The 28-year-old has played just once this season, against Empoli in September - a game that saw him withdrawn at half-time after suffering from dizziness.
Subsequent tests have revealed that an accumulation of fluid, resulting in swelling on the brain, was the route of Castan's discomfort.
Roma coach Rudi Garcia has opted not to risk the Brazilian since and he could now be set for a longer-term absence as he prepares to go under the knife, though his neurological issue is not life-threatening.
A club statement read: "After the Empoli v Roma match, Leandro Castan suffered a bout of acute dizziness.
"The player underwent medical tests in the days after the match, which revealed a congenital vascular alteration in his brain, a condition more commonly known as a cavernoma.
"Inside the cavernoma, the tests revealed a small edema and, as a consequence, temporary swelling. This was the cause of the symptoms reported by the player, which subsided of their own accord within a few days.
"Although the cavernoma does not represent a threat to the player's life - indeed, he is currently in peak physical condition - he is at risk of reoccurrences.
"Therefore, the medical staff, club and player have agreed that he will undergo neurosurgery at the beginning of December in order to definitively cure the problem. This will allow his to resume regular competitive sporting activity."
