Surgical pathology of subaortic septal myectomy: histology skips over clinical diagnosis. 
Subaortic septal myectomy is usually performed to mitigate obstruction in patients with the obstructive form of hypertrophic cardiomyopathy (HCM) or in those with congenital subaortic stenosis.  Moreover, it is combined with aortic valve replacement in patients with severe aortic valve stenosis (SAS) and asymmetrical septal hypertrophy causing concomitant left ventricular outflow tract obstruction.  When both conditions coexist, it is conceptually difficult to identify a cardiomyopathy beyond an adaptive myocardial hypertrophy, strictly related to pressure overload.  Myectomy histopathology might be useful to enlighten the cause of the obstruction and establish the diagnosis.  The aim was to describe the pathological findings of surgical septal myectomy specimens obtained from a group of patients with diverse clinical diagnosis, including HCM, severe aortic stenosis, and asymmetrical septal hypertrophy.  This was a retrospective study of 56 patients undergoing septal myectomy along a 10-year period at a tertiary cardiac surgical center.  Clinical, interventional, and anatomopathological findings between patients with and without a preoperative diagnosis of HCM were analyzed and compared.  Mean age at intervention was 67.5±20.5 years; 37 (66.1%) were female Preoperative diagnosis of sarcomeric obstructive HCM was assumed in 23 (41.1%) patients.  All the other patients (58.9%) were referred for surgery with preoperative diagnosis of asymmetric septal hypertrophy, mainly in the context of severe aortic stenosis (24 patients).  Twenty-seven (48.2%) patients had a greater than 30 mmHg intraventricular gradient at rest.  Patients with presumed HCM were significantly younger (56.5±15.8 vs. 70.2±13.3 years, P<.001), had higher prevalence of significant intraventricular obstruction at rest [20 (87.0%) vs. 8 (34.8%), P<.001], and more frequently had moderate or severe mitral regurgitation [9 (39.1%) vs. 5(15.1%), P=.043].  All patients with aortic valve stenosis underwent both aortic valve replacement and septal myectomy.  Twelve (52.1%) of the patients with obstructive HCM had isolated septal myectomy, while in the remaining 11, the procedure was combined with intervention on the mitral valve.  Histopathological final diagnosis was of nonspecific reactive myocardial hypertrophy in all but 4 (92.2%) patients.  In those, 2 (3.6%) had the final diagnosis of HCM and 2 (3.6%) the diagnosis of congenital subaortic membranous stenosis with reactive myocardial hypertrophy.  Different grades of subendocardial fibroelastosis and myocardial fibrosis, mainly interstitial, were present [27 (48.2%) and 18 (32%) patients, respectively].  When microscopic data were compared between patients with or without a preoperative clinical diagnosis of HCM, no significant differences were found.  In patients submitted to surgical septal myectomy, histology was mostly indistinctive among different clinical entities.  Since different myocardial hypertrophy etiologies may share similar pathological expression, there is a need for detailed clinical assessment when trying to define the best strategy for clinical management.