Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 61-year-old man is evaluated for a 10-month history of generalized weakness. He reports no pain or myalgia. History is significant for hypercholesterolemia treated with a stable dose of simvastatin for the past 3 years.

On physical examination, temperature is normal, blood pressure is 138/74 mm Hg, pulse rate is 70/min, and respiration rate is 16/min. BMI is 27. There is symmetric weakness of the arm and thigh muscles with slightly reduced grip and power of the finger flexors. No muscle tenderness is noted. There is no rash, skin thickening, or digital ulcers. Reflexes and the remainder of the physical examination are normal.

Laboratory studies are notable for a normal complete blood count, an erythrocyte sedimentation rate of 23 mm/h, and a serum creatine kinase level of 365 U/L.

Chest radiograph is normal. Electromyogram and nerve conduction studies show myopathic changes in the proximal and distal muscles of the extremities as well as some neurogenic changes.

Which of the following is the most likely diagnosis?

A: Amyotrophic lateral sclerosis
B: Inclusion body myositis
C: Myasthenia gravis
D: Statin-induced myopathy

MKSAP Answer and Critique

The correct answer is B: Inclusion body myositis.

The most likely diagnosis is inclusion body myositis (IBM), an insidious and slowly progressive inflammatory myopathy that occurs more commonly in men and in those over the age of 50 years. Muscle weakness may be diffuse and involve both the distal and proximal muscles. Although typically symmetric, IBM muscle involvement may be asymmetric in up to 15% of patients. Skin is generally spared. IBM is rarely associated with extramuscular manifestations such as rash, fever, or pulmonary involvement. Patients with IBM typically have only mildly elevated (typically <1000 U/L), or even normal, levels of muscle enzymes. The characteristic triad of electromyographic findings for myopathy includes short-duration, small, low-amplitude polyphasic potentials; fibrillation potentials at rest; and bizarre, high-frequency, repetitive discharges. This older male patient has developed slowly progressive weakness affecting both the proximal and distal muscles without any significant pain or stiffness. This presentation suggests a myopathy with weakness based on his history and physical examination, mild elevation of muscle enzymes, and abnormal electromyogram (EMG) results, all of which are most consistent with IBM.

Amyotrophic lateral sclerosis is characterized by progressive dysfunction of both upper motoneuron and lower motoneuron pathways in one or more areas of the body. Common upper motoneuron features are spasticity, hyperreflexia, and pathologic reflexes, including extensor plantar responses. Typical lower motoneuron features are muscle weakness, atrophy, fasciculations, and cramps. These findings are not present in the patient.

Myasthenia gravis is characterized by fluctuating, fatigable muscle weakness that worsens with activity and improves with rest. Neurologic examination may reveal bilateral asymmetric ptosis worsened by prolonged upward gaze, an expressionless or sagging appearance of facial muscles, a “snarling” smile, nasal speech worsened by prolonged speaking, and limb weakness that increases with exercise. None of these findings are present in this patient.

Statin-induced myopathy most commonly presents with muscle pain, tenderness, and cramping typically within the first 6 months of therapy, and EMG results are normal. This patient has no muscle pain, has an abnormal EMG, and has been taking a stable dose of a stain for years, making statin-induced myopathy unlikely.

Key Point

• Inclusion body myositis has an insidious onset, with muscle weakness that may be diffuse and involve both the distal and proximal muscles.

This content is excerpted from MKSAP 17 with permission from the American College of Physicians (ACP). Use is restricted in the same manner as that defined in the MKSAP 16 Digital license agreement. This material should never be used as a substitute for clinical judgment and does not represent an official position of ACP. All content is licensed to KevinMD.com on an “AS IS” basis without any warranty of any nature. The publisher, ACP, shall not be liable for any damage or loss of any kind arising out of or resulting from use of content, regardless of whether such liability is based in tort, contract or otherwise.