Spine-Specific Sarcopenia: Distinguishing Paraspinal Muscle Atrophy from Generalized Sarcopenia.

BACKGROUND CONTEXT: Atrophy of the paraspinal musculature (PM) as well as generalized sarcopenia are increasingly reported as important parameters for clinical outcomes in the field of spine surgery. Despite growing awareness and potential similarities between both conditions, the relationship between "generalized" and "spine-specific" sarcopenia is unclear.

PURPOSE: To investigate the association between generalized and spine-specific sarcopenia.

STUDY DESIGN: Retrospective cross-sectional study.

PATIENT SAMPLE: Patients undergoing lumbar spinal fusion surgery for degenerative spinal pathologies.

OUTCOME MEASURES: Generalized sarcopenia was evaluated with the Short Physical Performance Battery (SPPB), grip strength, and the psoas index, while spine-specific sarcopenia was evaluated by measuring fatty infiltration (FI) of the PM.

METHODS: We used custom software written in MATLAB® to calculate the FI of the PM. The correlation between FI of the PM and assessments of generalized sarcopenia was calculated using Spearman's rank correlation coefficient (rho). The strength of the correlation was evaluated according to established cut-offs: negligible: 0 - 0.3, low: 0.3 - 0.5, moderate: 0.5 - 0.7, high: 0.7 - 0.9, and very high ≥ 0.9. In a Receiver Operating Characteristics (ROC) analysis, the Area Under the Curve (AUC) of sarcopenia assessments to predict severe multifidus atrophy (FI ≥ 50%) was calculated. In a secondary analysis, factors associated with severe multifidus atrophy in non-sarcopenic patients were analyzed.

RESULTS: A total of 125 (43% female) patients, with a median age of 63 (IQR 55 - 73) were included. The most common surgical indication was lumbar spinal stenosis (79.5%). The median FI of the multifidus was 45.5% (IQR 35.6 - 55.2). Grip strength demonstrated the highest correlation with FI of the multifidus and erector spinae (rho = -0.43 and -0.32, p < 0.001); the other correlations were significant (p < 0.05) but lower in strength. In the AUC analysis, the AUC was 0.61 for the SPPB, 0.71 for grip strength, and 0.72 for the psoas index. The latter two were worse in female patients, with an AUC of 0.48 and 0.49. Facet joint arthropathy (OR: 1.26, 95% CI: 1.11 - 1.47, p = 0.001) and foraminal stenosis (OR: 1.54, 95% CI: 1.10 - 2.23, p = 0.015) were independently associated with severe multifidus atrophy in our secondary analysis.

CONCLUSION: Our study demonstrates a low correlation between generalized and spine-specific sarcopenia. These findings highlight the risk of misdiagnosis when relying on screening tools for general sarcopenia and suggest that general and spine-specific sarcopenia may have distinct etiologies.