In Reply: Fully Endoscopic Microvascular Decompression of the Trochlear Nerve for Treatment of Medically Refractory Superior Oblique Myokymia: Technical Case Instruction and Operative Video

To the Editor: We thank Dr Shusaku Noro1 and colleagues for their constructive letter to the editor regarding our recent publication,2 “Fully Endoscopic Microvascular Decompression of the Trochlear Nerve for Treatment of Medically Refractory Superior Oblique Myokymia: Technical Case Instruction and Operative Video.” Superior oblique myokymia (SOM) is a rare condition, and patients with this pathology stand to benefit from greater attention from the neurosurgical community. Below we will briefly address a few thoughtful comments brought up by Dr Noro and colleagues: 3. Recurrence and Pathophysiology: Although our patient had 50% recurrence of her preoperative symptoms at 1-year follow-up, it is important to note that she had complete symptom resolution immediately after surgery, which lasted for several months. At the time of surgery, the trochlear nerve was decompressed along the entirety of its observable course from its dorsal root exit zone (REZ) to the tentorial edge. Before closure during surgery, we did not observe any additional vascular anomalies or additional sites of compression from smaller arterial branches arising from the superior cerebellar artery (SCA). During preoperative clinic visits, our patient voiced a strong preference for trialing an as minimally invasive as possible intervention for her SOM. In fact, she requested nuchal muscle graft only as opposed to Teflon during surgery. It would certainly be reasonable to consider a more complex intervention such as slinging of the SCA if it was difficult to decompress the trochlear nerve at the time of surgery; however, we felt satisfied with our trochlear nerve decompression using nuchal muscle graft only. 4. Figure 1: General trochlear nerve anatomy, such as the fact that the REZ of the trochlear nerve is located along the dorsal midbrain, is well understood to readers of Operative Neurosurgery. We explicitly discuss the precise location of the trochlear nerve REZ in the limitations section of our manuscript, where we cite a radiologic study revealing that the trochlear nerve REZ is located on average 4.5 mm off the midline of the dorsal midbrain. In Figure 1, we simply sought to highlight that our patient's preoperative MRI suggested a potential site of neurovascular conflict involving the trochlear nerve within the perimesencephalic cistern between the free edge of the tentorial leaflet and the SCA. 5. Video Presentation: The trochlear nerve REZ was identified with relative ease from a standard retromastoid craniectomy position using the endoscope, despite its medial location as noted in more detail in the manuscript. The trochlear nerve REZ was specifically labeled in the operative video at the 5 min 0 seconds mark. The operative video shows placement of the nuchal muscle graft just lateral to the trochlear nerve REZ, at the site of greatest observed compression, from the 5 min 4 seconds to 5 min 30 seconds mark. We hypothesize that the symptoms of SOM may result from compression of the trochlear nerve anywhere along its course similar to trigeminal neuralgia, where although it is classically understood to be secondary to compression of the trigeminal nerve at its root entry zone, can still be caused by compression of the trigeminal nerve more distally.