Introduction

Shoulder pain is a non-specific symptom of multiple pathologies, both rare and common. What is more, this pathology significantly decreases the quality of life of our patients. It mainly presents as an upper extremity impairment but may also result in psychiatric disorders when the pain becomes chronic [1–3]. Rotator cuff disease is responsible for most shoulder pain cases [4–6], but little is been known about the etiology of shoulder pain at night. Therefore, this scoping systematic review aimed to analyze the most common reasons for shoulder pain.

Materials and methods

From September to December 2022, two independent researchers (N.P. and Z.G.) searched the following databases: PubMed, Scopus, Science Direct, and Google Scholar according to PRISMA Extension for Scoping Reviews guidelines. The following terms were used: “shoulder pain”; “sleep deprivation”; “sleep initiation maintenance disorders,” and “sleep quality”. All full-text English-language articles on the topic were included (except reviews), regardless of the year of publication. Articles were excluded if they lacked correlation between shoulder pain and its etiology or sleep quality. Non-peer reviewed publications and articles that did not specify the etiology of shoulder pain were also excluded from the search. Two researchers screened the records by titles and abstracts and independently decided if a record met the inclusion criteria. In the event of any disagreement, the opinion of the senior researcher (M.D.) was sought, and his decision was final. References of the included studies were checked for additional data. Data extraction was performed by two researchers (N.P. and Z.G.), while the third researcher (M.K.) supervised the process to ensure accuracy. The data was then sorted into subtopics based on the etiology of shoulder pain. Due to the scoping nature of the review and the heterogenous designs of the included studies, the meta-analysis could not be performed. The study selection is visualized in the flow diagram (Fig. 1).

Results

The initial search identified 1561 articles. After removal of duplicates, 401 articles were screened by the title and abstract. 1124 articles were excluded because they did not fulfill the inclusion criteria. Finally, 78 articles were included in the review, sorted into groups and presented in Table 1.
1. Injuries and pathologies
1.1. Rotator cuff disease
Rotator cuff disease (RCD) was the most prevalent in the “pathologies and injuries” group. RCD related shoulder pain is the most common cause of shoulder pain and accounts for 70% of shoulder pain cases. [4]. Nocturnal pain is reported by up to 91% of RCD patients [5, 6]. RCD related pain is a multidimensional disorder, and the role of sleep disorders in the chronification of this pain is well known [7, 8]. Sleep disturbances are the primary reason patients with RCD contact their doctor [5, 9]. However, the mechanisms of this nocturnal pain have not yet been fully established. One hypothesis describes it as a result of impingement and bursitis in the subacromial space. A higher level of proinflammatory cytokine – TNF-α was observed among patients with RCD who suffer from pain-related sleep disturbances [10]. Two studies pointed out that synovitis causes increased blood flow in the synovial membrane, which results in increased blood flow in the artery supplying the capsule – anterior humeral circumflex (AHCA). The conclusion is that night pain among RCD patients may be related to the hemodynamics, and it diminishes when the blood flow velocity decreases. [11,12]. A study by Mengi et al. showed that the presence of subscapularis tendinosis appears to be an independent predictor of nocturnal pain. In addition, patients with a positive Neer’s test may experience more nocturnal pain [4].
Vergili et al. tried to compare sleep quality in patients with RCTs (rotator cuff tears) of different sizes. The study showed no significant difference between patients with small and large RCTs [13]. Similar conclusions were reached by Reyes et al. [14], Serbest et al. [15], and Khazzam et al. [6]. Only tendon retraction showed a significant correlation with pain [14]. On the other hand, a study by Gumina et al. showed that patients with small tears have poorer sleep quality compared to patients with more severe tears. It was shown that they needed more time to fall asleep and had more disturbed sleep than patients with large and massive tears. [16]. As presented, this problem is still under debate; however, Ansok et al. described that patients even tend to overreport their time spent asleep when compared to objective time asleep measured by validated monitors [17].
The literature search showed two studies in which RCD was treated non-surgically with an evaluation of pain and sleep disturbances. The first was physiotherapy. Passive mobilization, manual ischemic trigger point therapy, and passive traction were performed. After 12 weeks of follow-up, the patient had decreased pain levels and improved sleep quality and quantity [18]. In the second study, the authors evaluated the therapeutic effect of subacromial platelet-rich plasma (PRP) injections in RCD. After 12 months of follow-up VAS, ASES, and Constant-Murley Score indicated improvement after the injection. Sleep disorders were resolved in 86% of the patients [19].
A review of the literature revealed 7 studies pertaining to rotator cuff repair (RCR) with evaluation of pain and sleep disturbances. One of the main goals of RCR is to restore normal sleep patterns. Dolan et al. indicated that 77% and 81% of patients reported resolution of sleep disturbances by 6 months and 2 years postoperatively, respectively [5]. In agreement with these results, Longo et al. proved that sleep disturbances improved three to six months after RCR [20], and Cho et al. reported that after RCR, sleep disturbances decreased among their patients from 6.6 ± 3.6 PSQI score before surgery to 4.2 ± 3.3 at 12 months after surgery. [21]. In a study by Horneff et al., sleep quality achieved in the sixth month post-surgery was maintained over a follow-up period of more than 24 months (5.4 vs 5.5 PSQI score). The interesting addition to their study is that the prolonged use of narcotic medication negatively affected sleep, with a greater effect seen over time. [9]. In all of the aforementioned studies, RCR resulted in a significant decrease in pain intensity.
According to Wu et al., preoperative sleep disturbances of patients are independently associated with the occurrence of moderate or above pain 1 month after surgery (OR = 3.794, 95% CI: 1.261–11.409, p = 0.018). Preoperative sleep disturbances should be given more attention and actively improved to effectively promote postoperative pain management in patients with rotator cuff tears. [22]. On the contrary, Cho et al. demonstrated that preoperative sleep disturbances did not predict poor outcomes (pain intensity or sleep quality) after RCR [21]. Bingöl&Biçici presented interesting results about rotator cuff re-ruptures. Sleep disturbances may indicate rotator cuff retear. Patients with re-rupture after 1 year follow-up had significantly higher postoperative PSQI scores and lower CSS and OSS scores compared to those without re-rupture (p < 0.001) [23].

