Ultrasonographic Evaluation of the Sciatic Nerve in Patients With Piriformis Syndrome

Overview

Piriformis syndrome (PS) is defined as a neuropathy of entrapment resulting from compression and irritation of the sciatic nerve at the level of the piriformis muscle (PM) at the pelvic outlet, causing sciatica radiating to the leg. The neuropathic component of PS is the compression of the sciatic nerve in or around the PM, while the primary cause of the somatic component is myofascial pain of the PM. PS accounts for 6-8% of all hip and sciatic pain cases. It is more common in middle-aged patients, and the most common etiological cause is trauma. There are various methods for the diagnosis of Piriformis Syndrome. Clinically, tenderness on palpation of the PM is the most common symptom. The FAIR test (passive flexion abduction and internal rotation of the hip), the Freiberg test (forceful internal rotation of the thigh in the supine position), the Pace test (abduction of the leg against resistance while sitting) and the Beatty maneuver (active abduction of the thigh on the affected side in the lateral decubitus position) are maneuvers that help in diagnosis. It is known that there is a delay in proximal nerve conduction (H-reflex) on the pathological side compared to the healthy side in the FAIR maneuver on EMG in PS, and it is not a sufficient test for diagnosis alone. Some studies have reported increased piriformis thickness on the pathological side in MRI. None of these clinical and imaging methods are sufficient for diagnosis alone, and today ultrasonography (US) has become one of the most important imaging methods for musculoskeletal clinicians in the diagnosis of PS. Imaging with US and the diagnostic injection test applied to the piriformis muscle with US guidance are quite important for diagnosis. There are a number of studies in the literature evaluating PM with US. These studies have shown that the piriformis muscle thickness and echogenicity increase on the pathological side. However, there is a lack of studies in the literature focusing on the evaluation of the sciatic nerve with US in patients with PS. Our aim in this study is to compare the thickness of the PM and the sciatic nerve by measuring US on both the pathological and painless sides in patients with unilateral hip pain who were diagnosed with PS clinically, to record the variations present in the sciatic nerve, and to determine whether the myofascial pain or the neuropathic component is dominant in the disease.

The sciatic nerve is the largest nerve in the body and is formed by the ventral roots of the lumbosacral plexus (L4-S3), passes through the foramen ischiadicum majus under the piriformis muscle (PM) and enters the gluteal region. It is controversial whether anatomical variations in the sciatic nerve and PM predispose to the development of Piriformis syndrome (PS). The sciatic nerve passes under the PM as a single branch in almost 90% of cases. However, there are some variations where it passes above and through the muscle or divides into tibial and peroneal branches, with one branch passing under the piriformis. There are a number of studies in the literature evaluating the PM with US. These studies have shown that the PM thickness and echogenicity increase on the pathological side. In addition, it has been mentioned that the sciatic nerve has a coarse and blurry image in patients complaining of sciatic pain. In one study, cross-sectional area measurement was performed instead of PM thickness measurement. However, this was criticized in later studies, and it was stated that it was difficult to calculate muscle volume exactly with two-dimensional US due to the irregular shape of the PM, which is anatomically pear-shaped, and that thickness measurement would provide more accurate results instead of cross-sectional area. In the first study in the literature focusing on the sciatic nerve in PS and the neuropathic pain it causes, it was found that the sciatic nerve was significantly enlarged (75%) on the symptomatic side and had decreased echogenicity in some patients when compared to healthy volunteers.