Response to Comment,

We read with interest the correspondence by Yalcin regarding our recently published study. Indeed, the remarks are relevant and interesting and warrant further consideration in a renewed debate about the role of electrophysiology both for diagnosis and for identifying refractory cases for specific management focusing on the piriformis muscle.

Electrophysiologic explorations are among the complementary examinations that are often cited in the context of piriformis muscle syndrome (PMS). Electromyography (EMG) is useful for the diagnosis of radiculopathy, to judge the potential severity and also to document the radicular damage in certain difficult cases. The presence of signs of acute denervation (spontaneous activity) in the muscles that depend on the nervous metamer territory involved confirms the diagnosis of radiculopathy. Suspected PMS usually shows no neurogenic changes because the disorder is functional, with the usual positional elements.

Therefore, nerve conduction studies are usually preferred, and in particular, investigation of the H-reflex (proximal loop studying the sciatic nerve) and its possible modifications during test manoeuvres . Investigating reduced conduction of the H-reflex , or even disappearance of the H-reflex , is useful in terms of specificity but only mediocre in terms of sensitivity. Indeed, the length of the nerve segment studied is substantial and therefore does not allow for identifying the affected site, contrary to distal focal disorders such as carpal tunnel syndrome.

The sciatic nerve is located deep in the gluteal region, and therefore transcutaneous stimulation provokes diffusion of the response and precludes specific study of the zone of interest in the infrapiriformis canal. Currently, electrodiagnostic tests are insufficient to confirm this diagnosis. However, the author raises an interesting point regarding the treatment of data , notably mentioning the double-crush syndrome (studied primarily in the upper limbs and mainly in the context of cervical radiculopathy with or without carpal tunnel syndrome). No specific studies have been performed in the context of PMS.

In addition to the H-reflex evaluating the S1 metamer, the proximal F-wave responses for the fibular (L5 metamer) and tibial nerves (S1 metamer) should also be investigated. However, previous reports investigating these responses in the context of S1 and L5 radiculopathies showed moderate sensitivity, ranging from 4 to 65% depending on the series . These studies aimed to identify abnormal F-wave latencies compared to reference values. These reference values vary widely, and depend primarily on the patient’s height, although temperature and age also play a role.

Therefore, delayed proximal conduction is certainly a specific sign but probably not very sensitive. Results need to be interpreted in comparison with reference values depending mainly on height but also with values obtained on the asymptomatic contralateral limb.

Clinical sensitisation manoeuvres should be coupled with electrophysiologic studies and prolonged for several tens of seconds to be able to elicit a block of proximal conduction. The idea is to try to identify even moderate proximal spinal disease, particularly with therapeutic failure focusing on the piriformis muscle, with isolated lesions of the piriformis muscle. A greater initial delay in response as compared to the healthy side, without significant modification of the FAIR manoeuvre, is an interesting avenue to explore. Yet, in our series, we did not find any significant difference between electrophysiologic values in poor responders and adequate responders to rehabilitation and toxin injection .

Therefore, with the current state of knowledge, it seems utopic to be able to demonstrate the role of electrophysiology for specific management focusing on the piriformis muscle, not only because of the large number of patients necessary for such a study but also because of the lack of sensitivity of H-reflex response analysis, or even F-wave analysis, both before and after the FAIR manoeuvre .