1.2 Frozen shoulder
Patients with adhesive capsulitis, also known as frozen shoulder (FS) are particularly vulnerable to reduced sleep quality. The incidence of sleep disturbance was reported to be around 85% in FS patients [24]. It is generally characterized by a three-phased evolution of the disease – freezing, frozen, and thawing. Stage 1 is the painful phase where sleep disturbances are common due to nocturnal shoulder pain [24,25]. In a cross-sectional study by Mulligan et al., it appeared that FS, in comparison to subacromial impingement syndrome (SIS,) led to poorer sleep quality (PSQI score 12.07 ± 5.7 vs 8.59 ± 4.73; p = 0.008) and function (SANE score 34% vs 53%, p= 0.007; ASES score 38 vs 51, p = 0.007) [26].
Pain in FS may lead to depression or anxiety and impairs patients’ quality of life (QoL) [24,25]. FS patients with depression or anxiety have significantly lower PROMs values and significantly higher prevalence of sleep disturbances compared with FS patients with normal psychological status [27]. Besides the excruciating pain, patients with FS struggle to reestablish a sense of normalcy as they cope and adapt to a profound emotional change. A sociological approach to FS was extensively described in a study by King et al. and referred to as “no-man’s land.” [28]. Considering all the aforementioned symptoms, it is critical not to delay surgical intervention. Arthroscopic capsular release significantly decreases nocturnal pain and sleep disturbances regardless of the timing of surgery [29].

