Brief Answer:
Read below

Detailed Answer:
I read your question carefully and I understand your concern.

The foramen magnum is the canal from which the brain continues with the spinal cord, meaning it is a hole in the most inferior part of the skull at the junction with the vertebral column.

Now a Chiari 1 malformation is a type of abnormality in which the cerebellum, the little brain may be downward displaced, descending through the foramen magnum when it should not be there. The 13 mm means it's that much inside the foramen.

Now management depends, some patients have no symptoms at all may be found by coincidence. Some have only some headache and can still be managed conservatorily. But in those cases with more symptoms which could indicate that the cerebellum is compressing the junction between the brain and spinal cord (as there's only limited space in that canal) then surgery may be needed.

So you need a good neurological exam to evaluate whether there are other signs (the symptoms can certainly be due to that) and if yes to consult a neurosurgeon.

I remain at your disposal for other questions.

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