# Clinical Cases: Muscle Physiology ## Case 1: Duchenne Muscular Dystrophy - Structural Protein Deficiency ### Clinical Image ![Duchenne Muscular Dystrophy](case_01_image.jpg) *Source: [Wikimedia Commons - Duchenne muscular dystrophy](https://commons.wikimedia.org/wiki/File:Duchenne-muscular-dystrophy.jpg) - Public Domain* ### Patient Presentation A 5-year-old boy is brought by his parents to the pediatric clinic because they are concerned about his motor development. He has difficulty keeping up with peers, falls frequently, and has trouble climbing stairs. His parents note that he uses his hands to "walk up" his legs when rising from the floor (Gowers sign). They also noticed that his calves appear unusually large compared to his thin thighs. Family history reveals that his maternal uncle died from a "muscle disease" at age 22. ### Demographics - Age: 5 years - Sex: Male - Family history: Maternal uncle died of muscle disease (X-linked inheritance pattern) ### Chief Complaint Progressive proximal weakness, frequent falls, difficulty climbing stairs, and calf enlargement ### Physical Examination - General: Alert, cooperative boy with waddling gait - Motor: - Proximal weakness: 3+/5 hip flexors, hip extensors; 4/5 shoulder girdle - Distal strength relatively preserved: 4+/5 distally - Positive Gowers sign: Uses hands to climb up legs when rising from floor - Pseudohypertrophy: Firm, enlarged calves bilaterally - Gait: Wide-based, lordotic, waddling (Trendelenburg) - Reflexes: Diminished at knees, present at ankles - Spine: Lumbar lordosis to compensate for pelvic girdle weakness - Cardiac: Regular rhythm, no murmur ### Workup - Creatine kinase (CK): 18,500 U/L (markedly elevated; normal <200) - Genetic testing: Deletion of exons 45-50 in DMD gene on X chromosome - Muscle biopsy: Absent dystrophin on immunohistochemistry, fiber size variation, increased connective tissue - ECG: Tall R waves in V1, deep Q waves in lateral leads (early cardiomyopathy) - Echocardiogram: Normal LV function currently (baseline for monitoring) - Pulmonary function tests: FVC 92% predicted (baseline for monitoring) ### Diagnosis Duchenne Muscular Dystrophy (DMD) ### Treatment 1. Glucocorticoids (prednisone or deflazacort) - prolongs ambulation by 2-3 years and preserves pulmonary and cardiac function 2. Physical therapy: Stretching to prevent contractures, moderate exercise 3. Cardiac monitoring: Annual echocardiogram, ACE inhibitor when EF begins to decline 4. Pulmonary monitoring: Annual PFTs, non-invasive ventilation when needed 5. Orthopedic management: AFOs for ankle contractures, scoliosis monitoring/surgery 6. Nutritional support: Monitor for obesity (steroids) and malnutrition (later stages) 7. Genetic counseling for family members 8. Consider emerging therapies: Exon-skipping antisense oligonucleotides (eteplirsen for exon 51), gene therapy trials ### Physiological Principles Demonstrated - **Dystrophin function**: Dystrophin is a cytoskeletal protein that connects intracellular actin to the dystrophin-associated glycoprotein complex (DAGC) in the sarcolemma, which anchors to the extracellular matrix. This creates a mechanical link that transmits force and stabilizes the membrane during contraction. - **Membrane damage without dystrophin**: Without dystrophin, the sarcolemma is vulnerable to contraction-induced damage. Membrane tears allow calcium influx, triggering necrosis and eventual replacement with fibrotic and fatty tissue. - **Pseudohypertrophy**: Calf enlargement results from replacement of muscle fibers with fat and connective tissue, not true muscle hypertrophy. The tissue is firm but weak. - **X-linked inheritance**: The DMD gene is on the X chromosome. Males (XY) with one mutant allele are affected; females (XX) are usually asymptomatic carriers but may have elevated CK or mild symptoms due to skewed X-inactivation. - **CK elevation**: Muscle damage releases creatine kinase into the bloodstream. Markedly elevated CK (>10,000) suggests ongoing muscle breakdown, characteristic of dystrophies. - **Cardiac muscle involvement**: Dystrophin is also expressed in cardiac muscle. Progressive cardiomyopathy is a major cause of death in DMD, requiring regular monitoring and early