Chronic Small Vessel Ischemic Disease: This is the most likely diagnosis given the description of mild nonspecific white matter disease related to mild small vessel disease in a 55-year-old male. The condition is common in this age group and can result from hypertension, diabetes, or atherosclerosis, leading to damage of the small blood vessels in the brain.

Hypertensive Encephalopathy: Although less likely than chronic small vessel disease, hypertensive encephalopathy can cause white matter changes, especially if the patient has a history of uncontrolled hypertension.
Multiple Sclerosis (MS): While MS is more common in younger populations, it can present at any age. The diagnosis would require further clinical and radiological evidence of demyelinating lesions disseminated in time and space.
Migraine: Chronic migraine can lead to white matter lesions, although this is less common and typically associated with other migraine-related symptoms.

Vasculitis (e.g., Primary Central Nervous System Vasculitis): Although rare, vasculitis can cause white matter disease and is critical to diagnose due to its potential for severe consequences and the need for immunosuppressive treatment.
Infectious Diseases (e.g., Neurosyphilis, Lyme Disease): Certain infectious diseases can cause white matter changes and are crucial to identify due to their treatability with antibiotics.
Leukoaraiosis due to CADASIL: Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL) is a hereditary stroke disorder leading to subcortical vascular dementia. It's essential to consider in patients with a family history of similar conditions.

Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes (MELAS): A rare mitochondrial disorder that can cause stroke-like episodes and white matter changes, more commonly seen in younger individuals.
Fabry Disease: A genetic disorder that can lead to vascular dysfunction, including small vessel disease in the brain, presenting with white matter lesions among other systemic symptoms.
Susac Syndrome: A rare condition characterized by microangiopathic changes in the brain, retina, and inner ear, leading to white matter lesions and other specific symptoms.