Delirium due to Medication Side Effects: This is often the most common cause of delirium in patients, especially the elderly, who are more susceptible to the side effects of medications. Muttering can be a manifestation of the altered mental status seen in delirium.

Alzheimer's Disease with Delirium: Patients with Alzheimer's disease can develop delirium due to various factors such as infections, medications, or environmental changes, leading to symptoms like muttering.
Dementia with Lewy Bodies: This condition is known for causing fluctuations in cognitive function, visual hallucinations, and delirium, which could present with muttering.
Urinary Tract Infection (UTI): Especially in the elderly, UTIs can cause delirium, which may manifest as muttering among other symptoms.

Meningitis or Encephalitis: Infections of the brain or meninges can cause delirium and muttering. These conditions are medical emergencies and require prompt diagnosis and treatment.
Septicemia: Sepsis can lead to delirium and altered mental status, including muttering. Early recognition and treatment are crucial.
Hypoglycemia or Hyperglycemia: Both low and high blood sugar levels can cause delirium and muttering, especially in diabetic patients. These conditions are easily treatable but can be fatal if not addressed promptly.

Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that can cause dementia and delirium, potentially leading to muttering.
Neurosyphilis: A rare condition caused by the spread of syphilis to the brain, which can lead to delirium and other psychiatric symptoms, including muttering.
Hashimoto's Encephalopathy: A rare condition associated with Hashimoto's thyroiditis, which can cause delirium, seizures, and other neurological symptoms, including muttering.