Abstract: A monitor-driven intracranial catheter assembly including dual lumen catheter for monitoring intracranial pressure via a pressure sensor located in one lumen while simultaneously draining cerebrospinal fluid through another lumen. A third lumen can also be provided with a side port for receiving a stylet. The stylet is used as a stiffening agent to aid in advancing the catheter into the patient's brain. The pressure sensor is preferably a spectral modulation sensor that communicates with a photodetector apparatus by means of a single optical fiber, thus minimizing the diameter of the catheter. The photodetector is self-correcting for signal errors caused by temperature change, changes in intensity of the input light, and transmission losses within the optical apparatus.