Dear Steve,
Thank you so much for your detailed reply.
I looked up Morel-Lavallee lesion in Bianchi book and it seems that there is quite a few differentials to its appearance such as haematoma and soft tissue tumour (in the case Morel lavalle appears complex). I guess we can never be specific with these when writing our reports.
I agree with you with regards to looking at whether sciatic nerve appears thickened due to oedema and the variations in its proximal branching.
I also read this recent article: Feasibility and Reliability of an Ultrasound Examination to Diagnose Piriformis Syndrome (2020) and they look at comparing piriformis thickness (which I think is low yield as Piriformis syndrome can be bilateral 10% of the time). Do you assess the muscle itself or just the appearance of the sciatic nerve alone?