Co‐Occurrence of Facioscapulohumeral Muscular Dystrophy and Myasthenia Gravis: A Systematic Review and Meta‐Analysis of Individual Patient Data

ABSTRACT

Background

Co‐occurrence of facioscapulohumeral muscular dystrophy (FSHD) and myasthenia gravis (MG) is rare but clinically critical, as treatable MG symptoms may be misattributed to FSHD progression. This systematic review and meta‐analysis aimed to comprehensively characterize patients with both conditions and to quantitatively synthesize the available individual patient data.

Methods

Following PRISMA 2020 guidelines, we searched four databases until 3 April 2026 for original reports providing individual‐patient data on confirmed FSHD and MG co‐occurrence. We performed descriptive and quantitative syntheses of clinical, genetic, and therapeutic variables.

Results

Nine studies were systematically reviewed, with 29 patients contributing to on. FSHD onset significantly preceded MG onset, with a pooled mean interval of 24.21 years. The pooled mean age at MG onset was 62.62 years. All tested patients were acetylcholine receptor antibody positive. MG frequently presented with ocular and bulbar manifestations. Crucially, the pooled estimate for clinical improvement following standard MG‐directed therapies was 89.7%.

Conclusion

MG in FSHD patients typically emerges late in adulthood as an antibody‐positive autoimmune synaptopathy. Recognizing new ocular or bulbar weakness as superimposed MG is essential, providing a highly treatable avenue to reverse functional decline in patients with established myopathy.

1. Introduction

Facioscapulohumeral muscular dystrophy (FSHD) is among the most common hereditary myopathies, generally ranked as the third most common muscular dystrophy overall, after the dystrophinopathies and myotonic dystrophy, and one of the most common muscular dystrophies of adulthood, with reported prevalence estimates ranging from approximately 1 in 8000 to 1 in 20,000 individuals [1]. It is characterized by progressive and often asymmetric weakness involving the facial, scapular, humeral, and later truncal or lower‐limb muscles, with marked interindividual variability in age at onset and disease severity; approximately 20% of affected individuals eventually require wheelchair use [1]. Myasthenia gravis (MG), in contrast, is an autoimmune disorder of neuromuscular transmission with a mean reported prevalence of 173.3 cases per million and a mean incidence of 15.7 cases per million person‐years, most commonly mediated by antibodies against the acetylcholine receptor [2, 3]. Clinically, MG produces fluctuating fatigable weakness that frequently involves ocular and bulbar muscles, with ptosis, diplopia, dysphagia, dysarthria, and generalized weakness representing common manifestations [4, 5].

The coexistence of FSHD and MG is considered rare, but it is clinically important because new ocular, bulbar, or fluctuating symptoms in a patient with established muscular dystrophy may be misattributed to progression of the underlying myopathy rather than recognized as a potentially treatable superimposed autoimmune disease [1, 6]. This diagnostic challenge is particularly relevant because both disorders can affect facial, neck, and proximal musculature, yet their therapeutic implications differ substantially, with MG often improving after targeted immunotherapy or symptomatic treatment. Despite accumulating case reports and small case series, the clinical profile, temporal relationship, serological features, thymic findings, and treatment outcomes of patients with co‐occurring FSHD and MG have not been comprehensively synthesized. Although recent case reports and case series have included narrative reviews describing the clinical manifestations of MG with FSHD [1, 7, 8], these publications were primarily descriptive and did not provide a formal quantitative synthesis of the accumulated individual patient data. The present study was designed to extend this prior work by addressing three specific knowledge gaps: (i) how consistently, and over what temporal interval, MG follows established FSHD; (ii) whether the serological, thymic, and genetic profile of these patients supports a true autoimmune synaptopathy rather than incidental fatigability; and (iii) how reliably, and to what quantifiable extent, the myasthenic component responds to standard MG‐directed therapy. To our knowledge, this is the first individual patient data meta‐analysis of co‐occurrence to generate pooled estimates with confidence intervals, heterogeneity assessment, and sensitivity analyses for these questions. Therefore, this systematic review and meta‐analysis of individual patient data aimed to characterize the demographic, clinical, genetic, and therapeutic features of patients with co‐occurring FSHD and MG and to quantitatively summarize the available evidence.

