Ivan Ilych felt himself abandoned by everyone, and that they regarded his position with a salary of 3,500 rubles as quite normal and even fortunate. He alone knew that with the consciousness of the injustices done him, with his wife's incessant nagging, and with the debts he had contracted by living beyond his means, his position was far from normal. (3.2)
This is the first and only time Ivan Ilych experiences a sense of isolation before his sickness. And it's for a silly reason – nobody's willing to give him money to cover the debts he's racked up by living beyond his means. No one else seems to think having a salary of 3,500 rubles and not getting the promotion he wanted is terribly tragic. It's like Ivan feels isolated from everything whenever his life isn't what he wants it to be.
And it seemed to him that the meaning of what the doctor had said was that it was very bad. Everything in the streets seemed depressing. The cabmen, the houses, the passers-by, and the shops, were dismal. His ache, this dull gnawing ache that never ceased for a moment, seemed to have acquired a new and more serious significance from the doctor's dubious remarks. Ivan Ilych now watched it with a new and oppressive feeling. (4.9)
Ivan just had his first meeting with a doctor. The exam is inconclusive, but this doesn't prevent Ivan from getting nervous. And look at what effect his fear has. Ivan is normally cheerful, yet now everything around him seems bleak and depressing. It's just going on as usual, while he might be facing the end. The world at large isn't concerned with him and can't help him anyway. Ivan's fear is beginning to cut him off from the normal world around him.
There was no deceiving himself: something terrible, new, and more important than anything before in his life, was taking place within him of which he alone was aware. Those about him did not understand or would not understand it, but thought everything in the world was going on as usual. That tormented Ivan Ilych more than anything. He saw that his household, especially his wife and daughter who were in a perfect whirl of visiting, did not understand anything of it and were annoyed that he was so depressed and so exacting, as if he were to blame for it. (4.17)
Now Ivan starts to experience the strong sense of isolation from other people that will only grow as his disease worsens. To him it seems like no one else sees or cares about what he's experiencing. In particular, he seems to get no help from his family. To Ivan, what's happening is the most important and terrifying thing in his life. His wife and daughter are happy to just keep shopping.
At the law courts too, Ivan Ilych noticed, or thought he noticed, a strange attitude towards himself. It sometimes seemed to him that people were watching him inquisitively as a man whose place might soon be vacant. Then again, his friends would suddenly begin to chaff him in a friendly way about his low spirits, as if the awful, horrible, and unheard-of thing that was going on within him, incessantly gnawing at him and irresistibly drawing him away, was a very agreeable subject for jests. Schwartz in particular irritated him by his jocularity, vivacity, and savoir-faire, which reminded him of what he himself had been ten years ago. (4.23)
At work things just go on as usual, with no one caring about what Ivan's illness. Ivan's friends are so incapable of coping with anything unseemly in life that all they can do is laugh off his disease. The most horrible and terrifying thing Ivan's ever experienced seems to be joke to them. It's telling, though, that Schwartz – the worst of the group – reminds Ivan of himself.
With this consciousness, and with physical pain besides the terror, he must go to bed, often to lie awake the greater part of the night. Next morning he had to get up again, dress, go to the law courts, speak, and write; or if he did not go out, spend at home those twenty-four hours a day each of which was a torture. And he had to live thus all alone on the brink of an abyss, with no one who understood or pitied him. (4.27)
Ivan now feels completely cut off from the whole world around him. The work place is alienating, and home is even worse. His whole former life, in other words, offers him no comfort. Everywhere he could possibly go, he's just alone with his pain and his fear. All that mattered to him before no longer matters.
He would go to his study, lie down, and again be alone with It: face to face with It. And nothing could be done with It except to look at it and shudder. (6.12)
By this time Ivan has had another realization and become sure that he's going to die. Facing his death makes him more isolated than ever before. He's going to cease to be, while everything else will keep going on as it has. He has to wonder what his own life will mean, once it's over. Nothing and no one else can help him wrap his mind around his death or do anything to change it.
Once when Ivan Ilych was sending him away he even said straight out: "We shall all of us die, so why should I grudge a little trouble?" – expressing the fact that he did not think his work burdensome, because he was doing it for a dying man and hoped someone would do the same for him when his time came. (7.34)
Ivan discovers in Gerasim someone who can comfort him in his isolation. Gerasim recognizes, unlike everyone else, that he himself will die. That enables him to understand what Ivan is going through; he can put himself in Ivan's place. Gerasim's honesty about his own death could show Ivan that death isn't something he has to face alone. Every human being faces it, and they can face it together if they're open about it. But Ivan doesn't see that right away because he's too focused on himself.
His son had always seemed pathetic to him, and now it was dreadful to see the boy's frightened look of pity. It seemed to Ivan Ilych that Vasya was the only one besides Gerasim who understood and pitied him. (8.51)
Vasya is the only person besides Gerasim who can break through Ivan's isolation. This might be because he genuinely feels sorry for his father. Ivan can see that. Unlike Gerasim, though, Vasya probably can't understand what's happening. He does know that his father is suffering, though, and is not able to ignore that suffering in the same way his other family members do. Vasya has not grown into the false world to which everyone else belongs.
He removed his legs from Gerasim's shoulders, turned sideways onto his arm, and felt sorry for himself. He only waited till Gerasim had gone into the next room and then restrained himself no longer but wept like a child. He wept on account of his helplessness, his terrible loneliness, the cruelty of man, the cruelty of God, and the absence of God. (9.10)
It seems as though Ivan has barely thought about God up to this point. But if there's anyone Ivan is isolated from, it's God. Does God feel absent to Ivan because Ivan doesn't believe in him, or because Ivan hasn't thought of him before?
Latterly during the loneliness in which he found himself as he lay facing the back of the sofa, a loneliness in the midst of a populous town and surrounded by numerous acquaintances and relations but that yet could not have been more complete anywhere – either at the bottom of the sea or under the earth – during that terrible loneliness Ivan Ilych had lived only in memories of the past. Pictures of his past rose before him one after another. (10.6)
Real loneliness doesn't require physical isolation. Even though Ivan is surrounded by people, including family and friends, he has no connections to them. That reveals the artificial quality of all of the relationships Ivan has spent his life building. His only recourse at this point is to go deeper into himself and his past for comfort. Perhaps he does this because in his past, he did have genuine connections to his parents.