Introduction

Buttock pain and posterior hip pain are very common symptoms caused by a variety of disorders [8]. The entrapment of the sciatic nerve in posterior hip area potentially causes buttock and posterior hip pain. Conventionally, the term “piriformis syndrome” has been used to cover entrapment of the sciatic nerve by the piriformis muscle [5]. However, accumulating evidence shows that various disorders cause entrapment of the sciatic nerve in addition to piriformis muscle, such as the obturator internus muscle, levator ani muscle, gemelli muscle, and coccygeus muscle [6, 19]. Therefore, the more comprehensive term, “deep gluteal syndrome” has been adopted. It has been almost 2 decades since McCrory et al. first used the term “deep gluteal syndrome”, expressing a pathophysiological understanding of chronic hip, groin, and buttock pain [18].

However, clinicians are not confident in diagnosing deep gluteal syndrome (DGS). The problem arises from the ambiguity of the DGS disease definition, deep gluteal space definition, and DGS diagnostic pathway. The purpose of this systematic review was to identify the DGS disease definition and deep gluteal space definition. Also, it was aimed at proposing a general DGS diagnostic pathway in reference to the included studies in this review.

Materials and methods

Eligibility criteria

Studies in which cases were diagnosed with DGS were included in this review. When the diagnostic pathway was not explicitly shown, the studies were excluded. Also, review articles and commentary papers were excluded.

Literature search

A comprehensive literature search was conducted using four electronic databases: MEDLINE (1946 to June 20th, 2019), EMBASE (1974 to June 20th, 2019), PubMed (1999 to June 20th, 2019), and Google Scholar (1999 to June 20th, 2019). We chose the key search term “deep gluteal syndrome”.

A 3-step screening process (title screening, abstract screening, and full-text screening) was adopted to select eligible articles. After duplicate articles were removed, two reviewers (**KK AND **AS) independently performed title and abstract screenings. If either of the reviewers included an article during title or abstract screening, it was moved to the next stage for screening. During full-text screening, discrepancies were resolved through discussion and consensus with the senior authors (**SU AND **OA). We did not register the protocol for this systematic review.

Data collection and analysis

All relevant data were collected in duplicate (**KK, **AD), including demographics of studies (authors’ name, country, publication year, patients number, the percentage of female in the study cohort, age), DGS disease definition, deep gluteal space definition, and DGS diagnostic pathway. When age was not expressed with mean, but expressed with median, minimum–maximum range, or interquartile range, we estimated the mean in reference to Wan et al. (2014) in order to output the pooled mean age [34]. The extracted data in DGS disease definition, deep gluteal space definition, and DGS diagnostic pathway were qualitatively synthesized.

Assessment of risk of bias

The methodological index for non-randomized studies (MINORS) appraisal tool was used to assess the quality of the included studies [28]. A score of 0, 1, or 2 was given for each of the eight items on the MINORS checklist with a maximum score of 16 for non-comparative studies. The MINORS were independently scored in duplicate by two reviewers (**KK, **CA). Methodological quality was categorized a priori as follows: a score of 0–8 was considered as poor quality, 9–12 was considered as fair quality and 13–16 was considered as excellent quality, for non-comparative studies in the MINORS.

Results

Study identification and characteristics

The frequency of use of the term “deep gluteal syndrome” has recently increased in medical literature. In Google Scholar, more than 280 articles included the term “deep gluteal syndrome” and especially more than half of papers have been published in the last 5 years, as shown in Fig. 1.

Fig. 1
figure 1

The number of publications mentioning DGS in Google Scholar. More than 280 articles included the term “deep gluteal syndrome” and more than half of the articles have been published in the last 5 years

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The initial literature search yielded 359 articles, of which 14 studies (e.g. 11 full papers and 3 conference proceedings) met the eligibility criteria pooling 853 patients clearly diagnosed with DGS [2, 3, 7, 9, 10, 12, 16, 22,23,24, 26, 27, 29, 33]. In the pooled population, the mean age was 48.1 years (range 5–80 years) and 57.6% of the patients were female, as shown in Table 1. The results of the screening process are described in the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) flow diagram (Fig. 2) [20].In most of the studies with the pooled

Table 1 Demographics of the included studies
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Fig. 2
figure 2

PRISMA flowchart. The initial literature search yielded 359 articles using four electronic databases: MEDLINE, EMBASE, PubMed, and Google Scholar. After a 3-step screening process, 14 studies met the eligibility criteria pooling 853 patients clearly diagnosed with DGS

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Risk of bias in included studies

The consensus mean MINORS score of the included studies was 10.7 ± 1.5, indicating fair methodological quality. Seven studies satisfied the domain of inclusion of consecutive patients, having potentially low risk of selection bias, however, there were no studies in which data were prospectively collected. The MINORS scores in the included studies are shown in Appendix Table 3.

