With the given information, I presume The MRI
was done for Brain and not for lower spine.
The Diagnosis of Chiari malformation
is made only when shift of brain stem is seen-
Therefore the diagnosis of Chiari Malformation is sufficient to tell that your brain stem is shifted.
The tethered Cord is diagnosis which can only be told after doing MRI for lower spine.
On cannot tell on Brain MRI about Tethered Cord.
Please get your whole spine MRI if your treating Doctor is concerned about tethered Cord.