Chronic Lacunar Infarct: This is the most likely diagnosis given the presentation of a lacunar stroke of indeterminate age. Lacunar infarcts are small (usually < 15 mm in diameter) and result from the occlusion of penetrating arteries that supply the deep structures of the brain. The indeterminate age suggests that the infarct could be chronic, which aligns with the natural history of lacunar strokes that may not always present with clear acute symptoms.

Subacute Lacunar Infarct: This could be considered if the clinical presentation suggests a more recent event than a chronic infarct but not acute enough to be clearly defined as an acute stroke. The timeframe for subacute is generally considered to be between a few days to a couple of weeks after the event.
Perivascular Space (Virchow-Robin Space): Enlarged perivascular spaces can sometimes mimic lacunar infarcts on imaging. They are benign and represent fluid-filled spaces surrounding blood vessels as they penetrate the brain.

Cavernous Malformation: Although rare, cavernous malformations can present with similar imaging findings to lacunar infarcts. They are vascular lesions that can bleed, leading to significant morbidity or mortality, making them a critical diagnosis not to miss.
Brain Tumor (e.g., Glioma): Certain brain tumors, especially low-grade gliomas, can have a non-specific appearance on imaging that might be confused with a lacunar infarct. Missing a diagnosis of a brain tumor could have significant implications for treatment and prognosis.
Infectious Processes (e.g., Brain Abscess): In rare cases, an infectious process like a brain abscess could present with imaging findings similar to a lacunar infarct, especially if the abscess is small and in a deep location. Missing this diagnosis could lead to severe consequences, including death.

Migraine Infarction: Although rare, migraine can very rarely lead to infarction, which might present similarly to a lacunar stroke on imaging. This would be considered in patients with a history of migraine, especially if there are other atypical features.
Vasculitis: Certain forms of vasculitis, such as primary central nervous system vasculitis, can lead to small vessel occlusion and infarcts that might resemble lacunar strokes. This diagnosis is rare but important to consider due to its treatment implications.
Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes (MELAS): This is a rare mitochondrial disorder that can cause stroke-like episodes, which might appear similar to lacunar infarcts on imaging. It's crucial to consider this in younger patients or those with a suggestive clinical history.