November 16, 2021 0 By Gopakumar

Among Indian surgeons, it is rare to find the skilful art of using Diffusion Tensor Imaging-Fiber Tractography (DTI-FT), a novel MRI technique, to locate the facial nerve on the surface of the brain tumour before surgery

Dr Biji Bahuleyan M.Ch. 

A 15-year-old girl came to us with a large brain tumour. It was diagnosed at another hospital when she was investigated for hearing loss, headache, and giddiness. Magnetic resonance imaging (MRI) characteristics of the tumour indicated that this could most likely have arisen from the cranial nerve that subserves hearing and balance, or the vestibulocochlear nerve, on the right side. These tumours are called vestibular schwannomas (VS). The massive size of the tumour distorted her cerebellum and brain stem and had created an obstruction to the flow of CSF, causing hydrocephalus. We decided to proceed with surgical removal of the tumour.   

Anatomically, tumours arising from the vestibulocochlear nerve are closely related to the cranial nerve that controls the muscles of the face (the facial nerve). With the slow growth of VS, the facial nerve gets stretched over them and can get displaced in any direction, draping the surface of the tumour. Thus, the facial nerve could be located either on the front, back, top or bottom of a VS. The diameter of the facial nerve is a few millimetres, and its colour simulates the colour of the tumour. Hence identifying the facial nerve on the surface of VS and preserving it anatomically and functionally is a major challenge while resecting VS. For this young girl, who has her entire life ahead of her, injury to the facial nerve during extirpation of the tumour was something our surgical team could never dream of.    

Conventionally, the location of the facial nerve on the surface of a VS is determined only during surgery, on the operating table. This is done with the aid of an instrument called the facial nerve monitor. In this patient, we used a novel MRI technique called diffusion tensor imaging fiber tractography (DTI-FT) to locate the facial nerve on the surface of the tumour before surgery. The patient’s DTI-FT demonstrated that the facial nerve was displaced forward and upward on the surface of the tumour (Fig 1). Thus DTI-FT gave us additional confidence about the location of the facial nerve on the surface of the tumour which we confirmed during surgery with a facial nerve monitor. We performed a near total removal of the tumour with anatomic and functional preservation of the facial nerve. We decided to perform only a near total resection of the tumour, as a total removal of the VS significantly increases the risk of damage to the facial nerve. She had no weakness of the muscles of her face after the surgery. For the residual small tumour, she underwent radiosurgery. 

Magnetic resonance imaging (MRI) technology has evolved by leaps and bounds as a useful tool in diagnosing and treating diseases of the brain. Repeating DTI-FT is one among the many MRI methodologies that helps study fiber tracts and cranial nerves within the brain. 

Structurally, the human brain is made of grey matter and white matter. The grey matter contains the cell bodies of neurons. The white matter is made of the axons of these neurons, which carry signals between the brain, the spinal cord and between various regions of the brain. These complex networks help the normal functioning of the brain. The white matter is organized in various bundles called “fiber tracts” or “fascicles”. The brain is made up of numerous such small and large bundles to create the complex networks that form the basis of brain functions. For example, the motor fiber tract sends signals from the brain to various muscle groups of the body for the voluntary movement of the body parts. The sensory tract carries sensory impulses like touch, pain, temperature etc from various regions of the body to the brain. Cranial nerves are those that arise directly from the brain and terminate in sense organs like eyes, nose and other regions of the head and the neck.  

DTI-FT helps beautifully identify these fiber tracts and cranial nerves and isolate them from the rest of the brain and brain tumours. In DTI-FT, the fiber tracts are colour coded. 

DTI-FT has been used in diagnostic neuroradiology for more than 30 years. With advancement in technology, its usefulness has increased significantly since then. It has also been shown to be useful in the diagnosis of neurological diseases like stroke, demyelination, neuro infections, brain tumours etc. 

The aim of brain tumour surgery is to maximize tumour resection and minimize brain damage. DTI-FT has shown to be extremely useful for neurosurgeons for planning surgical resection of brain tumours and other lesions within the brain. 

Fiber tracts and cranial nerves can either get displaced, distorted, or disrupted by brain tumours. Thus, during the origin and growth of a tumour, the fiber tracts and cranial nerves near it could get displaced either to the front, back, right or left. DTI-FT helps study the fiber tracts in the vicinity of brain tumours and its relation to the tumour. This is important for a surgeon to determine the surgical approach toward these tumours. If DTI-FT demonstrates a fiber tract on the right side of the tumour, the tumour is approached through its left side. This is because, if the tumour is removed through the right side, there is a high chance that the process of tumour removal could damage the motor tract, resulting in significant weakness on the opposite side of the body. Hence if such a relation is identified on DTI-FT, the tumour is approached through the left side.  

To aid the safe removal of brain tumours, in addition to DTI-FT, brain mapping techniques like the use of neuromonitoring, neuronavigation, awake brain surgery, intraoperative ultrasound etc are excellent surgical adjuncts to the safe resection of brain tumours. Today, DTI-FT has become an integral part of the MRI studies of the brain in all standard institutions for the diagnosis and planning surgery for  tumours of the brain.