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 statin for years, making statin-induced myopathy unlikely.