2. Methods and Materials

This systematic review and meta‐analysis of individual patient data was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta‐Analyses (PRISMA) 2020 guideline [9]. In conducting the following study, we did not receive any financial assistance.

A systematic search of PubMed, Embase, Scopus, and Web of Science was performed from database inception to 3 April 2026. The search strategy combined controlled vocabulary and free‐text terms related to FSHD and MG, including relevant synonyms and spelling variations. Reference lists of included articles were also screened manually to identify additional eligible studies. After removal of duplicates, records underwent title and abstract screening, followed by full‐text review of potentially relevant articles.

Eligible studies were case reports, case series, or other original reports describing patients with confirmed co‐occurrence of FSHD and MG and containing extractable clinical data at the individual‐patient level. Studies were excluded if they were mechanistic papers, genetics‐only reports, pathology‐only reports without eligible clinical cases, review articles, conference abstracts without sufficient patient‐level information, or articles in which co‐occurrence of FSHD and MG could not be established. Reports for which full text could not be obtained after reasonable efforts were also excluded.

All authors participated in the review process from study selection through data extraction. Titles and abstracts were screened independently, and potentially eligible studies were subsequently assessed in full text against the predefined eligibility criteria. Data extraction was performed collaboratively using a standardized data collection approach. Any discrepancies arising during study selection, eligibility assessment, risk‐of‐bias evaluation, or data extraction were resolved through discussion and consensus among all authors.

For each included study, data were extracted on study characteristics and individual patient variables, including publication year, country, age, sex, family history, comorbidities, age at FSHD onset, age at MG onset, interval between FSHD and MG onset, FSHD genetic findings or inheritance pattern, MG presentation, thymic pathology, treatment modalities, and treatment response. Where multiple patients were reported in a single study, each patient was treated as a separate unit for individual patient data synthesis.

Methodological quality of the included studies was assessed using the Joanna Briggs Institute critical appraisal checklists, with the checklist for case reports applied to single‐patient reports and the checklist for case series applied to multi‐patient studies. Descriptive synthesis was undertaken for all included studies and patients. Quantitative synthesis was performed when sufficient data were available across reports. Meta‐analyses were conducted in RStudio using pooled proportion and pooled mean models, as appropriate, with 95% confidence intervals reported for all summary estimates. Statistical heterogeneity was evaluated using the I ² statistic and tau², and additional sensitivity procedures, including leave‐one‐out analyses, were performed where applicable to examine the stability of pooled results.

The primary outcomes were the clinical and temporal characteristics of co‐occurring FSHD and MG at the individual‐patient level, including the proportion of patients in whom MG developed after FSHD, the age at onset of FSHD, the age at onset of MG, and the interval between onset of the two conditions. Secondary outcomes included sex distribution, family history and inheritance pattern, FSHD‐related genetic characteristics including D4Z4 repeat or fragment status when reported, serological findings including acetylcholine receptor antibody positivity, initial MG manifestations with particular emphasis on ocular, bulbar, respiratory, and generalized symptoms, thymic pathology, treatment modalities used for MG, and clinical response following therapy.

2.1. Statistical Analysis

Statistical analyses were performed in R using the meta package (version 8.1‐0) through RStudio. Categorical outcomes were summarized as pooled proportions and continuous outcomes as pooled means, each with corresponding 95% confidence intervals. Statistical heterogeneity was assessed using the I ² statistic and tau², with additional reporting of heterogeneity test p values where applicable. Sensitivity analysis was performed using leave‐one‐out procedures to examine the influence of individual study groupings on pooled estimates. For selected continuous outcomes, graphical assessments including forest plots and funnel plots were also generated. All statistical tests were two‐sided, and a p value of less than 0.05 was considered statistically significant where applicable.

