Neurosurgery

INTRODUCTION

Neurological surgery encompasses surgical disease of the central nervous system (the brain and spinal cord) and the peripheral nervous system. It is a specialty in its own right and is fast subdividing into sub-specialties such as neuro-oncology, vascular, paediatrics and spine. Neurosurgeons operate on every part of the body as the nervous system extends into every part of the body. There is extensive crossover with other specialties: ENT for skull base approaches; cardiothoracics for anterior approaches to the thoracic spine, thoracic sympathectomy and deep hypothermic cardiac arrest (DHCA) for complex neurovascular cases; general vascular for carotid disease; orthopaedics for spine and peripheral nerve; plastics for peripheral nerve repair. The only barrier intentionally breached by a neurosurgeon and by no other specialist is the dura mater.

Referrals stem from neurology, the emergency department, general practice, any surgical specialty, paediatrics and even obstetrics, both for the management of neurosurgical disease occurring during pregnancy and the prenatal diagnosis of congenital conditions such as spinal dysraphism that require neurosurgical management as soon as the child is born. There have even been recent forays into intra-uterine surgery for open spina bifida but these operations are currently beset by unacceptably high maternal mortality rates and are not recommended. Neurosurgical patients range from 25-week premature babies with intraventricular haemorrhage or spina bifida to 100-year-olds with chronic subdural haematoma. Many patients are unconscious preoperatively and many require intensive care postoperatively, hence strong links with the intensive care unit are essential.

Pituitary surgery and spinal surgery are intentionally not discussed since they fall outside the scope of this chapter and should only be undertaken by experts in the field.

THE ETHOS OF NEUROSURGERY

1. Damage to the brain will result in disability at remote sites which is not always proportional to the extent of brain damage: a massive lesion in a non-eloquent area such as the right frontal lobe can cause virtually no disability whereas a lesion no larger than a few millimetres in the internal capsule or brainstem can cause hemiplegia. Modern neurosurgical approaches make ingenious use of corridors through the skull base or non-eloquent areas of the brain: it is always preferable to remove more skull and retract the brain less.

2. Although the brain is 2% of the body weight, it receives 15% of cardiac output. It depends on this massive blood supply to support its extremely high metabolism and tolerates any disruption poorly. It can also bleed catastrophically and the usual haemostatic measures of applying firm pressure and ligating vessels cannot be used due to the risk of causing neurological injury.

3. Emergency neurosurgery generally involves removing pressure from some point of the neuroaxis to prevent extension of injury. This can ameliorate secondary injury, and if the primary injury can be survived this allows some chance of recovery:

Primary injury relates to structural neurological damage caused by transfer of kinetic energy, pressure, shearing, etc. This injury is static, but may be overwhelming; for example, a high-velocity bullet wound.

Secondary injury arises from local and systemic events that follow. There will be a local inflammatory cascade and subsequent oedema. In the enclosed space of the skull and spine this will create raised pressure resulting in impaired perfusion. Systemic hypotension and hypoxia will be equally deleterious.

In some patients the primary injury may be trivial: an extradural haematoma usually arises from a low-velocity injury. There is often no injury to the brain from the blow, but the expanding blood clot will produce a rapidly fatal secondary injury unless decompressed.

As a general principle, the speed of the surgical remedy needs to be proportional to the speed of onset. Trauma and haemorrhage require rapid intervention, whereas a slow growing tumour has a less dramatic effect on the brain despite occupying a similar volume.

TRAUMA

Trauma is the most likely emergency to be encountered but the same principles can be applied to most other neurosurgical problems:

1. The head is vulnerable to injury due to its position and its weight. Under normal conditions it is supported by powerful muscles in the neck, but in high-speed deceleration its weight results in massive forces acting on it about the moment of the top of the chest. Most assaults include blows to the head.

2. Patients with a severe head injury frequently have neck injuries and this must be assumed and the cervical spine protected unless the patient is fully conscious and the neck can be cleared of injury by clinical examination according to ATLS protocol.

3. Patients with severe head injury frequently require immediate surgery to save their life; however, adequate resuscitation is of greater importance: operating on a hypoxic hypotensive patient with an unprotected fractured cervical spine is far more likely to result in the patient’s death than delaying whilst they are resuscitated: an intubated, oxygenated patient with a sustainable blood pressure and a protected cervical spine is in a far better condition to tolerate a trauma craniotomy. Specific management must be preceded by adequate resuscitation.

Intracranial pressure (ICP) is defined as the pressure of CSF in the frontal horn of the lateral ventricle. The relationship between ICP and the volume of an expanding mass lesion within the brain is given by the Monro-Kellie doctrine (Fig. 35.1). This divides the intracranial contents into compartments: brain, CSF, venous blood, and arterial blood. There may be an extra compartment in the form of a mass lesion such as haematoma. The doctrine states that since the intracranial volume is fixed, an increase in one compartment must be compensated by a decrease in another or ICP will rise. In effect this means that normal ICP can be maintained with an expanding haematoma by displacing CSF and venous blood (the total volume is approximately 100 ml). Thereafter, ICP will rise steeply, clinically manifest by a sudden and often catastrophic deterioration in the patient’s conscious level. This explains the lucid interval seen in extradural haematoma (see below). Subsequent decrease in the volume of the arterial blood compartment will produce ischaemia, with further injury and swelling. As a haematoma expands, the brain is displaced towards the foramen magnum, and pressure builds up in the conical posterior fossa causing the brainstem to herneate through the foramen magnum, compressing the respiratory and cardiac centres in the medulla – this is ‘coning’. Ischaemia and herniation are the mechanisms of death from raised ICP.