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TN is diagnosed with an MRA scan, which is a high quality MRI scan with contrast.
Regular MRI scans do not always show compressions, especially not those that involve small blood vessels.
An expert neurosurgeon needs to interpret the scan, since regular neurologist do not always have the expertise to spot the compressions.
In case no compressions are found, TN is diagnosed on a person's medical history only.
Sometimes a neurologist prescribes Tegretol to test if a person really has TN, since this particular anticonvulsant can be diagnostic (i.e. it nearly always provides pain relief in case of TN).