cardioprotective therapy. --- ## Case 2: Malignant Hyperthermia - Ryanodine Receptor Dysfunction ### Clinical Image ![Rhabdomyolysis](case_02_image.jpg) *Source: [Wikimedia Commons - Rhabdomyolysis urine](https://commons.wikimedia.org/wiki/File:Rhabdomyolysis_urine.jpg) - CC BY-SA 3.0* ### Patient Presentation A 22-year-old male undergoes general anesthesia for an elective knee arthroscopy. Shortly after administration of sevoflurane and succinylcholine, the anesthesiologist notices masseter muscle rigidity during intubation. Over the next 15 minutes, the patient develops tachycardia (150 bpm), marked hyperthermia (temperature rising to 40.5C), muscle rigidity throughout the body, and mixed respiratory and metabolic acidosis. The end-tidal CO2 rises dramatically despite increased minute ventilation. His urine becomes dark brown. ### Demographics - Age: 22 years - Sex: Male - Procedure: Knee arthroscopy under general anesthesia - Triggering agents: Sevoflurane (volatile anesthetic) and succinylcholine ### Chief Complaint Intraoperative crisis: Hyperthermia, muscle rigidity, tachycardia, and acidosis ### Physical Examination (Intraoperative) - Vital signs: HR 150, BP 180/110, Temp 40.5C (rising rapidly), SpO2 88% despite 100% FiO2 - EtCO2: 85 mmHg (severely elevated, indicating hypermetabolism) - Muscle tone: Generalized rigidity, masseter spasm - Skin: Mottled, diaphoretic - Urine: Dark brown (myoglobinuria) ### Workup - Arterial blood gas: pH 7.12, PaCO2 72, PaO2 68, HCO3 14, lactate 12 (severe mixed acidosis) - Serum potassium: 7.1 mEq/L (dangerous hyperkalemia from muscle breakdown) - Creatine kinase: 45,000 U/L (massive muscle breakdown) - Myoglobin: Markedly elevated in serum and urine - Genetic testing (after recovery): RYR1 mutation identified - Caffeine-halothane contracture test: Positive (gold standard if genetic testing unavailable) ### Diagnosis Malignant Hyperthermia (MH) ### Treatment 1. STOP all triggering agents immediately (discontinue sevoflurane) 2. Call for MH cart and additional help 3. Hyperventilate with 100% oxygen using clean circuit (no volatile anesthetics) 4. DANTROLENE 2.5 mg/kg IV bolus, repeat every 5 minutes until symptoms resolve (may need 10+ mg/kg) 5. Active cooling: Ice packs, cold IV fluids, cooling blanket 6. Treat hyperkalemia: Calcium, insulin/glucose, bicarbonate, consider dialysis 7. Maintain urine output >2 mL/kg/hr with fluids and mannitol to prevent myoglobin-induced renal failure 8. Monitor and treat arrhythmias (avoid calcium channel blockers with dantrolene) 9. ICU admission for continued monitoring 10. Genetic counseling and testing for patient and first-degree relatives 11. MedicAlert bracelet and anesthesia warning letter ### Physiological Principles Demonstrated - **Excitation-contraction coupling**: Normally, action potentials traveling down T-tubules activate dihydropyridine receptors (DHPR), which mechanically couple to ryanodine receptors (RyR1) in the sarcoplasmic reticulum, causing calcium release that initiates contraction. - **RyR1 mutation effects**: In MH, mutant RyR1 channels have abnormal sensitivity to triggering agents. Volatile anesthetics and succinylcholine cause uncontrolled, sustained calcium release from the SR. - **Hypermetabolic state**: Sustained elevated cytoplasmic calcium causes continuous cross-bridge cycling, depleting ATP and generating heat. SERCA works overtime trying to pump calcium back into the SR, further consuming ATP. - **CO2 production**: The hypermetabolic state massively increases CO2 production, causing the characteristic dramatic rise in end-tidal CO2 often the first sign. - **Rhabdomyolysis**: Sustained contraction, ATP depletion, and membrane damage cause muscle cell death, releasing myoglobin, CK, and potassium. - **Dantrolene mechanism**: Dantrolene directly blocks RyR1 calcium release channels, interrupting the cycle of uncontrolled calcium release and muscle contraction. It is the specific antidote for MH. - **Temperature rise is a late sign**: The hyperthermia results from heat generated by sustained muscle contraction and metabolism; rising EtCO2 and muscle rigidity typically precede fever. --- ## Case 3: Myotonic Dystrophy - Chloride Channel Dysfunction ### Clinical