Fornix lesions as a complication of 3rd ventriculostomy are rare and almost not reported. However, in our series of 94 procedures we observed 5 fornix lesions. Although we did not find any clinical deterioration, we were alarmed by these unexpected incidences and analysed the mechanism. All fornix lesions occurred using an endoscope sheath with separated channels for the endoscope itself, the instruments and for rinsing and suction. The limited field of view suggests the surgeon to be already inside the 3rd ventricle while the tip of the scope is still in the lateral ventricle just before the foramen of Monro. The instrument enters the optic field--depending on the used optic--as lately as 2 to 3 mm. The analysis showed that the lesions happened when the instruments were in the blind angle of the endoscope's optic which itself was outside of the foramen of Monro. Being aware of this mechanism with its potential risks it did not occurred again.
