools and precautions. Inadequate plaque control, with consequent gingival inflammation and bleeding, is certainly due to tremors, rigidity, dementia, which do not allow adequate manual skills. Furthermore, an unbalanced diet in favor of carbohydrates and sugars (simpler and more palatable), as well as an altered quantity and quality of saliva, can lead to greater carious receptivity. In the light of these considerations, all issues evidenced for PD patients in this review necessarily imply the need for specific assistance. Caregivers should be directly involved in patients oral care after a targeted training given by dentists and dental hygienists. Visual instructions should be performed with the caregiver and the patient present together in the dental office (if the disease allow mobility), or even at home. Appropriate methods of brushing should be verified in patient’s mouth to suggest the easiest and most effective way to eliminate plaque deposits. Manual capacity should be determined in terms of interdental cleaning, and suitable interdental devices should be chosen according to the grade of autonomy and compliance. Finally, informative flyer could be designed with the support with Dental Associations, to widely release knowledge about the orofacial management of PD patients.

Finally, a limitation of the studies included in this review should be mentioned, as some authors [21–25, 27, 28, 32, 33] published on the topic in closed proximity, and probably the same populations were investigated in more than one study. Even if the results cannot be considered as representative of the countries they originated from, it must be underlined that multiple investigations by the same research group were conducted with different samples (PD patients only or PD patients in comparison with healthy group) and considering different factors influencing the disease (duration, stage severity). Despite the groups analyzed were almost homogeneous for age and type of study design (cross-sectional), differences were particularly evident regarding type of populations considered, indexes assessed for plaque and bleeding, oral parameters collected for examination of PD duration/severity. This heterogeneity does not allow a proper comparison between studies in terms of uniform analysis of multiple variables: taken for established the medical characterization of PD (with the known codified scales), this issue could be addressed with standardized methods, through the choice of universal clinical indexes for collection of parameters for an easy and practical evaluation of hard and soft tissues in the mouth.

Considering these findings, the challenges faced by PD patients in home oral hygiene underscore the need for specific care. Areas of future research should encourage longitudinal studies to better understand the impact of oral health interventions on the progression of PD, not only through the action of private practice, but also with the support of families, caregivers and public structures involved in the assistance of PD patients.