Adult and juvenile-onset inflammatory myopathy: autoantibodies and muscle MRI in a cohort of Egyptian patients

Idiopathic inflammatory myopathies (IIMs) include a wide range of clinical phenotypes, and their classical manifestation is muscle weakness resulting from inflammation in skeletal muscle [1]. Muscle effects can frequently be associated with the involvement of other organs, such as the skin, joints, lungs, gastrointestinal tract, and heart, indicating the systemic nature of this disease [2]. Distinct subgroups of IIMs, including dermatomyositis (DM), polymyositis, inclusion body myositis (IBM), and immune-mediated necrotizing myopathy (IMNM), have been identified based on histopathological features, muscle symptoms, and the presence of skin manifestations [3].

Studies on IIM in Egypt are scarce, and an epidemiological study from Upper Egypt in 2005 revealed a lifetime prevalence of 11.5/100,000 people [4].

Autoantibodies found in patients with myositis are either myositis-specific autoantibodies (MSAs) or myositis-associated autoantibodies (MAAs). MAAs are autoantibodies that are also found in other conditions along with myositis, including systemic sclerosis (SSc) and systemic lupus erythematosus [5].

There are several known MSAs, including antibodies against Jo-1, PL-7, PL-12, EJ, OJ, anti-TIF1γ/α, anti-MDA5 and Mi-2, each of which is associated with certain clinical manifestations. Both anti-Mi2, which targets the nucleosome remodeling deacetylase complex, and anti-TIF1γ play a role in transcription regulation. NXP2 acts against nuclear matrix protein 2, which is involved in transcription regulation and p53 activation. Anti-ARS, which is associated with anti-synthetase syndrome, targets transfer RNA (tRNA) synthetases, which incorporate amino acids into their cognate tRNAs [5‐12].

IMNM constitutes approximately 20% of all autoimmune myopathies and can manifest either abruptly or more gradually, characterized by progressive weakness in the proximal muscles of the upper and lower extremities. Facial muscle weakness may also be present in some patients. Antibodies associated with IMNM include those against 3-hydroxy-3-methylglutaryl-coenzyme proteins in the endoplasmic reticulum, whereas anti-HMGCR targets an enzyme crucial for cholesterol biosynthesis [5, 13].

The diagnosis of inflammatory myopathy involves assessing clinical symptoms and signs, conducting laboratory tests and electromyography, and utilizing muscle imaging and muscle biopsy [14].

Magnetic resonance imaging is a crucial diagnostic tool for evaluating neuromuscular disorders; it can reveal muscle edema, atrophy, subcutaneous issues, and fatty infiltration. Additionally, MRI provides insight into the distribution of muscle involvement, making it valuable for assisting in the diagnosis of inflammatory myopathies, guiding muscle biopsy, distinguishing IIM subtypes through a pattern-based approach, and monitoring disease activity over time [15].

This study aimed to ascertain the presence of different myositis subtypes based on serum antibodies in both adult and juvenile-onset Egyptian patients. Additionally, we compared the clinical, laboratory, and radiological findings between these two groups.

This was a cross-sectional observational study. Data were collected and analyzed over a period of 2 years from March 2020 to March 2022.

Patients of various ages and both sexes, presenting with history and clinical features indicative of inflammatory muscle disease according to the Bohan and Peter criteria [16, 17] were included, while patients with a positive family history of muscle disease were excluded. Seventy-seven (77) patients met the study criteria.

Detailed history of proximal muscle weakness and respiratory and dermatological symptoms was obtained, and clinical examination, including neurological examination with a special account of muscle power grading according to the Medical Research Center (MRC) muscle strength scale, was performed [18].

Three ml blood samples were collected from all patients enrolled in the study under complete aseptic conditions. After centrifugation of the samples, the sera were separated and stored at -70 °C until the time of the assay.

The anti-HMGCR CoA antibody titer was measured using the QUANTA Lite® HMGCR ELISA, and the antigen used in the QUANTA Lite® HMGCR ELISA test was an immunoreactive fragment antigen. This antigen is bound to the surface of a microwell plate. Prediluted calibrators, controls and diluted patient samples are added to separate wells, allowing any anti-HMGCR antibodies present to bind to the immobilized antigen. Unbound antibodies were removed by washing, and an enzyme-labeled anti-human IgG conjugate was added to each well. A second incubation allows the enzyme-labeled anti-human IgG to bind to any patient antibodies that have become attached to the micro wells. After washing away any unbound enzyme-labeled anti-human IgG, the remaining enzyme activity was measured by adding a chromogenic substrate and spectrophotometrically measuring the intensity of the color that developed. A standard curve was constructed using the ODs of the calibrators versus the corresponding concentrations. The anti-HMGCR CoA antibody titers of all the samples were obtained by blotting the corresponding OD of each sample on the constructed curve. The linear range of the kit was 3.125–200 standard units. MRI images of whole-body muscles, including axial cuts and T1, T2 and STIR images, were obtained for 57 patients. The MRI device used was a Philips Achieva dStream (3 Tesla, Philips Healthcare, 2020).

