Approach to differentiating lesions (muscle): Clinical sciences

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Introduction0:00–0:42

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.

Unstable Patient0:42–1:19

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.

Stable Patient1:19–2:31

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.

Critical illness myopathy2:31–3:14

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.

Hypokalemic & hyperkalemic periodic paralysis3:14–5:26

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.

Becker muscular dystrophy5:26–7:47

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.
With these findings, you should consider Becker muscular dystrophy, Next, check serum creatine kinase levels and obtain genetic testing.
If the creatine kinase level is high, and genetic testing reveals a deletion or duplication in the Duchenne muscular dystrophy gene, resulting in partially functional dystrophin, diagnose Becker muscular dystrophy.
Here's a clinical pearl to keep in mind! Duchenne muscular dystrophy is a more severe form of muscular dystrophy, associated with no functional dystrophin production.
In this case, signs of muscle weakness start shortly after infancy and include delayed motor development, such as the ability to walk.
When getting up from the floor, children may show the Gowers sign. In other words, the child will use their hands to push off their thighs to stand up.

Myotonic dystrophy type 17:47–10:08

Okay, when it comes to myotonic dystrophy, these patients report difficulties loosening their grip, such as after a handshake or letting go of a doorknob.
They also have difficulties with fastening buttons and opening bottles and jars. Other important history findings include walking difficulties, foot drop associated with frequent trips or falls, slurred speech, and difficulty swallowing.
Keep in mind that these symptoms are worse with cold temperatures. In some cases, they might have nonmotor symptoms, such as cognitive impairment, vision disturbances due to cataracts, and palpitations due to arrhythmias.
Finally, the patient will report a first-degree family member with similar symptoms. On the exam, there is prominent wasting and atrophy of the temporal and masseter muscles, ptosis, and dysarthria.
You’ll also see weakness in the muscles of the face, neck flexion, as well as finger flexion and foot dorsiflexion. During the exam, be sure to ask the patient to grip and release your hands because a delay in grip relaxation, which is also known as grip myotonia, is a common finding in this condition.
Finally, if you use a reflex hammer to tap the thenar eminence, you will notice a prolonged contraction of the thumb, known as percussion myotonia.
With these findings, consider myotonic dystrophy, so perform electromyography and obtain genetic testing. The electromyography will demonstrate electrical myotonia, which makes a characteristic “dive bomber” sound from waxing-waning frequency and amplitude due to spontaneous discharges of muscle fibers.
Finally, if the genetic testing shows CTG trinucleotide repeat expansion on the DMPK gene, diagnose myotonic dystrophy type 1.
There is also myotonic dystrophy type 2, which occurs due to mutations in the CNBP gene. Type 2 typically presents later in life and primarily affects proximal limb muscles.
Finally, let’s take a look at individuals with no family history of muscle weakness. In this case, you should consider non-hereditary causes of muscle weakness.

Statin-induced myopathy10:08–11:20

Your next step is to assess the pattern of weakness in the upper extremities. In most myopathies, the weakness in the upper extremities is predominantly proximal, meaning the patient will typically describe having trouble with lifting their arms, such as placing objects in high cabinets or washing their hair.
If this is the case, assess for statin use. These are usually the patients who didn’t use statins before but started using them after a recent stroke or myocardial infarction.
If the patient reports weakness and muscle pain after the initiation of a statin, the likely diagnosis is statin-induced myopathy.
In severe cases, statins can cause rhabdomyolysis and even renal failure. Keep in mind that patients with statin-induced myopathy get better weeks after stopping the statin.

Polymyositis11:20–11:52

On the flip side, if the patient is not on statins, consider idiopathic inflammatory myopathies, such as polymyositis and dermatomyositis.
To differentiate the two, examine the patient’s skin and look for cutaneous findings. If the exam reveals no cutaneous findings, consider polymyositis and obtain a muscle biopsy.
If the biopsy reveals endomysial inflammation, diagnose polymyositis. Alternatively, you might find some cutaneous findings.

Dermatomyositis11:52–13:18

For example, you might see a heliotrope rash, which is a reddish-purplish rash on the eyelids. Next, there might be a shawl sign that appears as erythematous or scaling patches on the shoulders, upper back, and neck.
However, if these patches are on the anterior chest exposed by a V-neck shirt, that’s the V sign. You might also find Gottron papules, which are reddish-purplish papules over the metacarpophalangeal and interphalangeal joints of the fingers.
If you see them over other extensor surfaces like the elbows and knees, that’s a Gottron sign. Finally, you might find mechanic’s hands, which appear as rough and cracking skin on the sides of fingers and fingertips.
If you identify any of these skin findings, consider dermatomyositis, so again, proceed with a muscle biopsy to make a definitive diagnosis.
If the biopsy shows perifascicular and perimysial inflammation, diagnose dermatomyositis. Here’s a high-yield fact!
Both polymyositis and dermatomyositis are associated with some cancers, especially non-Hodgkin lymphoma. Alright, let’s discuss one last presentation of a patient without a family history of muscle weakness, who has a distal pattern of weakness in the upper extremities.

Inclusion body myositis13:18–14:05

In this case, consider inclusion body myositis, which is also an idiopathic inflammatory myopathy and typically affects adults over the age of 50.
Patients will report slowly progressive, painless weakness that preferentially affects the finger flexors, causing issues with handgrip, fastening buttons, and opening jars.
Again, obtain a muscle biopsy, and if it shows rimmed vacuoles in myofibers, diagnose inclusion body myositis. Alright, as a quick recap...

Review14:05–14:55

Muscle lesions are conditions that affect the skeletal muscle system, eventually causing muscle weakness and atrophy. Critically ill patients with sepsis or multiorgan failure can develop a unique type of condition called critical illness myopathy.
In non-critically ill patients, consider hereditary conditions like hypo- and hyperkalemic periodic paralyses, as well as muscular dystrophy and myotonic dystrophy.
On the flip side, if there is no family history of muscle weakness, consider statin-induced myopathy; and idiopathic inflammatory myopathies, such as polymyositis, dermatomyositis, and inclusion body myositis.