Dorsal mri says.subtle intramedullary hyperintensity seen in cord extending from d5-d8 level-syrinx/myelitis. no cord expansion is seen means what?

There is not: enough specific info about the MRI finding or your clinical history to provide a very meaningful response. A syrinx is a longitudinal fluid collection in the spinal cord, small ones are usually incidental and of no concern, but they can be associated with other anomalies or tumors. Myelitis is inflammatory and very symptomatic. Other possibilities include demyelinating processe(e.g. MS).

Unclear: I'm not sure what levels are being defined here - usually see cervical, thoracic or lumbosacral spine defined on the spinal MRI. A hyperintensity would need to be defined by a given sequence to have more specific meaning. You should discuss this with the ordering physician in order to better have an idea of what the potential implications are of the findings.