Tissue changes, as manifested by doughy, ropy, thickened, or fibrotic interstitial tissue, at T11-L2 was the strongest and most consistent osteopathic palpatory finding associated with type 2 diabetes mellitus in this study. There are several potential explanations for this finding. First, it may reflect other phenomena in the pathogenesis or progression of type 2 diabetes mellitus, such as diabetic nephropathy. The latter would be expected to contribute to reflex viscerosomatic changes at the T11-L2 segmental level, albeit bilaterally [4
]. Epidemiologic studies have refuted the notion that renal prognosis is benign in type 2 diabetes mellitus [9
]. Most studies that assessed the presence of diabetic nephropathy in patients with type 2 diabetes mellitus found that at least two-thirds of patients were affected [10
]. In fact, some patients may manifest with diabetic nephropathy several years prior to a diagnosis of diabetes mellitus. Although the clinical features of 15 such patients have been summarized [11
], the incidence rate and natural course of this phenomenon remains unclear. Also, in our study, the significant associations with tissue changes at T11-L2 bilaterally in subjects with type 2 diabetes mellitus and hypertension may reflect an augmented viscerosomatic response to two underlying diseases that promote nephropathy.
A second possible explanation is that residual, uncontrolled confounding may have contributed to a spurious association between type 2 diabetes mellitus and osteopathic palpatory findings at the T11-L2 spinal segmental level. For example, if other diseases involving the T11-L2 spinal segmental level were more often found in subjects with type 2 diabetes mellitus than in subjects without type 2 diabetes mellitus, then these other diseases may have spuriously inflated the observed ORs. Theoretically, the anatomic structures most likely to be associated with osteopathic palpatory findings at the T11-L2 spinal segmental level include the adrenal medulla, large intestine, appendix, kidney, ureter, urinary bladder, prostate, and uterus [4
]. Subgroup analyses or multivariate modeling would help address the issue of confounding; however, such analyses were limited by sample size constraints.
Finally, it is possible the observed association between type 2 diabetes mellitus and tissue changes at T11-L2 on the right side may have simply occurred by chance and may not be clinically related to type 2 diabetes mellitus at all. This appears unlikely because of the strength and consistency of the finding across analyses and because the subgroup analyses of subjects with type 2 diabetes mellitus and hypertension not only corroborated the significant association with tissue changes at T11-L2, but also demonstrated that the association was much stronger and occurred bilaterally. Additionally, among subjects with type 2 diabetes mellitus and hypertension, a strong diabetes mellitus duration effect was observed, thereby suggesting a temporal relationship between type 2 diabetes mellitus and subsequent tissue changes at T11-L2 bilaterally.
There are several limitations of this study that should be mentioned. First, this was a case-control study with a relatively small number of subjects. The inability to adequately perform more subgroup analyses or to control for additional confounders because of small sample size has already been noted. The assessment of type 2 diabetes mellitus status and examinations for osteopathic palpatory findings were performed in a cross-sectional manner. Although we hypothesize that osteopathic palpatory findings such as tissue changes at T11-L2 may be a manifestation of viscerosomatic reflexes associated with type 2 diabetes mellitus, a temporal relationship cannot definitively be established. One could argue that such osteopathic palpatory findings preceded the development of type 2 diabetes mellitus and, much like a trigger point, may have initiated somatovisceral reflexes [12
]. Nevertheless, with respect to temporality, the observation of a strong diabetes mellitus duration effect for tissue changes at T11-L2 bilaterally, in conjunction with hypertension, provides a rationale to suggest the existence of a viscerosomatic reflex.
Second, the osteopathic palpatory examinations were performed by predoctoral osteopathic manipulative medicine fellows. Although these fellows elected to take an additional year of training in osteopathic manipulation during their medical curriculum and received additional study-specific training, they likely did not have the same level of proficiency in performing osteopathic palpatory examinations as more seasoned clinicians. Osteopathic students have reported more palpatory findings than physicians, presumably because more experienced examiners filter out insignificant findings [13
]. A study of palpatory diagnosis found somewhat greater agreement between two osteopathic physicians than between the osteopathic physicians and a predoctoral osteopathic manipulative medicine fellow [14
Finally, one might argue that the reliability of osteopathic palpatory findings reported in this study was not sufficiently high to ensure validity. Our initial intent was to have three examiners for each subject to allow for a majority decision on each of the 30 osteopathic palpatory findings; however, this was not logistically feasible at the time that the study was implemented. Using kappa as a frame of reference [15
], many osteopathic palpatory findings in this study would be classified as having fair or poor interexaminer reliability. Four findings, all of which involved the left side, were associated with a negative kappa. Of these, the poorest level of agreement involved tissue changes at T11-L2 on the left side. Such poor reliability for this particular osteopathic palpatory finding may help explain why only unilateral (i.e., right-sided) statistically significant findings were initially observed for tissue changes at the T11-L2 spinal segmental level. Nevertheless, at the aggregate level, interexaminer reliability in this study (kappa, 0.35) was comparable to that reported for other commonly used diagnostic tests, such as exercise electrocardiograms to identify ST-T responses (kappa, 0.30) and peripheral blood films to diagnose iron-deficiency anemia (kappa, 0.39) [16
]. To replicate the study findings and overcome the limitations described above, larger prospective studies with more experienced osteopathic examiners are needed.