In 'The Idiot', Dostoyevsky describes the main protagonist, prince Myshkin:
his eyes were large and blue, and had an intent look about them, yet that heavy expression which some people affirm to be a peculiarity. as well as evidence, of an epileptic subject.
However, this does not square with modern medicine, which rejects the notion that epilepsy can be traced in a person's looks, so I wonder if Dostoyevsky had something entirely different in mind?
I (think I) know that Epilepsy was considered a 'sacred disease' in antiquity and this seem to agree with the rest of Dostoyevsky's portrayal of Myshkin (especially given Myshkin's life-saving seizure when he is put under great stress by being threatened), but I doubt that medical science in Imperial Russia hadn't already given up any such notions of the disease?
The most relevant data point here is that Dostoevsky had epilepsy. He first started having seizures--generalized tonic-clonic (grand mal) ones--in prison in the 1850s. (Some people have speculated he presented with other types of seizures earlier; there's a whole debate about what specific seizure disorder he had, because of course there is). So we're working with a pretty specific data set given the immediacy of the issue: other people's descriptions of his own seizures to him, of which plenty survive.
In "The Insulted and the Injured," Dostoevsky writes in a character with epilepsy, Nelly, who (like his son) developed seizures early in life. He describes a tonic-clonic seizure...but then goes even more extensively into the aftermath. Often, tonic-clonic seizures and complex partial seizures are followed by a period of confusion and grogginess known as the "post-ictal" state; I generally describe it as swimming back up to full consciousness and passing out as soon as you get there. Dostoevsky, however, has a far more poetic way to describe it from the outside:
She looked at me for a long time without any motion, but with high tension, just as she tried to understand something. But it was obvious that it was troublesome for her. Finally, something like a thought enlightened her face. After an attack, she usually could not think clearly and only mumbled inapprehensible words.
That seems to track with the "heavy expression" of Myshkin--post-ictal grunkiness rather than a connection with mental illness (epilepsy is often comorbid with a whole range of neurological and psychiatric conditions) or the ictal (active) state of a complex partial seizure.
Just for kicks, I did a quick scan of Google Books from 1800-1850, and it looks like English-language literature uses "epileptic expression" to describe the post-ictal period as well.
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Addendum: Good heavens, there is a larger body of academic scholarship on Dostoevsky and epilepsy than there is on several of the authors I study in my dissertation. What a world.