Introduction
Statin-induced autoimmune Myositis (SIAM) is a rare clinical entity that can occur as a side effect after prolonged statin treatment, with a prevalence of 1 in 100,000 inhabitants [
1,
2]. A wide spectrum of musculoskeletal disorders associated with statin therapy is included in the definition of “Statin-induced myopathy spectrum” (SIMS). Clinical presentations may also range from asymptomatic elevation of creatine kinase (CK), sometimes associated with myalgia and exercise intolerance, to rarer forms characterized by proximal muscle weakness, marked serum CK elevation and histological evidence of myonecrosis. The necrotizing and autoimmune forms are commonly defined as “Statin-induced necrotizing autoimmune Myositis (SINAM) [
3]. Symptoms and signs of SIMS also include fatigue, cramping, stiffness, tendon pain, tenosynovitis and rare cases of dysphagia. [
4,
5]. Statin-induced myositis (SIM) recognizes two mechanisms: a more common cytotoxic mechanism, and a rarer autoimmune-mediated mechanism, associated with the presence of antibodies directed against the 3-hydroxy-3-methylglutaryl-coenzyme A reductase (Anti-HMGCR Ab). Normally, HMGCR is highly expressed in muscles and it represents the target enzyme on which statins act as competitive inhibitors, lowering the production of cholesterol [
6]. The autoimmune-mediated mechanism represents the pathogenetic substrate of SIAM. However, the exact pathophysiological mechanism leading to SIAM remains unclear. It is hypothesized to include mechanisms affecting membrane excitability, mitochondrial function, ubiquinone depletion, calcium-metabolism, and apoptosis. [
7]. The onset appears to be more common after the age of 50 years, with a slight preponderance for the female sex [
2].
Side effects induced by statins usually are dealt with discontinuation of statin therapy. SIAM is instead later suspected when resolution or improvement of symptoms does not occur despite statin discontinuation. Moreover, SIAM-affected patients treated after a time delay (months or years) in some cases require a prompt immunosuppressive treatment [
6].
In this article the authors summarize the most contemporary literature regarding SIAM and provide two illustrative case reports relevant to the topics discussed. To the best of authors’ knowledge, only a single SIAM diagnostic algorithm has been published so far [
8]. Therefore, to facilitate the diagnosis of nuanced SIAM clinical cases, here we propose a modified diagnostic algorithm of SIAM. The development of the present algorithm takes into account data from a total of 69 patients, collected both from literature and from our personal experience.
Discussion
SIMS defines a heterogeneous group of clinical presentations which occur as rare adverse events after the statin treatment. The criteria of the American College of Cardiology (ACC), the American Heart Association (AHA) and the National Heart, Lung, and Blood Institute (NHLBI) describe three types of SIM [
14]: (i) Statin-induced myalgia: muscle symptoms without CK elevations, (ii) Statin-induced myositis: muscle symptoms with CK elevations., (iii) Statin-induced rhabdomyolysis: muscle symptoms with marked CK elevations (over ten times the ULN) with an elevated creatinine value and the occasional presence of brown urine (myoglobinuria). Statin-induced myositis recognizes at first a most common cytotoxic mechanism; less frequently an autoimmune mechanism. The finding of anti-HMGCR Ab defines SIAM, that is considered a quite different clinical entity than statin-induced myositis. Whether SIAM represents a more severe clinical presentation of statin-induced myositis, or it should be considered a subtype of autoimmune myositis, remains a topic of scientific debate [
7]. The autoimmune mechanism should be suspected over the more common cytotoxic mechanism at the persistence of symptoms, even after discontinuation of therapy [
6]. As shown by the algorithm, HMGCR-ab negativity in the face of statin therapy should suggest other forms of myositis including statin-induced forms with cytotoxic mechanism.
SIAM onset can be acute (days to weeks) or sub-acute (< 6 months). Generally, symptoms occur after a prolonged statin treatment and their resolution can be very slow, even after several months. The term “prolonged” identifies a period of continuous statin intake of more than 6 months. This period was assessed by observing data extrapolated from the selected articles, in which the shortest time of statin treatment before symptoms was around 8 months. The definition of 6 months as "prolonged use of statin" is also consistent with other publications regarding SIAM [
15]. The broad spectrum of clinical manifestations of our patients’ cohort accurately depicts SIAM clinic [
16]. The algorithm we propose starts from the recognition of the symptoms during statin therapy. Symmetric-proximal muscle weakness was referred to as the most common initial symptom in our cohort.