As regards the demographic and clinical data, among our 77 studied patients, 34 patients tested positive for autoimmune myopathy autoantibodies. These patients were predominantly female (female-to-male ratio 3:2) and ranged in age from 6 to 58 years, with a mean age of 28.618 ± 14.346 years, while the mean age of onset and illness duration were 23.574 ± 13.58 and 5 ± 3.91 years, respectively. Fifty-two patients (with adult onset) had an onset of illness above 18 years, and 25 patients (with juvenile onset) had an onset of illness younger than or equal to 18 years. The mean age, sex distribution and type of antibody detected in the studied adult and juvenile-onset patients are described in Table 1.

Regarding the symptoms of the 34 seropositive patients, 21 presented with weakness, 4 patients complained of recurrent falls, 4 suffered from muscle pains, 2 had an accidently discovered elevated serum creatine kinase level, one patient’s initial complaint was fatigability, and another experienced a sense of muscle stiffness (Fig. 1).

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Our studied population included 34 patients seropositive for autoantibodies. Twenty-three (23) patients had adult-onset disease, and 11 patients had juvenile-onset disease; their demographic and clinical data are shown in Table 2.

As for the laboratory results, the serum creatine kinase at the time of diagnosis for the studied samples ranged from 64U/L to 19,000 U/L. Twenty-three (23) patients tested positive for a single autoantibody, 9 tested positive for more than one antibody, and 5 had antibodies associated with more than one subtype. The distribution of antibodies is illustrated in Fig. 2.

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Patient data were categorized according to antibody-associated subtypes into five subtypes: IMNM, DM, overlap syndrome, anti-synthetase syndrome and a group that tested positive for multiple antibodies. Their demographic, clinical and laboratory data are presented in Table 3.

The distribution of different autoantibodies in both adult-onset and juvenile-onset patients is illustrated in Fig. 3. A comparison of the different subtypes of both groups is shown in Table 4.

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Radiological data included MRI of whole-body muscles that was obtained for 57 patients, including 26 patients who were seropositive for antibodies and 31 who were seronegative. For patients with positive antibody results, those who presented within the first year of illness showed no MRI changes or muscle edema without fatty infiltration, and the presence of edema and fat in relation to the duration of illness is illustrated in Fig. 4.

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MR images of the different subtypes of patients in our study with different illness durations are shown in Fig. 5.

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Idiopathic inflammatory myopathies encompass a group of chronic, autoimmune conditions that primarily impact the proximal muscles. Various diagnostic and classification criteria have been proposed for myositis, incorporating diverse elements such as clinical presentation, laboratory tests including myositis-specific antibodies, electromyography (EMG), muscle biopsy, and muscle MRI [19, 20].

Our study revealed that in the adult-onset group of patients, there was a predominance of females, with dermatomyositis being the most common subtype. This finding aligns with the findings from the EuroMyositis Registry in 2017, which also reported a high prevalence of dermatomyositis and a predominance of female patients (69%). Conversely, the juvenile-onset group in the study showed a predominance of males, with overlap myositis being the most common subtype, which is different from the findings of Tansley and colleagues in 2017, who reported that dermatomyositis was the most common subtype in the juvenile cohort, with a predominance of female patients [21, 22].

One of the mentioned findings in a review carried out by Fernandez and colleagues was that patients diagnosed with IMNM have the greatest elevation in muscle enzyme levels among other forms of myositis, which is consistent with our results (Table 3) [23].

In our study, more than half of the tested patients (56%) were seronegative, potentially due to the presence of untested or novel unidentified antibodies, as suggested by Betteridge and McHugh (2016). In the juvenile-onset group, 44% of patients exhibited positive autoantibodies, 54% of whom had myositis-specific antibodies. This is comparable to the findings of Tansley and colleagues., who identified autoantibodies in 60% of the juvenile-onset cohort within a large British sample, with a similar MSA incidence of 49% [20, 22].

Similar methodology for antibody testing was not found in Middle Eastern studies but was partly related to a 2020 study in India, which included 250 IIM patients (both adults and children) and found myositis autoantibodies (MSAs/MAAs) in 59.2% of patients, with 60.7% in adults and 50% in juvenile IIMs. The ARS subgroup was the most common in the adult group, and anti-Jo-1 was the most common type of MSA, followed by anti-Mi2, which is different from our results in which anti-HMGCR was the most frequently encountered antibody, followed by anti-Ku and anti-Ro52 in our adult group, yet their patients were not tested for anti-HMGCR [24].

