The split elbow sign in amyotrophic lateral sclerosis: A systematic review and meta-analysis

Amyotrophic lateral sclerosis (ALS) patients can exhibit split phenomena, with preferential weakness of specific muscle groups. The aim of this review is to investigate the split elbow (SE) phenomenon (different weakness/wasting pattern between biceps and triceps) as a potential clinical and neurophysiological feature in ALS.

Our study was reported according to the PRISMA statement and registered in PROSPERO (CRD42024528359). MEDLINE, SCOPUS, Web of Science, and grey literature sources were searched using the terms "split elbow" and "amyotrophic lateral sclerosis" up to April 2025. English-written peer-reviewed, randomized, non-randomized, observational, diagnostic accuracy, and case-control studies were included. Study quality was assessed using Joanna Briggs Institute critical appraisal tool. Regarding muscle strength, we pooled the standardized mean difference of normalized Medical Research Council (MRC) scores using random effects. We used a bivariate random-effects model to evaluate SE index (SEI_CMAP, compound muscle action potential of biceps/triceps) in distinguishing ALS from controls.

Seven studies with 1941 ALS patients (61.8 % male) met inclusion criteria. Pooled standardized mean difference (triceps - biceps MRC scores) was -0.17 [95 % CI, -1.03 to 0.69], p = 0.63, indicating no significant difference in muscle strength between elbow flexion and extension. Between-study heterogeneity was high (I² = 97.1 % [95.5 %; 98.2 %], p < 0.0001). The SEI_CMAP demonstrated only moderate accuracy in distinguishing ALS from controls (pooled sensitivity, specificity, and AUC of 0.789 [0.655-0.880], 0.580 [0.487-0.668], and 0.661, respectively).

Current evidence does not support a consistent SE pattern in ALS. Methodological variability and small sample sizes limit the generalizability of available findings, indicating that the SE is unlikely to provide meaningful diagnostic utility in routine clinical practice.