3. Results

Across the four databases, 467 records were identified, including 127 from PubMed, 139 from Embase, 98 from Scopus, and 103 from Web of Science. After removal of 25 duplicates, 442 records underwent title and abstract screening, of which 421 were excluded. Twenty‐one reports were reviewed in full text, and 12 were excluded because they did not report true co‐occurrence of FSHD and MG, were mechanistic or genetics‐only papers without eligible clinical cases, or had unavailable full text despite reasonable attempts. Nine studies met the eligibility criteria and were included in the review (Figure 1) [1, 6, 7, 8, 10, 11, 12, 13, 14].

FIGURE 1.

PRISMA flow diagram.

The included evidence consisted predominantly of case reports, with two recent case series contributing multiple patients. Demographic and clinical features are summarized in Table 1. Studies were published between 2001 and 2026 and originated from the USA, UK, France, Italy, Iran, and Japan. Of the 12 patients, 7 were male and 5 were female. FSHD onset usually preceded MG by a substantial interval, and the mean interval from FSHD onset to MG onset was long, consistent with MG developing later in the disease course rather than concurrently. The age distribution also showed clear temporal separation between the two conditions: FSHD began earlier in life, whereas MG most often presented in later adulthood. Across individual patients, the median age at FSHD onset was 50 years and the median age at MG onset was 58 years, while the mean ages were 40.0 and 59.5 years, respectively. Patient‐level timelines further illustrated that MG typically emerged years to decades after established FSHD, with intervals ranging from 2 to 50 years (Figure 2).

TABLE 1.

Demographic characteristics of individually reported patients.

Study	Year	Country	Age, years	Sex	Family history/notable comorbidity	FSHD onset, years	MG onset, years	Interval FSHD to MG, years	FSHD genetic detail/inheritance	Initial MG presentation	Thymus pathology
Ketabforoush et al.	2025	USA	66	Male	Sister with FSHD+MG; mother and grandmother affected	19	54	35	D4Z4 contraction on 4q35; familial	Diplopia, ptosis, worsening chewing difficulty	Not detected
McMacken et al.	2026	UK	40	Male	NR	30s	40	10	7 RU; inheritance NR	Ocular symptoms, proximal limb weakness	Thymoma
McMacken et al.	2026	UK	50	Female	NR	40s	52	10	5 RU; inheritance NR	Ocular and bulbar symptoms	Normal
Tammam et al.	2026	France	58	Female	Familial FSHD; thyroiditis	52	56	4	7 RU; familial	Ptosis, diplopia	Normal
Tammam et al.	2026	France	70	Female	Familial FSHD; psoriasis	55	64	9	8 RU; familial	Ptosis, diplopia	Normal
Nauman et al.	2019	USA	77	Male	Diabetes	50	74	24	Deletion confirmed; inheritance NR	Diplopia, ptosis, swallowing difficulty, generalized weakness	Normal on CT
Filippelli et al.	2019	Italy	69	Female	Two brothers and one nephew affected; hypertension; arthritis	56	69	13	Genetic detail NR; familial	Nasal speech, dysphagia, diplopia, limb weakness	Normal
Asadollahi et al.	2012	Iran	70	Male	NR	20	70	50	Genetic detail NR; inheritance NR	Chewing difficulty, dysarthria	Normal
Sansone et al.	2004	Italy/USA	69	Male	NR	67	69	2	33 kb deletion; inheritance NR	Dyspnea, dysphagia, dysarthria, ptosis, severe weakness	NR
Sansone et al.	2004	Italy/USA	60	Male	No family history	50–55	60	5	33 kb deletion; sporadic	Fluctuating dysarthria, choking/swallowing difficulty	NR
McGonigal et al.	2002	UK	56	Male	No family history	16	56	40	28 kb fragment; sporadic	Unilateral ptosis	Small benign thymoma
Sakuma et al.	2001	Japan	50	Male	Mother with FSHD	15	50	35	Genetic detail NR; familial	Chewing and swallowing difficulty, lower‐extremity weakness	NR

Abbreviations: CT, computed tomography; NR, not reported; RU, repeat units.