1.3 Subacromial impingement syndrome
Subacromial impingement syndrome (SIS) causes sleep disturbances due to pain mediated by the free nerve endings in the subacromial bursa. The majority of included studies focused on a conservative treatment approach. Patients with shoulder pain due to SIS may benefit from painkillers and cognitive-behavioral interventions that specifically target sleep disturbances. [30]. Response to subacromial corticosteroid injections and lidocaine/tetracaine patches in patients with SIS can be predicted from pretreatment pain quality. Patients with the most to gain (i.e., those reporting the highest levels of various pain qualities) can expect the best response to effective treatments [31]. Joint mobilizations with supervised exercises in patients with SIS reduce pain, improve sleep quality, and increase conditioned pain modulation after 8 weeks of exercise [32]. Therapeutic ultrasound does not have any benefits on SIS. However, transcutaneous electrical nerve stimulation (TENS) combined with exercise programs are effective in pain, disability, and sleep disturbances in patients with SIS [33]. Low-level laser treatment (LLLT) and extracorporeal shock wave therapy (ESWT) treatments were reported to be effective in the treatment of SIS at 3 months extended follow-up [34].

1.4 Others
Myofascial pain syndrome (MPS) is characterized by regional or widespread myalgia with trigger points (TrP). The TrP is a painful, physical sign that occurs in various myalgic conditions. Latent TrPs are very common and have been identified in the shoulder girdle muscles in 45-55% of healthy young adults. The upper part of the trapezius muscle is one the most common sites affected by this condition. The relationship between the presence of active TrPs, pain intensity, disability, and sleep quality is significantly associated with MPS [35]. During the literature search, the authors found one study on dry needling, considered one of the most effective MPS treatment methods. It is widely used by physiotherapists and leads to improved sleep quality and decreased pain intensity [35].
In a case study by Scibek&Carcia, the authors presented a patient with calcific tendinitis with severe sleeping disturbances forcing the patient to sleep in an upward position. Pain control was achieved with codeine combined with ice massage after exercise and at the end of the day. By completing a physiotherapeutic regimen focusing on decreasing pain, improving range of motion and strength over an 18-day period, pain levels at rest and with activity decreased from 8-9/10 to 0/10, and scores for SST progressively increased from 0/12 to 12/12 with no recurrence of symptoms in 22 months follow-up [36].
Additionally, shoulder pain is a common and significant problem in world-class badminton players. It was correlated with consequences such as sleep disturbances, changes in training, and competition habits. Previous shoulder pain was reported by 37% of players, and ongoing shoulder pain by 20% of players [37].

2. Postoperative pain
Surgical interventions generally improve self-reported sleep quality and PROMs from before to after surgery (e.g., RCR, total shoulder arthroplasty, arthroscopic capsular release, comprehensive arthroscopic management, and sternoclavicular joint procedures) [15,38]. It was also reported that arthroscopic shoulder surgery performed in outpatient care also improves sleep quality and alleviates pain in 88% of patients [39]. Similar problems are also common after laparoscopic cholecystectomy (LC). Pain after laparoscopic procedure is complex with multifactorial etiology, but the most common theory is carbon dioxide persistence between the right diaphragm and the hepatic dome [40]. Pain was reported in 35-60% of patients after LC. Intravenous administration of lidocaine and magnesium reduced intraoperative and postoperative opioid requirement and pain scores while improving postoperative analgesia. Lidocaine was also associated with the earlier return of bowel function, and magnesium – with improved sleep quality. [41]. Nakhli et al. also demonstrated that premedication with gabapentinoids can enhance postoperative rehabilitation quality after LC by reducing postoperative shoulder pain and improving sleep quality during the first postoperative night [40].

2.1 Postoperative pain: total shoulder arthroplasty
During a 3-month follow-up, Total Shoulder Arthroplasty (TSA) has been shown to reduce patients’ pain and improve their sleep quality. These benefits are particularly noticeable during activities of daily living that involve forward elevation and waist-level movements, in that specific chronological order [43]. Sleep disturbances have the strongest associations with post-operative outcomes of physical functions and pain interference (p <  0.01) [44]. After surgery, sleep quality significantly improves within 6 weeks and returns to normal levels in 1 year. The negative correlation of PSQI with ASES score suggests that improved functional outcomes are related to enhanced sleep quality [45]. Postoperative pain control after TSA may be augmented by continuous cryotherapy or plain ice because they were proven to be similarly effective with no differences in pain control, quality of sleep, or narcotic usage [46]. A multimodal sleep pathway after TSA, including non-pharmacologic sleep hygiene interventions and the use of zolpidem and melatonin, improves patient analgesia and sleep. It also reduces opioid usage, increases sleep duration, and improves reported sleep quality during the postoperative recovery period [47].