Atorvastatin resulted the statin being more associated with SIAM in the case series we examined. Also simvastatin played a relevant role in SIAM, although prevalence rates were 20.28 vs 85.5% of atorvastatin, respectively (Table
2). Although a formal statistical analysis was not possible with the collected data, these suggest a more frequent occurrence of SIAM with atorvastatin, than with other statins. Simvastatin and atorvastatin have a predominantly hepatic metabolism and are metabolized by cytochrome CYP3A4, releasing their metabolites into plasma. This metabolic pathway lends itself to multiple interactions with drugs metabolized by the CYP3A4 pathway. Simvastatin and Atorvastatin also have greater lipophilic properties and penetrate peripheral tissue cells better as well as liver cells. Rosuvastatin, along with fluvastatin and others, is only partially metabolized via the cytochrome CYP2C9 pathway. Rosuvastatin is excreted both via bile and urea and also exhibits hydrophilic properties. These chemical characteristics result in a reduced penetration of Rosuvastatin into peripheral tissues [
16].
The Medical Research Council (MRC) scale for Muscle Strength represents the most frequent adopted criterion to clinically describe the grade of muscle affection (0: no contraction; 1: flicker or trace of contraction; 2: active movement, with gravity eliminated; 3: active movement against gravity; 4: active movement against gravity and resistance; 5: normal power) [
18]. Lower values at the MRC score have been noticed in females than males (2.77 vs 3.36, respectively); in addition, a recurrent denervation pattern at the EMG/ENG, adipose substitution at the MR, presence of ANA Ab (in addition to anti-HMGCR Ab) and larger inflammatory infiltrates at biopsy suggest a more severe disease in female patients. Women notoriously have a predisposition towards autoimmunity and it probably gives them an additional risk factor to develop SIAM and/or to present a more severe disease.
Once SIAM is suspected, CK assay is mandatory. Values > 10 folds the ULN are suggestive of SIAM and statin therapy must be stopped as soon as possible. As an addition to the only available proposed algorithm [
8], we propose a shorter interval between the first CK assay and the second evaluation (2 vs 8 weeks) with the aim to anticipate the suitable treatment as soon as possible. CK values > 15 folds the ULN in association with acute symptoms are suggestive of rhabdomyolysis, so our algorithm proposes to directly treat/hospitalize the patient. If the CK < 10 the ULN, the algorithm recommends follow-up. The algorithm considers an individualized and patient-tailored management in terms of timing, and follow-up schedule. The algorithm also emphasizes the exclusion of other causes of rhabdomyolysis (paraneoplastic, infectious, traumatic, toxic and metabolic causes).
In addition, we propose to add the EMG/ENG and musculoskeletal MRI to the diagnostic workup before proceeding with the antibody assay. These tests allow us to document the organ damage of myositis and contribute to define the severity of the organ damage itself. EMG/ENG is altered in different forms of myositis and shows signs of irritable myopathic pattern with signs of denervation [
19]. EMG/ENG also makes it possible to differentiate inflammatory forms of myositis from other forms of muscle weakness, including weakness associated with steroid use. However, EMG/ENG does not allow differentiation between the subtypes of myositis. It is also useful for evaluating 'acute' versus chronic forms of myopathies [
20].
MRI with STIR sequences has a sensitivity of 89–100% for detecting inflammatory changes, which is higher than muscle biopsy at 66% [
21,
22], while MRI specificity is estimated around the 80–88% against the specificity of 100% for muscle biopsy [
23]. The musculoskeletal MR of the involved limbs typically shows symmetrical and diffuse intramuscular and intrafascial oedema in anterior, medial and posterior compartments of the affected muscles. In addition to oedema, other pathological changes may occur within the muscle as a consequence of the chronic inflammatory myopathic process, including loss of muscle bulk (atrophy), fatty involution or replacement with connective tissue. The necrotizing form of SIAM is characterized by more widespread muscle involvement and signs of fibroadipose substitution [
21]. However, it must be emphasized that MRI alone cannot discriminate between the various forms of myositis but needs to be corroborated by the other clinical elements (symptoms, ck, autoantibodies, etc.).
