Before heading to Africa, Marlow has to visit a doctor. We only see the guy for a few minutes, but he gives us unpleasant feelings—and Marlow, too. he's not a symbol so much as an agent of foreshadowing, a reminder that the imperialist project affects everyone, and not always in good ways.
In particular, he seems to be a little too interested in whether there's any "madness" in Marlow's family, and he calls the information "my share in the advantages my country shall reap from the possession of such a magnificent dependency" (1.26). In other words—he sees the explorers and agents as one (completely unethical) scientific experiment. Key element? Measuring Marlow's skull, which he sees as something akin to taking scientific observations of his brain.
Will Marlow, too, be irrevocably changed from his journey into a murderous and obsessed madman? We're not sure. But toward the beginning of Marlow's journey, he remembers the doctor's words and says—in a rare moment of humor—"I felt I was becoming scientifically interesting" (1.51). He might not come back totally mad, but it's close.