Traditional Cardiac Imaging and Challenges in DMD Patients
TTE is a readily available non-invasive imaging modality and is the first line diagnostic test in most adult and pediatric cardiac centers. Traditionally, TTE global cardiac function by LVEF is use to monitor and guide therapy because it is widely available and easy to use. However, TTE is not the ideal modality for assessing early manifestations or progression in DMD-CM for several reasons16, 69-71. First, standard TTE imaging using LVEF rarely detects abnormal cardiac function during the first decade34, 39. Second, TTE acoustic window limitations is common in DMD patients due to altered body habitus, e.g. scoliosis, rib spacing abnormalities, and adipose deposition within the chest wall related to steroid treatment and disease progression34, 35, 72(Fig 1a-b). Third, regional assessment of cardiac deformation for myocardial strain with TTE uses indirect and relatively insensitive techniques and has significant limitations due to incomplete visualization of all myocardial segments32, 34, 53, 73 (Fig 1a-b). Lastly, TTE cannot provide assessment of myocardial tissue characterization including myocardial fibrosis by LGE which is the phenotypic mechanism of cardiac disease in DMD-CM (Fig 1c-d)33, 50, 51. To overcome these limitations, recent guidelines recommend the use of CMR when sedation is no longer need for primary screening in DMD patients 33-35, 53, 67, 72. CMR overcome the limitations of TTE even with poor acoustic windows when assessing the heart to determine disease status is vital (Fig 1e-f).
CMR has evolved from a research tool to a become a highly accurate and sensitive modality for cardiovascular imaging74. CMR has been shown to be more reproducible and provide accurate volumetric assessment without any geometric assumption and is less operator-dependent than TTE75, 76. Recent studies have shown that TTE is inadequate for detecting the presence of DMD heart disease in the first decade of life compared to CMR34, 39. TTE misclassified 20% of DMD patients as either normal or abnormal and 37% of the myocardial segments were not visible compared to CMR34, 35. In contrast to TTE, CMR is not affected by body habitus even in advanced stages of DMD skeletal muscle disease (Fig 1 a,b,e and f). It is independent of ventricular geometric assumptions and has been shown to be accurate and reproducible for global functional assessment of both ventricles43, 74. CMR offers a one-stop shop for cardiac assessment beyond cardiac anatomy, function, chamber sizes. CMR has many advantages over TTE and includes but not limited to assessing myocardial strain using a gold standard technique of tagged imaging, LGE for myocardial fibrosis and myocardial tissue characterization (Fig 1 e-f) 44, 76, 77. The 2017 Cardiac Care Considerations recommend CMR as the non-invasive imaging modality of choice when it can be performed without sedation64, 65, 67. CMR is now routinely use in many Certified Duchenne Care Centers based on recommended guidelines.