If EMG/ ENG and/or musculoskeletal MR are compatible with SIAM, the next step consists of the analysis of autoantibodies patterns, including the extended myositis panel (anti-Mi2/alpha and beta, anti-DNA, Ku, SRP, PL7, PL-12, EJ, OJ, Jo 1, PM-Scl100 and PM-Sc75, Ro-52, NXP2, TIF 1g, SAE1 and MDA5) and anti-HMGCR Ab, necessary to make a differential diagnosis between SIAM and other forms of autoimmune myositis [
24]. Anti-HMGCR Ab always results negatively in Polymyositis (PM), Dermatomyositis (DM) and Inclusion body Myositis (IBM) [
25]. Necrotizing myositis (paraneoplastic syndrome, drugs, toxicity and other) can mime SIAM, showing the presence of anti-SRP Ab and/or anti-HMGCR Ab regardless of the absence of statin exposure [
10‐
23].
ANA are usually detected in PM, DM and IBM but their positivity is inconclusive in SIAM. Among the described cases, ANA positivity was found only in 5 females out of 69 patients. An uncommon finding is a weak anti-Ku positivity. This was found in both cases described in this report and in one additional case previously described in which ANA were also positive [
26]. Currently, no clinical significance can be attributed to this finding.
Muscle biopsy is the last step to support the diagnosis Biopsy is able to assess the various forms of inflammatory myositis and to distinguish these from necrotizing forms. It also permits to assess of the degree of severity of the pathology by highlighting the type of inflammatory infiltrate. Being biopsy invasive and not always easy to perform, it becomes useful to confirm the diagnosis and it is reserved for doubtful cases [
20]. Cellular infiltrate is composed largely of macrophages over the CD4+ and CD8+ lymphocyte population (macrophages probably play a role in tissue repair). The hematoxylin and eosin staining of a paraffin-embedded specimen reveals abundant myophagocytosis [
27]. Diffuse or multifocal up-regulation of MHC I molecules is common. Nevertheless, sarcolemmal MHC I staining appears to be particularly specific to SIAM, such as staining is only rarely noted in metabolic or genetic muscle disorders [
28].
In some patients, muscle weakness persisted even after the muscle enzyme levels have returned within the normal range [
27]. Symptoms resolution time (SRT) had required a mean duration of 12.14 months, longer in females (16.65 months) than in males (7.88 months). The last one data confirms the suggestion of more severe disease in females than in the male group.
Once few consensus and/or diagnostic criteria for myositis have been assessed and only a single diagnostic algorithm specific for SIAM has been published in 2016 [
8]. Moreover, given the continuous modernization of imaging/instrumental techniques, few diagnostic criteria for myositis take into account them. For example, the MRI finding of oedema on STIR imaging has been inserted as one of the variables of the European Neuro-Muscular Centre classification criteria from myositis of 2004 [
29] but it has been recently excluded by the diagnostic criteria of the International Myositis Evaluation and Clinical Trials Group (IMACS) of 2016, as MRI was not widely used in the study population from which the criteria variables were derived [
30]. Our algorithm is also updated to include the most modern diagnostic/imaging techniques in the diagnostic pattern-.
However, to date the study presents many limits. Once the study is not designed as a ‘systematic review’ of the literature, the methodological quality of the work was not assessed and we have not applied statistical analyses as we disposed only data extrapolated by articles but not the raw data. In fact, this does not allow us to provide clear associations/correlations between the various clinical elements of SIAM examined to date. Moreover, this work does not deal with the topic of SIAM therapy. The reviewed data did not allow to make a clear statement on treatment options. Further works have to be done to provide treatment guidelines to the medical community.
In conclusion, we think that our “experience-based algorithm” can provide a modern and flexible tool to early achieve a diagnosis of SIAM as specific as possible, adding a relevant contribution to the current clinical practice in the management of SIAM.
The different clinical presentations of SIAM, and the diagnostic-therapeutic approaches described in the various studies that we have reviewed, provide the scientific community a clarifying contribution to this not fully acknowledged clinical entity. As a final consideration, the take-home message should be to monitor closely any symptoms occurring during statin treatment so as to recognize and treat adverse events as early as possible.
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