Table 2 DGS diagnostic pathways in the included studies
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Definition of DGS disease and deep gluteal space

In the included studies, 11 studies explicitly described the DGS disease definition [2, 3, 10, 12, 16, 22,23,24, 26, 27, 33]. The most prominent DGS disease definition was signs and symptoms caused by non-discogenic entrapment of sciatic nerve in deep gluteal space. The DGS disease definition comprised three characteristics: (1) non-discogenic, (2) sciatic nerve pain, and (3) entrapment in deep gluteal space. Seven studies described deep gluteal space [2, 9, 10, 16, 23, 24, 26], as shown in Appendix Table 4. The most recognizable definition of deep gluteal space is as follows:

  • Anterior border: posterior acetabular column

  • Posterior border: gluteus maximus muscle

  • Medial border: sacrotuberous ligament

  • Lateral border: gluteal tuberosity

  • Superior border: sciatic notch

  • Inferior border: Ischial tuberosity

The definition of the deep gluteal space is illustrated in more detail in Fig. 3.

Fig. 3
figure 3

Deep gluteal space definition. The most accepted definition of deep gluteal space in the included studies is as follows: anterior border (posterior acetabular column), posterior border (gluteus maximus muscle), medial border (sacrotuberous ligament), lateral border (gluteal tuberosity), superior border (sciatic notch), inferior border (ischial tuberosity)

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DGS diagnostic pathway

In the included studies, the DGS diagnosis was performed in reference to diagnostic procedures categorized into five domains: (1) history taking, (2) physical examination, (3) imaging tests, (4) response-to-injection, and (5) nerve-specific tests, as seen in Table 2. In most of the studies with the pooled 544 patients, history taking, physical examination, and imaging tests were all performed. Response-to-injection was also performed in eight studies with the pooled 470 patients. Nerve-specific tests were also performed in five studies with the pooled 336 patients.

In history taking, posterior hip pain, radicular pain, and worsening when sitting for more than 20–30 min were the most common symptoms in patients clinically diagnosed with DGS. In physical examination, tenderness in deep gluteal space, pertinent positive results with seated piriformis test (recreated posterior hip pain at the level of the piriformis or external rotators when examiners extend the knee and passively move the flexed hip into adduction with internal rotation in seated position) [33], and positive Pace sign (recreated posterior hip pain with active or resisted abduction and external rotation of the hip) [33] were predominantly seen. In qualitative synthesis, active piriformis test and active extension of the hip against resistance were counted as the Pace sign. In imaging tests, pelvic radiographs, and pelvic magnetic resonance imaging (MRI) were commonly used to find abnormalities in deep gluteal space (e.g. sciatic nerve entrapment in piriformis muscle or obturator internus muscle in deep gluteal space). Spinal MRI was also performed to rule out discogenic sciatic nerve entrapment. In response-to-injection, five studies with the pooled 218 patients used ultrasound-guided injection. Several medications were used in injection and the most common medications were local anaesthetic (e.g. mepivacaine and lidocaine) combined with corticosteroids (e.g. methylprednisolone). A general diagnostic pathway for DGS is demonstrated in Fig. 4.

Fig. 4
figure 4

DGS diagnostic pathway. The most prominent DGS disease definition was non-discogenic entrapment of sciatic nerve in deep gluteal space. To confidently diagnose DGS, history taking, physical examination, imaging tests, response-to-injection, and nerve-specific tests were conducted

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Discussion

The most important finding of the present study was identification of the DGS disease definition, and also a clear definition of deep gluteal space. It was discovered that the DGS disease is defined by three characteristics: (1) non-discogenic, (2) sciatic nerve disorder, and (3) nerve entrapment in the deep gluteal space. In the diagnosis of DGS, we found five diagnostic procedures: (1) history taking, (2) physical examination, (3) imaging tests, (4) response-to-injection, and (5) nerve-specific tests. History taking, physical examination, and imaging tests were generally performed in cases clinically diagnosed with DGS, followed by response-to-injection and nerve-specific tests.

