Effects of C2 Hemisection with Daily Acute Intermittent Hypoxia or Chronic Intermittent Hypoxia on Cancellous Bone

Abstract

Daily exposureto “low dose” acute intermittent hypoxia (dAIH) is emerging as a novel therapyto promote recovery of breathing and other motor functions (without detectablepathology) after spinal cord injury (SCI). In contrast, those with SCIexperience sleep disordered breathing, leading to “high dose” chronic intermittenthypoxia (CIH), which can cause pathology. It is unknown if either dAIH or CIH influence bone.PURPOSETo assess 1) the effects ofC2 hemisection (C2Hx) on cancellous bone loss at the proximal humerus and 2) whetherdAIH or CIH elicit bone deficits at this skeletal site.METHODSMale Sprague‐Dawley rats were assigned to one of fourgroups: 1) SHAM animals in normoxia (21% 02, n=13); 2) chronic C2Hx (12 wks) innormoxia (C2Hx+Norm, n=9); 3) C2Hx+dAIH (Ten total 5‐min hypoxic bouts, 10.5%O2 x 5‐min normoxic bouts, n=6); or 4) C2Hx+CIH (2‐min hypoxic bouts, 10.5% x 2‐minnormoxic bouts, 8 hrs/day, n=8). All rats were preconditioned with normoxia for8 wks post‐surgery and then received continuous normoxia, dAIH, or CIH for 28days. Cancellous bone was quantified at the left (impaired) and right(unimpaired) proximal humeri via high‐resolution ex vivo microCT. Separate univariate 2 (Side: Left vs Right) x 4(Groups) ANOVAs were used to assess differences for all outcomes, with Tukey’s post hoc tests used when applicable. Severaloutcomes were also selected a prioriand evaluated with t‐tests, using the Holm‐Bonferroni correction to control TypeII error rate.RESULTSANOVA main effectswere present for Group, with post hoccomparisons indicating that all C2Hx groups displayed 22–30% lower proximalhumerus cancellous bone mineral density (BMD), lower cancellous bone volume (BV/TV),and lower trabecular number (Tb.N) and 30% higher trabecular separation (Tb.Sp)vs SHAM (all p<0.001). C2Hx+Norm and C2Hx+dAIH displayed no differences intrabecular thickness (Tb.Th) vs SHAM. In comparison, C2Hx+CIH exhibited 7%lower Tb.Th than SHAM (p<0.001). All C2Hx groups also displayed highertrabecular pattern factor (Tb.Pf) vs SHAM (p<0.01), indicating a lessconnected trabecular network. ANOVA main effects for Side indicated that across all groups, the left (impaired) limbs displayed 8–13% lower BMD, BV/TV, Tb.N, and Tb.Th (all p<0.01 to <0.001) and higher Tb.Sp and Tb.Pf than the right (unimpaired) limbs (both p<0.01). Paired samples t‐tests comparing the impaired vs unimpaired humerus within each group indicated no differences were present for any skeletal outcome in SHAMs. In comparison, C2Hx+Norm displayed 12–13%lower BV/TV and Tb.N (p<0.001) and higher Tb.Pf (p<0.01) in the impaired limb. No differences in skeletal outcomes were observed between the limbs in theC2Hx+dAIH or C2Hx+CIH groups, with the exception of higher Tb.Sp in the impaired limb after CIH (p=0.006).CONCLUSIONC2Hx produced cancellous bone deficits in both impaired and unimpaired humeri,with more pronounced bone loss in the impaired limb. dAIH produced minimalimpact on bone. Further investigation is needed regarding the potential deleterious effects of CIH on bone loss after C2Hx.Support or Funding InformationFunding: NIH SPARC grant OT2OD023854 and by resources from Malcom Randall VA Medical Center.