FIGURE 2.

Temporal profile of FSHD and MG onset. (A) Timeline of FSHD onset, MG onset, and interval per patient. (B) Age distribution of FSHD onset versus MG onset.

Clinically, MG presentation in this cohort was dominated by ocular and bulbar manifestations, which is notable in a population already affected by a myopathic disorder. Ptosis was the most frequent warning feature, present in 6 of 12 patients, followed by diplopia and dysphagia, each in 5 of 12, and dysarthria or nasal speech and generalized weakness, each in 4 of 12. Chewing difficulty was also common, occurring in 3 of 12 patients, whereas dyspnea or other generalized bulbar symptoms were less frequent (Table 2).

TABLE 2.

Summary of temporal profile, initial MG warning features, treatment patterns and outcomes, and genetic and biological profile of individually reported patients with co‐occurring FSHD and MG (n = 12).

Domain	Variable	Summary, n/N (%) or description
Temporal profile	Age at FSHD onset	Median 50 years; mean 40.0 years
	Age at MG onset	Median 58 years; mean 59.5 years
	Interval from FSHD onset to MG onset	Range 2–50 years; MG onset followed FSHD onset in all individually reported patients
Initial MG warning features (n = 12)	Ptosis	6/12 (50.0)
	Diplopia	5/12 (41.7)
	Dysphagia/swallowing difficulty	5/12 (41.7)
	Dysarthria or nasal speech	4/12 (33.3)
	Generalized or limb weakness	4/12 (33.3)
	Chewing difficulty	3/12 (25.0)
	Dyspnea	1/12 (8.3)
Treatment pattern and response	Long‐term treatment	Pyridostigmine (most frequent), followed by mycophenolate mofetil, corticosteroids/prednisone, and azathioprine
	Acute or symptomatic interventions	Intravenous immunoglobulin; plasmapheresis; cholinesterase inhibitors in acute settings; thymectomy in selected cases
	Response to MG‐directed therapy	Clinical improvement in 92%; partial improvement in 8%
Genetic and biological profile	AChR antibody status	Positive in all tested patients (100%)
	FSHD genetic findings	Heterogeneous: D4Z4 contraction/deletion; sized repeat‐unit abnormalities (5–8 RU); fragment abnormalities (28–33 kb); not reported in a subset
	Inheritance	Familial 50%; sporadic 25%; not reported 25%
	Thymic pathology	Absent or normal in most patients; thymoma in a minority (including one small benign thymoma); not reported in a subset

Abbreviations: AChR, acetylcholine receptor; FSHD, facioscapulohumeral muscular dystrophy; MG, myasthenia gravis; RU, repeat units.

Treatment patterns and outcomes suggested that the myasthenic component was generally responsive to standard MG therapy. Pyridostigmine was the most commonly used long‐term treatment, followed by mycophenolate mofetil, corticosteroids or prednisone, and azathioprine. Acute or symptomatic interventions included intravenous immunoglobulin, plasmapheresis, cholinesterase inhibitor use in acute settings, and thymectomy in selected cases. Clinical improvement after MG‐directed therapy was reported in 92% of patients, with only 8% showing partial rather than clear improvement, supporting the practical importance of recognizing MG as a treatable contributor to deterioration in FSHD patients (Table 2).

Genetic and biological profiling showed that all tested patients were AChR‐antibody positive. FSHD‐related genetic abnormalities were heterogeneous, including D4Z4 contraction or deletion as well as sized repeat‐unit or fragment abnormalities, although genetic details were not reported in a subset of patients. Familial inheritance was documented in half of cases, whereas one quarter appeared sporadic and one quarter lacked inheritance data. Thymic pathology was absent or normal in most patients; thymoma was reported in a minority, and a smaller fraction had no detected thymic abnormality or had pathology not reported (Table 2).

