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Neuronavigation is a guidance system that identifies lesions and plans the trajectory to the lesion to improve surgical outcomes. Frame-based systems, which involve head fixation by means of framing before imaging, are highly accurate. However, due to some limitations of frame-based systems, frameless systems that do not require frame fixation of the head have become popular and are widely used. Frameless systems require a registration step to establish the spatial relationship between the patient’s head and the image. Multiple types of registration methods and device tracking systems have been developed. Frameless systems are technically advanced and have demonstrated reliable accuracy, but these systems still have many occurrences of error and inherent limitations. Therefore, it is important to understand the principles and pitfalls of frameless systems to use them effectively and safely during surgical operations.

Objective : A chronic subdural hematoma (CSDH) is a collection of bloody fluid located in the subdural space and encapsulated by neo-membranes. An inner subdural hygroma (ISH) is observed between the inner membrane of a CSDH and the brain surface. We present six cases of CSDH combined with ISH treated via endoscopy.Methods : Between 2011 and 2022, among the 107 patients diagnosed with CSDH in our institute, six patients were identified as presenting with CSDH combined with ISH and were included in this study. Preoperative computerized tomography (CT) and magnetic resonance imaging (MRI) were performed simultaneously, and endoscopic surgery for aspiration of the hematoma was performed in all cases of CSDH combined with ISH.Results : The mean age of patients was 71 years (range, 66 to 79). The patients were all male. In two cases, the ISH was not identified on CT, but was clearly seen on MRI in all patients. The inner membrane of the CSDH was tense and bulging after draining of the CSDH in endoscopic view due to the high pressure of the ISH. After fenestration of the inner membrane of the CSDH and aspiration of the ISH, the membrane was sunken down due to the decreasing pressure of the ISH. There was one recurrence in post-operative 2-month follow up. The symptoms improved in all patients after surgery, and there were no surgery-related complications.Conclusion : CSDH combined with ISH can be diagnosed on imaging, and endoscopic surgery facilitates safe and effective treatment.

Hydranencephaly is a rare congenital disease defined as an absence of cerebral hemispheres with an intact thalamus, brain stem, and cerebellum. Generally, patients with hydranencephaly require cerebro-spinal fluid diversion due to progressive hydrocephalus. An excellent view of the choroid plexus and advancement of the neurosurgical endoscope were possible, leading to easy coagulation of the choroid plexus to balance cerebro-spinal fluid production without the use of a shunt device. We present this rare case and good treatment outcome from endoscopic coagulation of the choroid plexus.