Deformity angular ratio is associated with neuromonitoring changes without a vertebral column resection: spinal deformity is more influential than type of surgery

Purpose The purpose of this study is to investigate the effect of the deformity angular ratio (DAR) on intra-operative neuromonitoring (IONM) signal changes during posterior spinal fusion (PSF) without vertebral column resection (VCR). Methods Retrospective review of severe pediatric spinal deformity patients treated with PSF without VCR or three-column osteotomy from 2008 to 2018. Exclusion criteria were prior instrumentation, lack of IONM, and incomplete radiographic data. Coronal DAR (C-DAR), sagittal DAR (S-DAR), and total DAR (T-DAR) were calculated and compared between patients with IONM signal loss and those without. Results Two hundred and fifty-three patients met inclusion criteria. Forty-seven of two hundred and fifty-three (19%) patients had IONM signal loss. Intra-operative wake-up test was performed in seven cases; three of seven (43%) had a neurological deficit on wake-up test. All neurological deficits resolved at a mean of 41 days postop. IONM loss was associated with increased kyphosis ( p = 0.003) and was not associated with Cobb angle ( p = 0.16). S-DAR ( p = 0.03) and T-DAR ( p = 0.005) were associated with IONM signal loss but C-DAR was not ( p = 0.06). Increased incidence of IONM signal loss was seen with S-DAR > 7 ( p = 0.02) or T-DAR > 27 ( p = 0.02). Twenty-four of ninety-two (26%) patients with S-DAR > 7 had IONM signal loss compared to twenty-three of one hundred and sixty-one (14%) with S-DAR ≤ 7 (OR, 2.1; 95% CI, 1.1–4.0). Seven of sixteen (44%) patients with T-DAR > 27 had signal loss compared to forty of two hundred and thirty-seven (17%) patients with T-DAR ≤ 27 (OR, 3.8; 95% CI, 1.3–10.9). Conclusion Patients with S-DAR > 7 or T-DAR > 27 have a higher risk of IONM loss during pediatric PSF even in the absence of a VCR or three-column osteotomies. Level of evidence II.