Hi, As mentioned by you that the exact cause is un-diagnosed so I think it might be difficult to predict the prognosis. Proprioception and sensory loss are the symptoms and it would be very much essential to know the underlying cause. I think this may be a red flag indicator where in treating with regular Physiotherapy interventions may not be sufficient. I would suggest to refer this patient to a Neurologist so that exact diagnosis is confirmed. After ruling out various progressive neurological disorders (by Neurologist) we may be in a better position to manage this patient.
Rgds
Nikhil