McArdle's (Glycogen Storage Disease Type 5) lecture for USMLE

Описание: Simple handwritten lecture on the genetic disease known as Glycogen Storage Disease Type V also known as McArdle's Disease for Medical Students taking the USMLE. Discussing the pathophysiology, signs and symptoms of Glycogen Storage Disease as well as diagnosis, treatment and management of the condition

PATHOPHYSIOLOGY
McArdle's is a Glycogen Storage Disease which is a genetic disease which is inherited in an autosomal recessive fashion. Deficiency of the enzyme Myophosphorylase enzyme which is responsible for converted glycogen to glucose in the muscle. Muscle first prefers to use glucose, then fatty acid and then glycogen.

CLINICAL
Only with high intensity or short burst exercises they become symptomatic. Even low intensity but sustained gives them symptoms. The primary symptom is exercise intolerance, muscle cramping, fatigue and pain depending on the muscle being used. Immediately afterwards they get post exercise red urine due to rhabdomyolysis. This leads to myoglobinuria and then released in urine and leads to Renal Failure. Therefore the use of statin is contra-indicated in these types of patients. There is also a second-wind phenomenon they tend to be okay. This is becuase they can replenish glucose and Fatty Acid store and use that instead of glycogen. Patients with McArdle's syndrome are diagnosed with the ischemic forearm test. This involves measuring initial Creatine Kinase, Ammonia and Lactate level. Place blood pressure cuffs and inflate it. THen ask them to do repetitive grasping 1-2/sec for 2-3 mins. Therefore it forces the muscle to depend on glycogen stores rather than glucose. Then take post venous samples looking at CK Ammonia, and lactate in 5, 10 20 minute increments and take urine sample for myoglobinuria. Creatine Kinase is measure of muscle destruction. Normally, an increase in lactate and increase ammonia. If after the tests there is decreased lactate and decreased ammonia. This means that the exercise was not strenous enough. If there is a decrease in lactate and increase in ammonia. This means that there is not enough glucose in the blood suggesting pathway distrubance. This will prompt for a muscle biopsy.

TREATMENT
McArdle's is generally treatd with diet, such as increasing protein. sucrose before exercise will also help to have long exercise.