2.2 Postoperative pain: drugs
Effective postoperative pain management after arthroscopic shoulder surgery enables patients to leave the hospital earlier, lowers the chance of hospital readmission, and speeds up recovery time. Preoperative, intraoperative, and postoperative measures should all be considered as a comprehensive approach to pain control as they also improve patients’ sleep quality [48–52]. Painless sleep after surgery may be achieved with oral analgesics, sleep medications, or continuous cold-flow therapy [53]. Other drugs that help in the induction of painless sleep are: local anesthetics injections (ropivacaine [48,50] or bupivacaine [51] blocks), steroid injections (betamethasone), and, if necessary, opioids like oxycodone [53].
If possible, pain management should be multimodal. Elkassabany et al. presented the Multimodal Perioperative Pain Protocol (MP3), which consisted of preoperative oral acetaminophen and gabapentin, intraoperative intravenous ketorolac, and postoperative oral acetaminophen, gabapentin, and ketorolac with oxycodone as needed for breakthrough. Implementation of the MP3 after shoulder arthroscopy improved pain management and sleep quality two days after ambulatory shoulder surgery.  Additionally, this protocol significantly reduced opioid consumption up to three days after surgery [45].

3. Head and neck-related pain
Neck and shoulder pain (NSP) is a common work-related musculoskeletal disorder with unclear mechanisms [54,55]. High strain, active jobs and sleep disturbances are prognostic factors that increase the risk of NSP. Furthermore, sleep disturbances may modify the association between high strain and troublesome NSP [55]. Autonomic regulation is involved in the pathogenesis of chronic NSP. NSP patients have elevated heart rates, reduced heart rate variability during sleep, higher levels of stress and fatigue, and reduced sleep quality [54].
Significant correlations exist between aerobic exercise, uninterrupted sleep, and the absence of NSP [56]. Moreover, individuals with poor sleep quality are less likely to recover from NSP, and the severity of NSP is directly proportional to the decline in sleep quality [57]. Conversely, sleep problems are associated with an increased risk of chronic NSP. Regular exercise and maintenance of normal body weight may break this vicious cycle [56,58]. The NSP patients with the most pronounced sleep problems have significantly higher OR (OR 2.00; CI 1.09-3.67) for long-term sickness absence from work (> 90 days) [59]. The literature search revealed several pathologies coexisting with NSP, including temporomandibular disorders [60], concussions or cervical spine injuries [61], common headaches [62], and thoracic outlet syndrome [63].
The authors found several methods alleviating the NSP-related sleep disturbances. People who regularly exercise (≥1 hour per week) have a lower risk of chronic NSP (OR: 0.8) [56,58]. Furthermore, people with BMI <25 kg/cm(2) and sleep problems have a lower risk of chronic NSP than people with BMI ≥25 kg/cm [58]. He et al., in their randomized controlled trial, proved that intensive acupuncture may also improve the quality of sleep among NSP patients. The effect lasted for at least three years during the follow-up period [64].
4. Rare conditions and diseases
Joint hypermobility in Ehlers-Danlos syndrome (EDS) was correlated with poor sleep quality. According to questionnaires, 90% of patients reported having pain most frequently localized in the neck, shoulders, hips, forearms, and legs (>40% of all patients). 50% of the patients suffering from pain were awakened during the night due to pain [65].
Two articles were found on spinal cord injury (SCI). Adults who experienced it before the age of eighteen, individuals with shoulder pain, and people with tetraplegia are more likely to report poor sleep [66]. Paired associative stimulation (PAS) and motor rehabilitation were reported to improve patients’ functioning after SCI [67]. The literature search also indicated two studies about shoulder pain in fibromyalgia (FM) affecting sleep quality. FM patients have shorter total sleep time, longer sleep latency, and reduced sleep efficiency in polysomnography. The associations between heart rate variability, sleep quality and shoulder pain were found to be the most pronounced during non-REM stage 3 of sleep [68]. The use of high-intensity laser therapy (HILT) as an alternative therapy for pain management in FM was reported to effectively decrease pain, improve ROM, mood, activity level, and sleep quality, lasting for 1 week. After 1 week, only 50% of the initial pain intensity returned [69].
Patients suffering from postherpetic neuralgia (HSP) localized in cervical/upper extremity dermatomes may additionally suffer from insomnia [70]. The same phenomenon was reported among patients with neuronal amyotrophy. It is a relatively uncommon inflammatory brachial plexopathy that often manifests as severe pain followed by weakness in the shoulder girdle muscles. A 45-degree angle and noninvasive ventilation may be required for better sleep [71].

