It is possible to raise a drooping eye brow, but not lower the other one, sooooo, if you have one lower than the other, a surgical procedure can attempt better symmetry.
See an oculoplastic specialist for evaluation. A common cause of eyebrow asymmetry is underlying upper eyelid asymmetry which may be masked. If that is the case, eyelid surgery would be the answer. There are other causes of brow asymmetry.
I suggest you post a photograph so we can see what is going on!
The question is why do you have "unilateral brow ptosis?" Usually the answer is that there is something going on with one of the eyes that makes the higher eyebrow go up. If that is the case, the brow ptosis is not brow ptosis but a unilateral eyebrow compensation and the answer would be to fix the eyelid causing the issue. I hope you can understand why a photograph is so important.