Studies on inflammatory myositis in Egypt are limited. One notable study by EL Sherif (2020) on immune-mediated necrotizing myopathy included 18 patients, 83% of whom were seropositive. In the Middle East, research is sparse. A study performed in the year 2023 in Saudi Arabia reviewed 26 patient records over 12 years, focusing on clinical and serological results, specifically anti-Jo-1 antibody results, and identified dermatomyositis as the most common subtype. Similarly, Chatti and colleagues in 2023 conducted a 21-year retrospective study in Tunisia involving 85 idiopathic inflammatory myopathy patients, of which 46 were classified as having dermatomyositis. This study reviewed epidemiological, clinical, and laboratory data, revealing MSAs in 16 patients and treatment responses. In Morocco, a retrospective study over 15 years involving 14 patients focused on clinical, laboratory, and muscle biopsy data, although autoantibodies were not tested [25‐28].

In our studied IMNM subtype, the total number of anti-HMGCRRs was greater than that of anti-SRP; in the adult group, anti-HMGCR antibodies were present in 9% of the total cases, surpassing the prevalence of anti-SRP antibodies, which were found in 2% of the cases. Conversely, in the juvenile-onset group, the prevalence was reversed, with anti-SRP antibodies being more common. This finding is consistent with the results of Tansley and colleagues study on autoantibodies in juvenile-onset myositis, where anti-SRP antibodies were detected in 2% of the patients, and anti-HMGCR antibodies were identified in 1% of the patients. Additionally, in the 2020 El Sheriff study on IMNM patients in Egypt, the number of anti-SRP (45%) patients was greater than that of those who tested positive for anti-HMGCR (33%) [22, 25].

In our patients, pain was reported more often in the IMNM (n = 4) and dermatomyositis (n = 4) subtypes, while skin manifestations and bulbar and respiratory manifestations were highest in the dermatomyositis subtype (Table 3). The presence of myalgia has been previously studied and linked to disease activity and outcome, as concluded by Pillai and colleagues in 2024, who examined 50 myositis patients for the presence and degree of pain longitudinally over a period of 6 months; however, this was not correlated with different IIM subtypes [29].

We included 5 patients in our study, two with juvenile onset and 3 with adult onset with the coexistence of antibodies of different subtypes. Anti-SRP and anti-Ku were the most common in this group, each occurring in 3 patients, and both were detected in two patients (Table 4). This was studied by Huang and colleagues, who performed additional immune assays to confirm double-positive and false-positive cases. Although the coexistence of more than one MSA is rare, it still exists. Additionally, the clinical features tend to skew to one subtype instead of a mixed phenotype [30]. In a 2023 study by Zheng and colleagues, three patients were shown to have both anti-SRP and other antibodies, including anti-TIF1-γ, anti-Jo1, and anti-EJ. The study concluded that this coexistence led to diverse clinical symptoms and outcomes, primarily driven by one of the myositis-specific antibodies or through interactions in a complex syndrome, thereby broadening the clinical spectrum of idiopathic inflammatory myopathy. Additionally, three patients were identified with coexisting anti-SRP and other antibodies (PL-7/12, anti-Ku, Jo-1, PM-Scl75, and MDA5). These patients exhibited varying clinical severities; one patient was wheelchair-bound at the time of diagnosis with respiratory muscle involvement, while the other two patients remained ambulatory [31].

The MRI findings of our patients who presented within the first year of onset showed normal findings or edema with no fat replacement. Patients who were investigated one year after onset showed variable degrees of muscle fat infiltration (Fig. 4). It has been shown recently that in all types of myositis, including IMNM, fatty replacement can begin early after disease onset, suggesting that early diagnosis and initiation of therapy may help to decrease long-term disability [23] this can be seen in MR images from our study that are shown in Fig. 5. Numerous publications have discussed MRI findings in immune-mediated necrotizing myopathy in relation to muscle involvement and pre- and posttreatment observations. For instance, Mohassel and colleagues (2019) assessed six patients with anti-HMGCR antibodies using MRI before and after treatment to monitor disease remission. Despite these studies, the relationship between the duration of IIM at diagnosis and MRI findings remains poorly documented [32].

Our MRI findings in the IMNM group concurred with the fatty infiltration pattern, but posterior thigh muscles were more frequently affected by edema (Table 4), which partly concurs with the findings of the 255th ENMC workshop (2022) on muscle imaging in IIM, which described a suggested MRI pattern in IMNM as edema involving proximal muscles, including the anterior compartment of thigh muscles, while fatty replacement predominantly affected posterior and medial muscle groups; however, they concluded that there is no distinctive pattern for IMNM [14].

This is the first study performed in Egypt in a cohort of adults and children in whom we were able to diagnose and detect IMNM in both age groups. The incorporation of serum autoantibody testing has proven to be important for confirming the diagnosis and categorization of myositis patients. Early implementation, coupled with MRI muscle assessments, not only facilitates a more accurate determination of the actual prevalence of these diseases but also plays a pivotal role in formulating an effective management strategy. This comprehensive approach enables us to delineate appropriate lines of therapy and investigate the potential involvement of extra-muscular organs, thereby impacting disease mortality and morbidity outcomes.