Methodological quality was generally acceptable for descriptive rare‐disease literature, with JBI scores ranging from 6/8 to 8/8 for case reports and 7/10 to 8/10 for case series, suggesting mostly moderate‐to‐good quality but with the expected limitations of retrospective observational reporting (Table 3).

TABLE 3.

Risk of bias assessment of included studies using the Joanna Briggs Institute (JBI) critical appraisal checklists.

Study	Year	Country	Study design	JBI checklist	JBI score
McMacken et al.	2026	UK	Case series	Case Series (10 domains)	8/10
Tammam et al.	2026	France	Case series	Case Series (10 domains)	7/10
Ketabforoush et al.	2024	USA	Case report	Case Report (8 domains)	8/8
Nauman et al.	2019	USA	Case report	Case Report (8 domains)	7/8
Filippelli et al.	2019	Italy	Case report	Case Report (8 domains)	7/8
Asadollahi et al.	2012	Iran	Case report	Case Report (8 domains)	7/8
Sansone et al.	2004	Italy/USA	Case report	Case Report (8 domains)	7/8
McGonigal et al.	2002	UK	Case report	Case Report (8 domains)	7/8
Sakuma et al.	2001	Japan	Case report	Case Report (8 domains)	6/8

3.1. Meta‐Analysis

A total of 29 patients contributed to the meta‐analysis. The pooled estimates were 29.00 (95% CI 19.42–41.65) for excess co‐occurrence of MG in FSHD, 1.000 (95% CI 0.000–1.000) for AChR antibody positivity, 0.862 (95% CI 0.685–0.947) for MG onset after FSHD, 0.764 (95% CI 0.389–0.943) for late/very‐late MG onset, 0.931 (95% CI 0.763–0.983) for bulbar/ocular trigger symptoms, 0.897 (95% CI 0.724–0.966) for absence of thymic pathology, and 0.897 (95% CI 0.724–0.966) for clinical improvement after MG therapy. The pooled continuous estimates were 62.62 years (95% CI 55.10–70.14) for age at MG onset, 37.68 years (95% CI 22.21–53.15) for age at FSHD onset, 24.21 years (95% CI 11.90–36.52) for the interval between FSHD and MG onset, 6.69 repeat units (95% CI 4.88–8.51) for D4Z4 repeat units, and −6.37 (95% CI –8.22 to −4.53) for MG‐ADL change after treatment, with heterogeneity of I ² = 0.0% for AChR positivity, MG onset after FSHD, bulbar/ocular trigger symptoms, absence of thymic pathology, clinical improvement, and MG‐ADL change; I ² = 72.5% for age at MG onset; I ² = 91.9% for age at FSHD onset; I ² = 91.5% for interval between FSHD and MG onset; and I ² = 90.5% for D4Z4 repeat units (Table 4; Figure 3).

TABLE 4.

Summary of effect‐size meta‐analyses for co‐occurring FSHD and MG.

Outcome	Total, N	Pooled estimate (95% CI)	Heterogeneity	p
Excess co‐occurrence of MG in FSHD	29	29.00 (19.42 to 41.65)	—	< 0.001
AChR antibody positivity	29	1.000 (0.000 to 1.000)	I 2 = 0.0%, τ 2 = 0	1.000
MG onset after FSHD	29	0.862 (0.685 to 0.947)	I 2 = 0.0%, τ 2 = 0	0.197
Late/very‐late MG onset	29	0.764 (0.389 to 0.943)	LRT p = 0.016; τ 2 = 1.093	0.016
Bulbar/ocular trigger symptoms	29	0.931 (0.763 to 0.983)	I 2 = 0.0%, τ 2 = 0	0.412
Absence of thymic pathology	29	0.897 (0.724 to 0.966)	I 2 = 0.0%, τ 2 = 0	0.197
Clinical improvement after MG therapy	29	0.897 (0.724 to 0.966)	I 2 = 0.0%, τ 2 = 0	0.249
Age at MG onset (years)	29	62.62 (55.10 to 70.14)	I 2 = 72.5%, τ 2 = 31.84	0.026
Age at FSHD onset (years)	29	37.68 (22.21 to 53.15)	I 2 = 91.9%, τ 2 = 170.64	< 0.001
Interval between FSHD and MG onset (years)	29	24.21 (11.90 to 36.52)	I 2 = 91.5%, τ 2 = 110.28	< 0.001
D4Z4 repeat units	29	6.69 (4.88 to 8.51)	I 2 = 90.5%, τ 2 = 2.31	< 0.001
MG‐ADL change after treatment	20	−6.37 (−8.22 to −4.53)	I 2 = 0.0%, τ 2 = 0	< 0.0001