Image ![Myotonic Dystrophy](case_02_image.jpg) *Source: Clinical illustration of myotonia and muscle physiology* ### Patient Presentation A 35-year-old male presents to the neurology clinic with a 10-year history of difficulty releasing his grip after shaking hands or opening jars. He also complains of weakness, particularly in his hands and lower legs. He notes that symptoms are worse in cold weather but improve after repeated muscle use ("warming up"). His father and grandfather had similar symptoms, and his father required a pacemaker for heart block at age 50. On questioning, he reports excessive daytime sleepiness, swallowing difficulties, and recent diagnosis of cataracts at an unusually young age. ### Demographics - Age: 35 years - Sex: Male - Family history: Father and grandfather affected (autosomal dominant pattern), father with cardiac conduction disease ### Chief Complaint Difficulty releasing grip (myotonia), progressive weakness in hands and lower legs ### Physical Examination - General: Hatchet-shaped facies (temporalis and masseter wasting), frontal balding - Cranial nerves: Ptosis, weakness of facial muscles, dysarthria, bilateral cataracts - Motor: - Grip myotonia: Cannot release handshake for several seconds (demonstrates prolonged muscle contraction) - Percussion myotonia: Tapping thenar eminence causes sustained thumb adduction - Weakness: Distal > proximal, finger extensors and ankle dorsiflexors most affected - Reflexes: Diminished throughout - EMG at bedside: "Dive bomber" sound characteristic of myotonic discharges ### Workup - Electromyography: Myotonic discharges - waxing and waning amplitude and frequency producing characteristic "dive bomber" sound - Genetic testing: Expanded CTG trinucleotide repeat in DMPK gene (>1000 repeats; normal <35) - ECG: First-degree AV block, prolonged QRS - Echocardiogram: Mild LV dysfunction - Sleep study: Central sleep apnea, excessive daytime sleepiness - Glucose tolerance test: Impaired glucose tolerance (insulin resistance common) ### Diagnosis Myotonic Dystrophy Type 1 (DM1, Steinert disease) ### Treatment 1. Cardiac monitoring: Annual ECG, Holter monitor; consider pacemaker/ICD for conduction disease 2. Myotonia treatment (if symptomatic): Mexiletine (sodium channel blocker), avoid triggering drugs 3. Excessive daytime sleepiness: Modafinil 4. Cataract surgery when visually significant 5. Physical and occupational therapy 6. Monitor for respiratory insufficiency: PFTs, consider BiPAP 7. Avoid depolarizing muscle relaxants (succinylcholine) - can trigger prolonged myotonia 8. Anesthetic precautions: Increased sensitivity to sedatives and respiratory depressants 9. Genetic counseling: Anticipation (earlier onset and increased severity in successive generations due to repeat expansion) ### Physiological Principles Demonstrated - **Normal muscle relaxation**: After an action potential, muscle relaxation requires calcium reuptake by SERCA and termination of electrical activity. Chloride channels (ClC-1) stabilize the membrane potential near the chloride equilibrium potential, preventing spontaneous activity. - **Myotonia mechanism**: In myotonic dystrophy, abnormal RNA accumulation from the expanded repeat sequesters splicing factors, causing mis-splicing of chloride channel (CLCN1) mRNA. Reduced chloride conductance destabilizes the resting membrane potential. - **Repetitive firing**: With reduced chloride conductance, the membrane is hyperexcitable. A single action potential triggers repetitive depolarizations, causing sustained muscle contraction (myotonia) even after voluntary effort ceases. - **Warm-up phenomenon**: Repeated muscle activation eventually depletes sodium channel availability through inactivation, reducing hyperexcitability. This is why myotonia improves with repeated use. - **Cold worsening**: Cold temperatures slow sodium channel recovery from inactivation, but the membrane instability from chloride channel dysfunction persists, worsening myotonia. - **RNA toxicity**: Unlike DMD where the protein is absent, DM1 is caused by toxic gain-of-function of expanded CUG repeat RNA, which affects splicing of multiple genes, explaining the multi-system involvement (cardiac, CNS, endocrine, cataracts).