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I have had a sudden appearance of a skull indentation in my crown. I have had migraines for 15 years. I've had no head injury that I can recall. What could have caused the indentation? It is four fingers wide and about 1/4 inch deep.
Most people have various indentations in the skull. The skull is not one solid bone, but made up of several plates that aren't originally fused at birth, but do so later in development and up into adulthood. The one on the crown is usually caused by the "posterior fontanels." At birth, this is the "soft spot or area where the skull bone plates don't yet meet. It's covered with cartilage at birth, and will usually harden as the child gets older. If the fontanels is unusually wide or large, the filling in might be incomplete. It would be extremely rare for it not to harden at all. However, certain conditions are known to cause a sinking of the fontanels in infants: dehydration, malnutrition, or it can simply be a normal variant. Most likely, if there was no injury, you've just never noticed this subtle, yet completely normal, imperfection in the past. If you don't believe this is the case, a CBC, blood studies, urinalysis, and imaging studies are needed to determine whether or not a nutritional disorder or structural skull deformity is present.
I noticed it a few weeks ago when I washed my hair and it's so pronounced that I can't believe that I would have never noticed it before. My mom is a nurse so I asked if she had ever noticed a malformation of my head as a child. She said that after my soft spot closed up as an infant that my head was always perfectly rounded. She said she would have definitely noticed a spot of this size and depth in my head.
No one's head is perfectly rounded, though some are prettier than others, to be sure. I had the same revelation in the same area a few years ago, and wondered how it was possible I never noticed it before, but when I review a profile picture of me as a baby (and without hair), the indentation was clearly visible.
The skull usually fuses completely by two years of age, and aside from blunt trauma or injury to the head (which you would have certainly noticed). There aren't any other conditions that would cause it to sink in unless you had a rare, incomplete fusion of sutures/fontanels and were also severely malnourished or dehydrated. Growths on the brain or atrophy of the brain, or even having chunks of your brain removed entirely would not alter structure of the skull in this way... it's like a durable helmet. Unless you are having any other unusual symptoms, or have reason to believe you're severely dehydrated or malnourished, this is most likely a normal variant you simply haven't noticed yet.