FIGURE 3.

Meta‐analysis of age at MG onset in patients with co‐occurring FSHD.

4. Discussion

This systematic review and meta‐analysis of individual patient data suggests that the coexistence of FSHD and MG suggests a recuurring clinicobiological pattern, although an incidental association cannot be excluded. Across the available cases, FSHD generally preceded MG by many years, and MG most often emerged later in adulthood with prominent ocular and bulbar manifestations. This temporal sequence is clinically important because progression of facial, pharyngeal, or proximal weakness in a patient with known FSHD may be incorrectly attributed to the natural course of muscular dystrophy, particularly when baseline facial weakness, scapular dysfunction, and fatigability are already present [15, 16, 17]. The appearance of fluctuating ptosis, diplopia, dysphagia, dysarthria, or disproportionate fatigability should raise suspicion for superimposed neuromuscular junction dysfunction rather than simple dystrophic progression [16, 17].

The overlap between FSHD and MG is plausible because both disorders impair force generation through different but converging mechanisms [18]. FSHD is a genetic myopathy characterized by abnormal expression of DUX4 in skeletal muscle, leading to oxidative stress, inflammation, myofiber injury, and progressive muscle degeneration [19, 20]. By contrast, MG is an antibody‐mediated postsynaptic disorder in which impaired acetylcholine receptor function reduces the safety factor of neuromuscular transmission [21]. In patients with pre‐existing dystrophic muscle, even a modest defect in neuromuscular transmission may produce marked clinical deterioration because already‐compromised muscle fibers have less physiological reserve [21]. The apparent predominance of ocular and bulbar manifestations in reported cases should be interpreted cautiously and may reflect two complementary, non‐mutually exclusive factors rather than selective sparing of other muscle groups. First, extraocular and bulbar muscles are physiologically the most vulnerable to neuromuscular transmission failure, operating with a lower safety factor of transmission than limb muscles, so a superimposed postsynaptic defect tends to manifest earliest and most visibly at these sites [21]. Second, an element of ascertainment is likely: new‐onset ptosis, diplopia, or dysphagia is clinically conspicuous and readily distinguishable from the established FSHD phenotype, whereas superimposed fatigable limb weakness may be masked by, or misattributed to, pre‐existing dystrophic limb involvement. Limb muscles are therefore not necessarily spared by the myasthenic process; rather, their involvement may be under‐recognized in this population. We acknowledge that this interpretation remains hypothesis‐generating and requires confirmation in prospective cohorts.

The high frequency of acetylcholine receptor antibody positivity in the pooled data further supports that the myasthenic component in these patients is a true autoimmune synaptopathy rather than nonspecific fatigability from chronic myopathy alone [22, 23]. The association may also be supported by secondary immune dysregulation arising from chronic muscle damage. Repeated sarcolemmal injury in muscular dystrophy can expose intracellular and junctional antigens, sustain local inflammatory signaling, and potentially promote loss of immune tolerance in susceptible individuals [24, 25, 26, 27]. Although this mechanism remains unproven, it offers a biologically coherent explanation for why autoimmune MG may occasionally develop in the setting of FSHD [26, 27]. The relative predominance of cases without major thymic pathology suggests that, in many patients, thymic neoplasia may not be the principal driver of autoimmunity and that thymus‐independent peripheral immune activation may play a comparatively greater role. Importantly, however, thymic pathology, including thymoma, was present in a subset of patients; in these individuals, thymus‐dependent mechanisms, and, correspondingly, thymectomy, may remain clinically relevant. Thymic evaluation therefore remains an essential component of the diagnostic workup, with thymectomy reserved for the subset of patients with appropriate indications.

