Ambiguity in US

[Linh]

Hi Steve,

Hope you are doing well.

I have looked into differentiating the different phases of an infective process such as cellulitis, phlegmon and abscess or entities such as seroma and granulation tissues in healing. I was wondering if you can break it down the most important features to differentiate each of these.

Also, for referrals querying piriformis syndrome and sciatic neuropathy, I know that US is not the appropriate imaging modality, so what should I do when scanning/write in the report to cover myself in the medicolegal aspect?

Best regards,
Linh

[Stephen Bird]

Hi Linh,
I think as a general rule we can not always be completely specific and this is a common theme with ultrasound for example solid breast lesions and ovarian lesions.
But as in these examples we can give some guidance,
With cellulitis I expect to see subcutaneous oedema and hyper-vascularity with no discreet collection. If it is in the lower limb you may note some reactive groin nodes and a hyper dynamic superficial venous return to build your diagnosis. The clinical appearance of shiny red skin also helps (never look at the ultrasound in isolation, always take the clinical presentation into account).
With a phlegm you will see an inflamed area and centrally within it there will be an area of reduced vascularity compared to the hyper-vascularity of the infective surrounding tissue. This indicates some breakdown heading towards abscess formation. If you compress this tissue you will not see much tissue movement and I would conclude it is a phlegm.
In an abscess the general tissue is inflamed and hypervascular and in the middle of this tissue where the abscess has formed you will have an avascular area. If you compress it (squash it) wit the transducer you may see the puss move within the abscess. I call this my “+ve for sloshy sign” I use the same technique when I am looking at hydroxyapatite deposits in the MSK system to see if they are aspiratable.
A seroma is a post operative collection which is filled with serous material. The fluid within is usually anechoic (if it is a haematoma or abscess the fluid will have internal echoes) The wall is usually thin and the internal contents are avascular. If you drain these they may re-accumulate.Another interesting type of serum is the Morel Lavallee lesion in the MSK system where the superficial and deep fascias of the sub-cutaneous fat slide agains each other during trauma and a seroma results full of blood and lymphatic fluid. Once again these often re-accumulate following aspiration.
If it is a haematoma it will have some internal echoes and may have some clot forming within or separation of the blood components (plasma Vs red blood cells).
Granulation tissue is scar tissue, so it is solid, hypo echoic, firm on palpation, avascular or hypo vascular and in the expected location following surgery or trauma.
So in summary I think these entities have a variety of appearances that are characteristic of the correct diagnosis but perhaps not specific, with some overlap.

Steve

[Stephen Bird]

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

[Linh]

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?

[Stephen Bird]

I am more interested in the nerve and its relationship to the piriformis muscle. The size of the muscle can vary a lot and it is not possible to accurately and reproducibly measure the muscle size or volume with ultrasound. I am concentrating on the nerve looking for it passing through or over the muscle (remembering there are 2 components to th sciatic nerve and one may pass beneath and the other through or over etc) and also looking for neural oedema compared to the other side. I also test for a tinnels sign with transducer pressure.

Steve.

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