5. Psychological factors
A decrease in the amount and quality of sleep affects the relaxation and recovery of shoulder girdle muscles. An increase in muscle tension and activity may also result from psychological factors such as depression, anxiety, or stress, all of which may create a vicious cycle. Shoulder pain can reduce the quality of sleep and cause psychological disorders, and poor sleep quality and psychological disorders can further affect shoulder dysfunction and pain [1].
Stressful factors in the work environment among health-care and hospital workers are correlated with a lack of adequate sleep and pain in the shoulder [2,3]. Results of a study by Sigursteinsdóttir et al. revealed a positive correlation between mental and physical exhaustion at the end of the workday and musculoskeletal pain. Adequate sleep had a significant negative correlation with all stressful factors in the work environment and shoulder pain [2]. Almost one in four healthcare professionals surveyed reported a severe or very severe musculoskeletal disorder (MSD), and nearly one in seven reported a severe sleep disorder (SD). General stress, work stress, physical effort at work, and particularly a painful or tiring posture at work were found to be clear and strong risk factors for MSDs, whereas only general and work-related stress were found to be significantly associated with SDs [3]. Also, academic pressure in students who experience a heavy learning burden and high level of stress is closely related to chronic shoulder pain, which results in sleep disturbances [72].
During the COVID-19 pandemic, social isolation potentiated painful symptoms in various parts of the body. Working and learning from home, along with using portable devices that require flexing the neck and repetitive movements, have led to a significant rise in the prevalence of shoulder pain, which is further correlated with sleep latency. In addition, compelling evidence suggests that reduced physical activity during the pandemic has been linked to a decline in sleep quality. This, in turn, has been associated with depression and musculoskeletal pain [73,74].

6. Neoplasms
Four articles about neoplasms impairing sleep quality due to shoulder pain are characterized in Table 3.
Discussion

Rotator cuff disease
The most important result of this systematic review was that the most frequent cause of nocturnal shoulder pain mentioned in the literature was RCD. Results of systematic review performed by Castro-Contreras and Valdez-Pardo show that 100% of patients with rotator cuff injury suffer from sleep disturbances, and up to 93% feel pain at night [79]. Both of these findings are in accord with the results reported in studies included in this review [5,6]. The resolution of sleep disturbances and pain reduction among RCD patients may be achieved by both conservative [18,19] or surgical treatment [5,9,20–23]. However, Longo et al. presented in their meta-analysis that surgical management is more effective in terms of pain alleviation in comparison to the conservative treatment group at 12 months follow-up (VAS score = − 1.08, 95% CI − 1.58 to − 0.58; p < 0.001) [80]. Unfortunately, there was no possibility of assessment and comparison of sleep quality among the groups due to the heterogeneity of the included studies.