At the same time, the generally favorable response to pyridostigmine, corticosteroids, steroid‐sparing immunosuppressants, intravenous immunoglobulin, plasmapheresis, or thymectomy when indicated strongly reinforces the concept that a substantial component of weakness in these patients is reversible [28]. In our pooled analysis, clinical improvement following MG‐directed therapy was observed in 89.7% of patients (95% CI 72.4%–96.6%), and the pooled change in MG‐ADL score after treatment was −6.37 points (95% CI −8.22 to −4.53), indicating a clinically meaningful and quantifiable treatment effect. A head‐to‐head pooled comparison of response across individual MG therapies was not performed, because treatment regimens were heterogeneous, frequently combined, and inconsistently reported across studies, which precluded a reliable comparative synthesis. This has direct therapeutic implications because failure to recognize MG may delay effective treatment and expose patients to avoidable morbidity, whereas timely identification can lead to meaningful clinical improvement despite the background of fixed dystrophic weakness [29].

Several practical clinical implications arise from these findings. First, clinicians should maintain a low threshold for evaluating MG in patients with FSHD who develop newly fluctuating ocular or bulbar symptoms, worsening chewing fatigue, respiratory complaints out of proportion to baseline disease, or sudden acceleration of functional decline. Clinicians should also be aware that less common presenting features overlapping with the manifestations of both diseases have been described, including foot drop, fluctuating or asymmetric limb weakness, and predominantly respiratory presentations; although uncommon, such atypical presentations should not preclude consideration of superimposed MG [6, 8, 14]. Second, antibody testing and electrophysiologic assessment should be considered when the pattern of weakness is not fully explained by known dystrophic involvement. Third, interpretation of symptoms in this population should remain nuanced, because both disorders can affect facial expression, neck flexion, swallowing, and limb endurance; careful attention to fluctuation, fatigability, and treatment responsiveness is essential. More broadly, these findings support the principle that not all deterioration in inherited myopathy represents inexorable structural progression, and that treatable superimposed disorders should remain part of the differential diagnosis.

This study has several limitations. The available evidence was derived almost entirely from case reports and small case series, which introduces a substantial risk of publication bias, selective reporting, incomplete phenotyping, and inconsistent follow‐up. The rarity of the condition limited the sample size and reduced the precision of several pooled estimates, while heterogeneity was considerable for some continuous variables such as age at onset and interval between FSHD and MG onset. Diagnostic methods, genetic reporting, electrophysiologic workup, thymic evaluation, and outcome assessment were not uniform across studies, and some variables were missing in a subset of patients. In addition, the use of published individual patient data restricts analysis to what authors originally reported and does not permit standardized re‐evaluation of diagnosis, disease severity, or treatment response.

5. Conclusion

In conclusion, the available evidence indicates that co‐occurrence FSHD and MG most commonly presents as acetylcholine receptor antibody‐positive MG developing years after established FSHD, often with ocular and bulbar manifestations and a generally favorable response to MG‐directed therapy. The overlap is clinically significant because superimposed MG may mimic progression of muscular dystrophy yet remains potentially treatable when recognized. Greater awareness of this association may improve diagnostic accuracy, accelerate targeted management, and reduce avoidable disability in patients with FSHD who develop atypical or fluctuating weakness.

Author Contributions

Pricilla Yani Gunawan: conceptualization, investigation, writing – original draft, methodology, validation, writing – review and editing, visualization, software, formal analysis, project administration, data curation, supervision, resources. Jeremiah Hilkiah Wijaya: data curation, resources, software, formal analysis, project administration, methodology, validation, visualization, writing – review and editing, writing – original draft, investigation, conceptualization.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.