BackgroundThe NASS guideline cannot recommend any of the surgical treatment options toward adult isthmic spondylolisthesis (AIS) since 2014. After the introduction of endoscopic decompression, instead of treating the spondylolysis itself, treatment can specifically target the refractory radicular pain developed during the degeneration progress without devastating the peripheral soft tissue. However, we noticed that endoscopic transforaminal decompression seems to be less effective in AIS compared to other types of degenerative spondylolisthesis. Thus, we came up with a novel craniocaudal interlaminar approach, utilizing the proximal adjacent interlaminar space to perform bilateral decompression and observed the pathoanatomy of pars defect directly and tried to identify the cause of decompression failure.MethodsFrom January 2022 to June 2022, 13 patients with AIS underwent endoscopic decompression via the endoscopic craniocaudal interlaminar approach and were followed up for at least 6 months. Visual Analogue Scale, Oswestry Disability Index and MacNab scores were recorded to monitor patients’ clinical recovery. All endoscopic procedures were recorded and reviewed to illustrate the pathoanatomy.ResultsFour patients required minor revision via the same technique. One of them required it due to incomplete isthmic spur resection, two due to neglected disc protrusion, and the other due to root subpedicular kinking in higher grade anterolisthesis. All patients’ clinical condition improved significantly subsequently. After reviewing the endoscopic video, we have observed that the hook-like, ragged spur originating from the isthmic defect extends beyond the region around the foramen. Instead, it extends proximally into the adjacent lateral recess, resulting in impingement along the fracture edge above the index foramen and, in some cases, even in the extraforaminal area.ConclusionsThe broad spanning isthmic spur extending to the proximal adjacent lateral recess might be the reason why the transforaminal approach yielded less satisfactory results due to the incomplete decompression result from approach related restriction. Our study demonstrated an optimistic outcome by applying decompression from the upper level. Therefore, we propose that the craniocaudal interlaminar approach might be a better route for decompression in adult isthmic spondylolisthesis.