Hi Linh,
In my opinion piriformis syndrome gets more attention than it deserves.
I get referrals on old folk with a pain in the buttock suggesting piriformis syndrome is the diagnosis. This is in my opinion an inappropriate referral.
I do take the diagnosis seriously in some circumstances and in these cases I think ultrasound is a great way to investigate it.
The patient has to be youngish with a normal lumbo-sacral spine and they have to be suffering from typical sciatica symptoms.
Piriformis syndrome is an extra axial cause of sciatica.
Extra axial sciatica is uncommon as mostly it comes from axial / spine related nerve irritation.
If they have a normal lumbo-sacral spine MRI and have sciatica we need to seriously consider an extra-axial cause, especially in the fit, active patient.
What I look for is the relationship of the sciatic nerve to the piriformis muscle.
I locate the ischial tuberosity and the hamstring origin in the axial plane. Just lateral to the ischial tuberosity you will identify the sciatic nerve sitting between G-Max above and quadrates femurs below. Follow the sciatic nerve proximal where it passes over inferior gemellis, Obturator internus and superior gemellis. The two gemelli muscles are muscular but the obturator interns is pure collagen like a tendon as its muscle belly is within the pelvis. Once you get to the sciatic nerve sitting on the superior gemelli I turn into long axis of the nerve and the nerve is seen passing beneath a triangular muscle which is piriformis (with g-max overlying). The normal sciatic nerve should pass deep to the piriformis muscle and not change in size or echo texture.
Abnormal cases of piriformis syndrome are characterised by the sciatic nerve NOT passing beneath the piriformis muscle. The sciatic nerve is actually 2 nerves bundles together (peroneal and tibial), the 2 components can seperate and one go beneath the piriformis muscle and the other component passing through or passing over the piriformis muscle. Many variations are possible and anything other than a single sciatic nerve passing beneath the piriformis muscle is a risk factor for neural irritation. When the nerve (or one of its components) passes through or over the piriformis muscle it risks irritation. When the patient exercises (e.g.cycling) they activate piriformis and this results in compression or traction on the nerve leading to neural oedema.
These are the things I look for, I agree MRI is also a great way to investigate, however we do have great spatial resolution and the ability to dynamically activate the piriformis muscle while scanning to see it it re-creates the symptoms and neural tension / compression. The piriformis is a deep lateral rotator of the hip , so resisted lateral rotation is the correct provocative manoeuvre.
Neil Simmons, one of my great mentors called it the “tick-tock manoeuvre”, patient prone, knee flexed and move the calf like a pendulum of a grandfather clock through internal and external rotation will move the piriformis muscle through its range. You can then easily resist the movement and see what happens to the nerve.
I think this is a challenging area of ultrasound and only successful if the patient selection is appropriate,
Happy scanning,
Steve