The term "silent migraine" is sometimes used for visual disturbances like auras (which precede a migraine) in the eyes, when no headache follows.

What is really happening is that the blood vessels in the retina of the eye are spasming and causing visual symptoms.

These vessels are very small and prone to damage from other cardiovascular disease states, or diabetes as well.

I think in your case the doctor is applying this concept of vascular spasms to the inner ear. You can have balance and dizziness problems with viral infections, blows to the head or
problems with blood flow to the inner ear where balance is sensed. Inside the inner ear are tiny hairs which detect position of the body. They can be broken off and fail to work if you expose yourself to repetitive high decibel sound, or get an infection there, or if the blood supply is reduced. This reduction can be a spasm of the vessel, which then passes. Or a complete blockage like a stroke (blood clot) which may be permanent.
Drugs, toxins, also can damage the nerves themselves in the inner ear, but if a doctor said "silent migraine" then he/she is referring to spasms of the blood vessels.

The term "silent migraine" is rather old fashioned, and really pertains to a vascular event that is temporary.

Poor blood vessel status can be due to diet, low B12, low B6 and low folate. It can be inflammatory and secondary to autoimmune disease like lupus or vasculitis.

I get the impression your doctor was just trying to explain in easier terms a very complex situation. Many patients don't understand the more complex terms doctors use to describe medical situations.