# Clinical Cases: Muscle Tissue - Cardiac and Smooth ## Case 1: Hypertrophic Cardiomyopathy ### Clinical Image ![Hypertrophic Cardiomyopathy - Echocardiogram](case_01_image.jpg) *Source: [Wikipedia - Hypertrophic Cardiomyopathy](https://en.wikipedia.org/wiki/Hypertrophic_cardiomyopathy) - CC BY-SA 3.0* ### Case Presentation A 19-year-old college basketball player collapses during practice and is found to be in ventricular fibrillation. He is successfully resuscitated with an automated external defibrillator. In the hospital, his echocardiogram reveals asymmetric septal hypertrophy with the interventricular septum measuring 22 mm (normal <11 mm) and systolic anterior motion of the mitral valve causing left ventricular outflow tract obstruction. Family history reveals his father died suddenly at age 42 during a tennis match. Genetic testing identifies a mutation in the MYH7 gene encoding beta-myosin heavy chain. Histologically (from autopsy studies of similar cases), hypertrophic cardiomyopathy shows characteristic myocyte disarray - cardiomyocytes are hypertrophied and arranged chaotically rather than in parallel, with increased interstitial fibrosis. He is diagnosed with hypertrophic cardiomyopathy (HCM) and an implantable cardioverter-defibrillator (ICD) is placed. He is advised to discontinue competitive athletics. ### Key Learning Points - HCM is often caused by mutations in sarcomeric proteins (beta-myosin heavy chain, myosin-binding protein C, troponins) - The mutations lead to abnormal sarcomere function, triggering compensatory hypertrophy - Histologically, myocyte disarray and interstitial fibrosis create electrical instability, predisposing to fatal arrhythmias - HCM is the leading cause of sudden cardiac death in young athletes - Understanding cardiac muscle structure (sarcomeres, intercalated discs) explains how sarcomere protein mutations produce both mechanical and electrical dysfunction --- ## Case 2: Arrhythmogenic Right Ventricular Cardiomyopathy ### Clinical Image ![ARVC - Fibro-fatty Replacement](case_02_image.jpg) *Source: [Wikipedia - Arrhythmogenic Right Ventricular Cardiomyopathy](https://en.wikipedia.org/wiki/Arrhythmogenic_right_ventricular_dysplasia) - CC BY-SA 3.0* ### Case Presentation A 32-year-old man presents after an episode of palpitations and near-syncope during jogging. His ECG shows T-wave inversions in leads V1-V3 and an "epsilon wave" (small positive deflection at the end of the QRS complex). Cardiac MRI reveals fatty and fibrous replacement of the right ventricular free wall with regional wall motion abnormalities. Genetic testing identifies a mutation in PKP2, the gene encoding plakophilin-2, a desmosomal protein. He is diagnosed with arrhythmogenic right ventricular cardiomyopathy (ARVC). The pathophysiology is explained: desmosomal proteins (plakophilin, desmoplakin, desmoglein, desmocollin) are essential components of the intercalated disc, specifically the desmosomes that provide mechanical adhesion between cardiomyocytes. When defective, cardiomyocytes detach and die during the mechanical stress of contraction, being replaced by fibro-fatty tissue. This creates a substrate for life-threatening ventricular arrhythmias. He receives an ICD and is restricted from competitive sports. ### Key Learning Points - ARVC results from mutations in desmosomal proteins of the intercalated disc - Intercalated discs contain three junction types: fascia adherens (anchors actin), desmosomes (mechanical strength via intermediate filaments), and gap junctions (electrical coupling) - Defective desmosomes cause cardiomyocyte detachment and death with fibro-fatty replacement - The fibro-fatty tissue disrupts normal electrical conduction, creating re-entry circuits and arrhythmias - This disease demonstrates the critical importance of intercalated disc integrity for cardiac function --- ## Case 3: Bronchial Asthma ### Clinical Image ![Asthma - Airway Smooth Muscle Constriction](case_03_image.jpg) *Source: [Wikipedia - Asthma](https://en.wikipedia.org/wiki/Asthma) - CC BY-SA 3.0* ### Case Presentation A 25-year-old woman with a history of allergies presents to the emergency department with acute shortness of breath, wheezing, and chest tightness that began after exposure to a cat. She is using accessory muscles of respiration and can only speak in short phrases. Physical examination reveals diffuse expiratory wheezing throughout both lung fields and prolonged expiratory phase. Peak expiratory flow rate is 40% of predicted. She is treated with nebulized albuterol (a beta-2 adrenergic agonist), systemic corticosteroids, and supplemental oxygen. Within an hour, her symptoms improve significantly. The pathophysiology is explained: allergen exposure triggers mast cell degranulation, releasing histamine and leukotrienes that cause airway smooth muscle contraction (bronchoconstriction). Albuterol binds beta-2 receptors on airway smooth muscle, activating adenylyl cyclase, increasing cAMP, which inhibits myosin light chain kinase (MLCK) and causes smooth muscle relaxation. She is discharged with an albuterol rescue inhaler and started on inhaled corticosteroids for maintenance therapy. ### Key Learning Points - Airway smooth muscle is regulated differently from skeletal muscle - contraction requires MLCK phosphorylation of myosin light chains - Beta-2 agonists cause smooth muscle relaxation by increasing cAMP, which inhibits MLCK - Smooth muscle lacks troponin; instead, calcium-calmodulin activates MLCK to enable cross-bridge cycling - Understanding the calcium-calmodulin-MLCK pathway explains why bronchodilators target this regulatory system - Smooth muscle can maintain sustained contraction (latch state) with minimal ATP expenditure, contributing to prolonged bronchoconstriction --- ## Summary: Cardiac and Smooth Muscle Disorders These cases demonstrate how understanding cardiac and smooth muscle histology informs clinical practice: | Disorder | Muscle Type | Structure Affected | Pathophysiology | |----------|-------------|-------------------|-----------------| | **Hypertrophic CM** | Cardiac | Sarcomeric proteins | Sarcomere dysfunction, myocyte disarray, hypertrophy | | **ARVC** | Cardiac | Intercalated discs (desmosomes) | Cell detachment, fibro-fatty replacement, arrhythmias | | **Asthma** | Smooth | Airway smooth muscle | MLCK-mediated bronchoconstriction, treated by beta-2 agonists | Understanding the unique structural and regulatory features of cardiac and smooth muscle - intercalated discs, calcium-induced calcium release, MLCK regulation - is essential for managing these conditions.