Approach to differentiating lesions (muscle): Clinical sciences

Muscle lesions are conditions that affect the skeletal muscle system, eventually causing muscle weakness and atrophy. Based on the underlying cause, muscle lesions can be subdivided into hereditary conditions, which include periodic paralysis, muscular dystrophy, and myotonic dystrophy; and non-hereditary conditions, such as statin-induced myopathy, polymyositis, dermatomyositis, and inclusion-body myositis. Another type of acquired muscle lesion is called critical illness myopathy, which is seen in individuals requiring ICU management.

Now, if your patient presents with chief concerns suggestive of a muscle lesion, first perform an ABCDE assessment to determine if they are stable or unstable. If unstable, stabilize the airway, breathing, and circulation. At this point, you might have to intubate the patient and start mechanical ventilation. Next, obtain IV access, consider starting IV fluids, and place your patient on continuous vital signs monitoring, including heart rate, blood pressure, pulse oximetry, and cardiac telemetry.

Alright, let’s go back to the ABCDE assessment and focus on stable patients. Your next step here is to obtain a focused history and physical exam. History typically reveals symptoms of muscle involvement, primarily limb weakness. However, if there is weakness in the bulbar and respiratory muscles, the patient might report difficulty swallowing and breathing. In some cases, muscle weakness might be associated with pain.

On the exam, you will notice symmetric, bilateral limb weakness, sometimes in combination with weakness of neck and facial muscles. In advanced stages, you might even see muscle atrophy. Since these lesions only affect muscles and not sensory neurons or nerves, the sensory exam will be normal, while muscle tone and deep tendon reflexes could be normal or decreased. Finally, due to weakness of the hip and pelvic muscles, you might see a waddling gait. With these findings, consider a muscle lesion and assess the level of care of the patient.

If your patient is currently in the intensive care unit or was treated there recently, consider critical illness myopathy. Next, assess for risk factors associated with this condition, which include sepsis, multiorgan failure, and high-dose glucocorticoids with or without paralytics. If any of these are present, the likely cause of muscle weakness is critical illness myopathy.

Here’s a clinical pearl! Individuals requiring ICU management can also develop critical illness polyneuropathy, which is characterized by muscle weakness and sensory deficits.

Now, let’s go back and take a look at individuals not requiring ICU management. In this case, you should assess for a family history of similar symptoms. If present, consider hereditary causes of muscle lesions and then assess the tempo of symptoms. If the symptoms are episodic, consider hypo- and hyperkalemic periodic paralyses, which are autosomal dominant conditions with ion channel dysfunction. To determine the specific type, check serum potassium during an episode of weakness and obtain genetic testing.

In hypokalemic periodic paralysis, the patient will typically report that their episodes of weakness occur after eating meals rich in carbohydrates, alcohol use, or when resting after exercise. These episodes usually last hours to days. Labs will reveal low serum potassium during an episode of weakness, while the genetic testing will reveal a mutation in either a calcium channel gene, CACNA1S, or a sodium channel gene, SCN4A. With these findings, diagnose hypokalemic periodic paralysis.

On the flip side, in hyperkalemic periodic paralysis, the patient describes episodes that are triggered by potassium-rich meals, fasting, cold temperature, and rest after exercise. In contrast to hypokalemic episodes, hyperkalemic ones usually last hours, not days. They might also report baseline muscle stiffness due to impaired muscle relaxation after sustained muscle contractions.

The labs will reveal normal or high potassium levels during an episode of weakness, while on genetic testing, there is a mutation in the sodium channel gene called SCN4A. If you see these findings, that’s hyperkalemic periodic paralysis.

Now, let’s take a look at individuals with persistent and progressive symptoms. In this case, you can rule out periodic causes of muscle weakness. Next, assess the cause, thinking of Becker muscular dystrophy or myotonic dystrophy.

Becker muscular dystrophy is an X-linked recessive condition. Symptoms typically start during adolescence or early adulthood. Patients often report progressive difficulty walking, running, and climbing stairs. Next, they will report having biological male family members with similar symptoms, and in some cases, their history might reveal cardiomyopathy.

On the exam, you will notice calf pseudohypertrophy, which refers to an enlargement of the calves, initially due to compensatory growth of muscle fibers and later due to accumulation of fat and connective tissue. Next, you will notice weakness and atrophy of the limbs, which is more prominent in proximal than distal parts. Finally, the weakness of paravertebral and abdominal muscles will result in lumbar lordosis, while the weakness of hip and pelvic muscles will lead to a wide-based and waddling gait.