Differential Diagnosis
The patient presents with a 6-week history of progressive fatigue, difficulty walking up steps, and brushing her hair, with muscle strength 4/5 in the proximal aspects of the upper and lower extremities. Given her history of breast cancer treated with chemotherapy, the differential diagnosis can be organized into the following categories:
- Single most likely diagnosis
- F) Lymphocytic infiltration of muscle: This option is the most likely diagnosis, as the patient's symptoms and muscle weakness are consistent with polymyositis, an inflammatory muscle disease that can be associated with cancer, including breast cancer. The patient's history of breast cancer and the absence of other systemic symptoms support this diagnosis.
- Other Likely diagnoses
- C) Drug-induced myocyte inflammation: Although the patient is not currently taking any medications that are commonly associated with myopathy, it is possible that a medication she is taking, such as a statin, could be causing her symptoms.
- E) Inherited type 2 muscle fiber atrophy: While less likely, given the patient's age and lack of family history, an inherited myopathy could still be considered.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- D) Immune-mediated demyelination: Although the patient's symptoms are more consistent with a myopathic process, an immune-mediated demyelinating disease, such as multiple sclerosis, could present with similar symptoms and would be important to rule out.
- G) Presence of anti-acetylcholinesterase antibodies: Myasthenia gravis, an autoimmune disease characterized by the presence of anti-acetylcholinesterase antibodies, could present with similar symptoms, particularly proximal muscle weakness, and would be important to consider.
- Rare diagnoses
- A) Deficiency of carnitine palmitoyltransferase: This is a rare genetic disorder that affects the metabolism of fatty acids and could present with muscle weakness, but it is less likely given the patient's age and lack of other symptoms.
- B) Destabilization of myelin metabolism: This option is less relevant to the patient's symptoms, as myelin metabolism is more closely related to demyelinating diseases, which are less likely in this case.