Differential Diagnosis for Fatigue in a 59-year-old with Metastatic Lung Cancer

Single Most Likely Diagnosis

• Adverse effect of medications: The patient is on multiple medications, including opioids (MS Contin and immediate-release oxycodone) and gabapentin, which are known to cause fatigue as a side effect. The recent increase in pain medication could also contribute to increased fatigue.

Other Likely Diagnoses

• Obstructive Sleep Apnea (OSA): Given the patient's BMI of 32, which falls into the obese category, and the fact that they are finally able to sleep through the night, it's plausible that untreated OSA could be contributing to their fatigue. The ability to sleep through the night could be a result of increased sedation from pain medications rather than improved sleep quality.
• Endocrine dysfunction: Metastatic cancer and its treatment can lead to various endocrine disorders, such as hypogonadism, adrenal insufficiency, or thyroid dysfunction, which can cause fatigue. The fact that the patient has metastatic lung cancer and is undergoing immunotherapy increases the likelihood of endocrine disturbances.
• Anemia or other hematologic effects: Although the CBC is reported as normal, anemia of chronic disease or other hematologic effects from cancer or its treatment could still be present and contribute to fatigue.

Do Not Miss Diagnoses

• Cardiomyopathy: While less likely given the normal physical exam and vital signs, cardiomyopathy ( potentially induced by chemotherapy or other factors) could lead to fatigue due to decreased cardiac efficiency. It's crucial not to miss this diagnosis due to its significant impact on prognosis and treatment.
• Adrenal insufficiency: This could be a consequence of metastatic disease affecting the adrenal glands or a side effect of certain medications. Adrenal insufficiency can be life-threatening if not recognized and treated promptly.

Rare Diagnoses

• Statins-induced myopathy: Although atorvastatin can cause muscle pain and weakness, outright myopathy leading to significant fatigue is less common. However, given the patient's report of fatigue worsening over the last month, and considering they are on atorvastatin, this possibility, while rare, should not be entirely dismissed without further investigation.
• Paraneoplastic syndromes: These are rare disorders triggered by an altered immune system response to a neoplasm. Certain paraneoplastic syndromes can cause endocrine dysfunction, neurological symptoms, or other systemic effects that might lead to fatigue. Given the patient's metastatic lung cancer, considering paraneoplastic syndromes is important, although they are rare.