Acute Vestibular Syndrome (Labyrinthitis or Vestibular Neuritis): This condition is characterized by sudden onset of vertigo, often accompanied by nausea, vomiting, and balance problems. The symptoms can be severe and debilitating, matching the presentation of sudden vertigo and truncal ataxia.

Posterior Inferior Cerebellar Artery (PICA) Stroke: A stroke in the area supplied by the PICA can cause sudden vertigo, ataxia, and nystagmus. Given the age of the patient, vascular diseases are a significant consideration.
Cerebellar Hemorrhage or Infarct: Similar to a PICA stroke, any lesion in the cerebellum can cause ataxia and vertigo. The cerebellum's role in coordinating movements makes it a prime area for lesions causing ataxia.
Multiple Sclerosis (MS): Although less common in a 60-year-old male as a new diagnosis, MS can present with almost any neurological symptom, including vertigo and ataxia, especially if there are lesions in the cerebellum or its connections.

Brainstem Stroke: A stroke in the brainstem can have a varied presentation, including sudden vertigo, ataxia, and potentially life-threatening complications if not promptly treated. The brainstem's critical role in vital functions makes early diagnosis crucial.
Vertebrobasilar Insufficiency (VBI): This condition involves transient or chronic ischemia in the posterior circulation of the brain, which can cause vertigo, ataxia, and other symptoms. It's a "do not miss" diagnosis due to its potential for leading to a stroke.
Medullary Thyroid Cancer with Metastasis: Although rare, metastasis to the cerebellum or brainstem from medullary thyroid cancer can cause ataxia and vertigo. Given the potential for aggressive disease, it's essential not to miss this diagnosis.

Acoustic Neuroma (Vestibular Schwannoma): A benign tumor on the vestibular nerve can cause gradual onset of vertigo, hearing loss, and ataxia. While the onset is usually slower, it's a consideration in the differential diagnosis.
Spinocerebellar Ataxia (SCA): This group of genetic disorders can cause progressive ataxia. However, some forms may present later in life, and sudden worsening can occur, making it a rare but possible diagnosis in this context.
Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that can cause ataxia among other neurological symptoms. Its rapid progression and unique features make it a rare but important consideration.