Psychological factors
According to our results, psychological factors were associated with shoulder pain at night [1–3,72–74]. This topic may seem difficult for an orthopedic surgeon or a physiotherapist to cope with as it is not in the field of these specialties. However, a deeper understanding of our patients’ minds would help guide them through the treatment process and break the vicious cycle created by psychological factors [1]. Maxwell et al. performed a qualitative synthesis exploring the experiences of individuals living with shoulder pain [81]. In the study, the authors describe 3 main types of patients. The first group comprises individuals who experience shoulder pain that has a negative impact on their emotional and social lives, as well as their daily activities. These individuals are fully focused on their negative emotional state, experiencing feelings of frustration and depression. It may be beneficial to include a psychiatrist or psychologist on the medical team to provide appropriate care for them. The second group are patients with a desperate need of a definite diagnosis, with a feeling of “structural fault,” which rapidly needs permanent treatment. The surgical approach may be the most beneficial to them. Finally, the third group consists of patients who manifest apprehension of movement because they anticipate shoulder pain. A rehabilitation program under the supervision of a physiotherapy specialist with consistent psychological support may be the most adequate for such people. Knowledge of our patients’ minds, their emotions and empathy from the medical team is known to have
a significant therapeutic impact, far beyond the effects
of prescribed drugs, surgeries and other treatment
methods [82].

Environmental and socioeconomic factors
When considering the causes of sleep disorders, one should not forget environmental factors comprising sound insulation, temperature and room humidity [83]. Also, socioeconomic factors like night clothes, bedding quality, and mattress rigidity or elasticity have significant impacts on sleep quality [83]. While searching the literature, our attention was drawn to studies that noted the effect of mattress and pillow type on sleep quality; however, they were excluded due to the lack of a specified shoulder pain etiology [84–86]. Jacobson et al. reckon that most healthcare practitioners have patients who complain about sleep disturbances and shoulder pain probably caused by the type of mattress they sleep on. To investigate this issue, they designed a study, which proved that using a prescribed bedding system more efficiently reduces shoulder pain and stiffness and improves sleep quality than a personal (patient’s own) bedding system. Significant improvement was observed in shoulder pain (by 60.83%) and sleep quality (by 60.73%) [84].
People tend to spend about one-third of their lives sleeping. Werner et al. hypothesized that different shoulder positions during sleep could significantly affect nocturnal subacromial pressure. As a result, atrophy and ruptures during sleep may affect tendon healing after RCR [87]. In our opinion, this can also apply to patients with chronic shoulder pain who have not undergone surgery. In the study mentioned above, subacromial pressures were lower among people sleeping supine and higher among people sleeping in prone and lateral decubitus positions [87]. These results seem interesting, especially in relation to the Kempf et al. study, which found that most of their subjects were more likely to sleep on the painful shoulder [88]. A new question arises: Why do people with shoulder pain unconsciously tend to increase their subacromial pressure by lying in the lateral decubitus position? that the answer to this question may be a promising subject for future studies.

Strengths and limitations
This review is of great clinical importance. It does not only summarize the most common etiologies of night shoulder pain, but also highlights the significance of having a broader perspective when patients report sleep disturbances. The presentation of the sleep assessment scales (Tab.2) may also facilitate their introduction into the daily routine of the surgical ward. There are several limitations to this review. Firstly, there is a lack of consistency in the measuring sleep quality. One should bear in mind that sleep quality is impaired in most chronic pain cases but rarely monitored. The six pathology groups are a relatively small number and it may be due to a lack of awareness of the problem among patients and medics. The second limitation is that the study did not have a registered protocol. The third limitation pertains to heterogeneity of scales assessing shoulder pain. This limitation, in conjunction with the first, made it impossible to perform a metanalysis.
Conclusions

This scoping systematic review proves that shoulder pain may have a highly diverse etiology, and may interfere with sleep in many different ways. Sleep quality should be monitored and effectively treated in patients with chronic shoulder pain, because it may significantly impact the effectiveness of the treatment of the primary pathology. And conversely, proper diagnosis of pain etiology and problem-specific therapy may effectively increase sleep quality.

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