Brief Answer:
No other investigations required, symptomatic treatment.

Detailed Answer:
Hello. I read your question and I understand your concern. I imagine such a report might seem daunting at first.
However as you might already imagine from the fact that this is a congenital condition, meaning you have had it during your whole life, it is not associated with any acute risk. What it could mean though, is that it limits somewhat the full range of movements of your spine and because it creates different load dynamics it could contribute to augmented strain to your vertebrae enhancing degeneration of the spine. Degeneration of the spine is to be expected in most people with aging, but in your case it is even more probable.
There is no necessary direct treatment for the vertebral fusion itself so in the absence of other data it is to be expected that a CT or MRI have not been considered necessary as they would not add anything to its management (that is if the vertebral fusion report is the only motive for CT/MRI being requested, if there is some other reason please let me know) .
I am assuming your complaint must have been back pain and rigidity for which your doctor has prescribed Tizanidine which is a drug used to relax your muscles and facilitate movement. If the pain is an issue over the counter analgesics like acetaminophen or non steroidal anti-inflamatory drugs should help (but of course their possible side effects like high blood pressure, gastric tract ulcers must be kept in mind, so discuss their use with your doctor first as I am sure he's familiar with them). If your symptoms are acute, some days of bed rest and later exercise to strengthen your back muscles are also helpful.
If you have further questions (with some more data on your signs and symptoms as well as other medical conditions), I remain at your disposal. I wish you a good health.

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