Previously, Kay et al. (2017) undertook a systematic review of the causes of DGS [11]. In this review, the most common cause of DGS was iatrogenic (from a previous intervention such as injection or debris from wear of arthroplasty prostheses) (30%) followed by piriformis syndrome (entrapment of the sciatic nerve by the piriformis muscle) (26%), and trauma (15%). Also, ischial tunnel syndrome is a non-discogenic sciatic nerve entrapment in the ischial tunnel (between ischial tuberosity and lesser trochanter) among the deep gluteal space, and it would be potentially categorized into DGS [17].

The term, “deep gluteal syndrome”, in itself, potentially covers cases with posterior hip pain in the deep gluteal space caused by anything outside of sciatic nerve entrapment, such as bursitis in the deep gluteal space, proximal hamstring tendinopathy, and piriformis muscle pyomyositis [13, 30]. Also, posterior femoral cutaneous nerve entrapment, inferior gluteal nerve entrapment, and superior gluteal nerve entrapment may be broadly categorized into DGS if these nerves are compressed in the deep gluteal space [21, 31, 32]. In searching the literature, we could not find any DGS cases outside of sciatic nerve entrapment. However, a future framework is required regarding whether DGS is specific to sciatic nerve entrapment or includes other pathological conditions.

A clear disease definition is essential in the provision of best clinical practice. DGS is currently clinically diagnosed based on the judgement of comprehensive information. However, many clinicians do not have confidence about the diagnosis and a clear definition of the DGS is needed. Here, definition of the DGS disease and the deep gluteal space was explicitly described. The most noticeable strength in this review lies in the identification of a potential DGS diagnostic pathway. A general diagnostic pathway for DGS in clinical practice was introduced in this review. The diagnostic pathway seems to comprehensively cover the three characteristics of the DGS disease definition. For instance, history taking and physical examination capture the signs/symptoms of sciatic nerve disorder. Martin et al. examined diagnostic test accuracy of seated piriformis test and Pace sign for cases with sciatic nerve entrapment in the gluteal region. It showed high accuracy of these tests with sensitivity 0.52 (95% confidence interval (95%CI 0.33–0.71) using seated piriformis test and 0.78 (95%CI 0.58–0.90) using Pace sign, and specificity 0.90 (95%CI 0.60–0.98) and 0.80 (95%CI 0.49–0.94), respectively [14]. Physical examination and pelvic radiographs comprehensively examine the possibility of the osseous biomechanical influences and spinal MRI excludes discogenic disorders [15]. Pelvic MRI confirms the presence of nerve entrapment in the deep gluteal space and excludes possibility of intra-pelvic entrapment. Among patients with posterior hip pain, lumbar spine or sacroiliac (SI) joint disorder should be ruled out [8] and, therefore, physical examinations and imaging tests specific to the lumber spine and SI joint should be considered in the DGS diagnostic pathway. Also, consultations with experts such as neurologists and gynaecologists can aid in differential diagnoses.

This review has several limitations. First, this review has risks of publication bias, since cases clearly diagnosed with DGS were published and listed in medical literature search engines, whereas underdiagnosed cases are probably less likely published in the medical literature. Publication bias may potentially oversimplify the DGS disease definition and the diagnostic pathway, since only typical cases are included aside from underdiagnosed cases [1]. To minimize risks of publication bias, we did a comprehensive search using Google Scholar. In our search, 280 articles were available in Google Scholar, in contrast only 22 articles were screened in PubMed. A second limitation is that the diagnostic pathway is not validated. Each diagnostic procedure should be assessed to confirm whether it accurately reflects three components of the DGS disease definition to satisfy content validity [4]. In future research directions, the conceptualization of a clear DGS disease definition is needed in the consideration of other pathophysiologies such as non-sciatic nerve entrapment in the deep gluteal space. An expert panels meeting may be necessary to clarify DGS diagnostic criteria using a Delphi consensus method [25].

The clinical relevance of this review is that it aids clinicians in the diagnosis of DGS with more confidence. Also, clarification of the DGS disease definition and the general diagnostic pathway guides future establishment of a clear diagnostic criteria.

Conclusion

The DGS disease definition was identified as being a non-discogenic sciatic nerve disorder with entrapment in the deep gluteal space. Also, we proposed the general diagnostic pathway for DGS using history taking (posterior hip pain, radicular pain, and difficulty sitting for more than 30 min), physical examination (tenderness in deep gluteal space, pertinent positive results with seated piriformis test, and positive Pace sign), and imaging tests (pelvic radiographs, pelvic MRI, and spine MRI).