The surgical anatomy of the pedunculopontine nucleus cannot be disputed, buried or exhumed

Abstract

Dear Editor,Our prior correspondence has already addressed the Romegroup’s perceived concerns about lead location vis-a-vis thepedunculopontinenucleus(PPN)andpontinemicturitioncen-tre (PMC), the size of the MRI artefact generated by theimplanted lead and the suitability of our patient for PPN deepbrain stimulation (DBS) [2, 10]. We agree that friendly dis-cussionpromotesscientificdebate.However,itshouldalsoberecognisedthatwild speculation and inaccuratestatements donot make a positive contribution. The Rome group does notthink it made wild speculations. Nevertheless, they made aspeculativeandincorrectassumptionthatweusealargemetalcannula despite the fact that we never use any cannula duringDBS surgery [7, 10].Wefailtounderstandwhyanyonewould“keepwondering(why) the electrode artefact … is larger than the aqueduct”.We reiterate that the size of the lead artefact on MRI dependsonseveralfactors,includingtheparticularMRIsequenceusedduring imaging [18].Notwithstanding the Rome group’s conjecture, we arefully aware of the published literature on somatosensoryevoked potentials (SEPs) and the PPN. Within that bodyof work, the Rome group states: “P16 potentials recorded bythe intracranial electrode contacts are generated by the vol-ley travelling along the medial lemniscus” [1]. We againassert that a lead lying within, or lateral to, the mediallemniscus would still be expected to record such SEPs.Contrary to their belief, “electrophysiological recordingsand functional evaluations” do not provide accurate anatom-ical information on lead location. This is especially true of anovel target where such surrogate markers are relativelypoorly defined in relation to anatomical location. Accurateinterpretation of stereotactic MRI currently provides themost reliable method of assessing the anatomical locationof implanted leads in patients.It was the correspondence from Rome that re-introducedthe topic of accurate lead location within the PPN. Yet,we are reproached for “exhumation of an already burieddispute”. Furthermore, inaccuracy of anatomical fact persistsin their recent correspondence. Contrary to their description,the rostrocaudal extent of the PPN straddles the pontomesen-cephalic junction (PMJ) and is not confined inferior to thePMJ [14, 15].Another group has also noted that: “in the study byStefani et al., electrodes were lateral to the medial lemniscus”[5]. Nevertheless, when referring to their own postoperativeMRI, the Rome group does “not understand … the claim that(their) electrode was placedlaterally to the lateral lemniscus”.To facilitate this understanding, their own published MRI isnow reproduced in the accompanying figure, together with asummary of the radiological anatomy of the PPN (Fig. 1).