Abnormal septal convexity into the left ventricle occurs in subclinical hypertrophic cardiomyopathy

Study population

A collaborative group at the Heart Hospital (University College London, United Kingdom)
have previously created and published a case–control cohort 7] composed of G?+?LVH- HCM participants matched to healthy volunteers on the basis
of age (±8 years), sex, body surface area (BSA ±10 %), and ethnicity. Inclusion criteria
for the G?+?LVH? group included: (1) maximal LV wall thickness 13 mm by CMR and mass
within the normal range relative to BSA, age, and sex; (2) sinus rhythm, no LVH, and
no pathological Q waves/T-wave inversion on 12-lead electrocardiography; and (3) no
causes of secondary LVH (valve disease, hypertension). Healthy volunteers had no history
of cardiovascular disease or hypertension, a normal physical examination, no family
history of inheritable cardiomyopathy or sudden cardiac death, and no personal history
of cardiac symptoms or cardiovascular disease (including hypertension) and with a
normal physical examination and ECG. Exclusion criteria for all participants were
the presence of conventional contraindications for CMR, claustrophobia, and arrhythmias
(e.g., atrial fibrillation, frequent atrial or ventricular ectopics). An ethics committee
of the UK National Research Ethics Service approved the generic analysis of anonymized
clinical scans. The genotyping project was approved by the UCL/UCLH Joint Research
Ethics Committee. All participants gave written informed consent conforming to the
Declaration of Helsinki (fifth revision, 2000). Study data were collected and managed
using REDCap electronic data capture tool (Research Electronic Data Capture, REDCap
Software – Version 5.9.6, http://www.project-redcap.org/).

Electrocardiography

Standard 12-lead electrocardiography was performed in the supine position during quiet
respiration. LVH was evaluated with the Romhilt-Estes 21] and electrocardiographic European criteria 22], 23]. Electrocardiographic images were analyzed by an experienced observer blinded to
clinical and CMR data.

Genetic screening

Genomic analysis of this cohort has been previously described in detail 7], 24], 25].?G+ individuals were defined as the ones carrying a known disease causing mutation
in one of the following sarcomere genes: myosin-binding protein C (MYBPC3), ?-myosin
heavy chain (MYH7), troponin T (TNNT2), troponin I (TNNI3), myosin regulatory light
chain (MYL2), myosin essential light chain (MYL3), tropomyosin (TPM1), and cardiac
?-actin (ACTC1).

Cardiovascular magnetic resonance image acquisition

Standard clinical scans (localizers, 3 long-axis views, black and white blood images,
full LV short-axis stack) were performed using a 1.5-T magnet (Avanto, Siemens Medical
Solutions®, Erlangen, Germany). CMR short-axis volumetric studies 26] were acquired from retrospectively gated, breath-held, balanced, steady-state free-precession
cines (slice thickness, 7 mm; interslice gap, 3 mm; flip angle, 60°–80°; repetition
time, 3.0 ms; echo time, 1.33 ms; field of view read typically, 380 mm; phase resolution,
75 %; typical acquired voxel size, 1.5?×?1.7 mm; 12 lines per segment). Late gadolinium
enhancement images acquired through an inversion recovery turbo fast low-angle shot
sequence were obtained 7 to 15 min after injection of 0.1 mmol/kg gadolinium-diethylenetriamine
penta-acetic acid.

Cardiovascular magnetic resonance analysis

The morphology, systolic function and the structure of the LV were evaluated by cardiologists
experienced in CMR (PR, MM). All CMR measurements were blinded to gene status. The
presence of fibrosis and the structure of the LV were evaluated by other cardiologists
experienced in CMR (JCM, GC, DMS).

Standard CMR measurements

LV volumes, LV ejection fraction (EF), LV outflow tract diameter, and LV mass were
determined according to standardized CMR methods 27] (papillary muscles were included in the LV mass). LV wall thickness was measured
at the septum and posterior wall on end-diastolic short-axis cine frames. The ratio
of maximal septal diastolic wall thickness to posterior wall thickness was calculated,
as well as relative wall thickness according to echocardiographic guidelines 11].

Measurement of septal convexity

The convexity of the interventricular septum into the LV was measured from the apical
4-chamber view as the maximal distance between LV septal endocardial border at mid
LV level and a line connecting mid-wall points at the level of tricuspid valve insertion
and at the level of apical right ventricular (RV) insertion on the LV (Fig. 1). Septal convexity (SCx) was expressed positively. In case of concavity (corresponding
to convexity into the RV – the normal arrangement), the measure was expressed negatively.
In addition, on the 3 consecutive short axis views at papillary muscle level, an evaluation
of the septal convexity was performed measuring the maximal distance between LV endocardial
border and a reference line joining the epicardium of the LV-RV insertion points (B
to A) (Fig. 2).

Fig. 1. Measurement of septal convexity (SCx) in apical 4 chamber view: performed as the maximal
distance (A to B) between the LV endocardial border (B) and the intersection point
(A), perpendicularly to a reference line joining at mid-wall the level of tricuspid
valve insertion (C) and the apical right ventricular insertion point into the LV (D)
in a 49-year old G?+?LVH- male (a), and in a matched healthy control (b)

Fig. 2. Measurement of septal convexity in short axis: views at papillary muscle level as
the distance (A to B) between septal LV endocardial border (B) and the perpendicular
intersection point (A) of a reference line connecting the epicardial RV insertion
points into LV (anteriorly: C, and inferiorly: D)

Measurement of other previously-described markers of HCM

LV mitral angle, mitral papillary muscles angle, and LV-aortic root angle were analyzed
on the open source software OsiriX® (http://www.osirix-viewer.com) as previously described 20]. The angle between LV outflow tract and basal septum was also analyzed. Left ventricular
end-diastolic sphericity index was measured (as previously described), as the ratio
of LV end-diastolic diameter measured in short axis view at papillary muscle level
(septal to lateral wall distance) divided by the LV end-diastolic long axis diameter
measured in the apical 4 chamber view 12]. LV end-diastolic and end-systolic eccentricity indexes were calculated in the short-axis
at the level of papillary muscles as the ratio of septal to lateral wall distance
divided by inferior to anterior wall distance 11].

LV end-systolic left atrial areas were measured by planimetry on 4-chamber view. Anterior
mitral valve leaflet (AMVL) length was measured using the method previously described
by Maron et al. 2] Additionally, the mitral valve annulus was measured in mid diastole in the apical
3 chamber view. The 3 long-axis views and a modified 2-chamber view (transecting RV
insertion points) were evaluated for the presence of myocardial crypts defined as
focal myocardial defects with a depth of ?50 % of the adjacent myocardium 5].

Statistical analysis

Descriptive data were analyzed for normality using the Shapiro-Wilks test and were
expressed as mean?±?standard deviation except where otherwise stated. Categorical
variables were compared using ?
²
tests. Non-categorical data were directly compared using paired t test. An optimal
threshold value for SCx within this case–control population was calculated as the
Youden Index derived from the area under the receiver operating characteristics curve.
Mean variability within and between readers was evaluated using the mean of differences
between two measurements. Paired measurements for repeatability of SCx were evaluated
using the Bland–Altman method 28]. A 2-sided p value 0.05 was considered significant. Statistical analysis was performed
using SPSS for Windows version 20.0 (Chicago, IL).