Anaesthesia for spinal surgery (Neuroanaesthesia) Part 1

The scope of spinal surgery

Surgical interventions may be required from the craniocervical junction to the sacrum. he approaches include maxillotomy, mandibular and tongue split, thoracotomy and anterior abdominal incision, as well as the more usual posterior route. Pathology includes disc protrusions, primary and secondary tumours, infections, trauma, arthritides and congenital disease. Most surgical endeavours are directed towards relieving stenosis of root canals or the spinal canal and/or stabilizing the spinal column. Correction of spinal curvatures (kyphosis and scoliosis) is one of the major endeavours of spine surgeons. here has been an explosion in technology in the last 20 years and there are now many different fixation systems and implants to facilitate stabilization of the spinal column (Fig. 15.1).

The language of spinal surgery

Anaesthetists unfamiliar with spinal surgery will find it helpful to be aware of some of the terms in common usage (Table 15.1). Some, such as the ‘clivus’ – the bone forming the base of the skull anterior to the foramen magnum – are common parlance in spinal units but are rarely used elsewhere.

Spinal instability

Symptomatic spinal instability has been defined as the ‘loss of the ability under normal physiological loads to maintain relationships between vertebrae in such a way that there is neither initial nor subsequent damage to the spinal cord or nerve roots, and there is neither development of incapacitating deformity or severe pain.’However, instability can be asymptomatic.


For example, in rheumatoid arthritis, up to 50% of patients with anterior atlanto-axial subluxation (AAS) may be unaware of their abnormality. Symptoms of AAS include neck, occipital and facial pain, which is sometimes lancinating (L’Hermitte’s phenomenon). Neurological impairment related to AAS is characteristically subtle, although sudden death has been described. he question is often asked whether symptomless rheumatoid patients should have flexion/ extension radiographs before anaesthesia. Although there is no evidence of benefit in terms of outcome, it is sensible to get the radiographs because of ‘outcome bias’ and also because current opinion recommends early fixation of AAS.

The spine can be regarded as two columns – anterior and posterior. he anterior column comprises the ligaments and bones back to the posterior longitudinal ligament (PLL) and the posterior column the elements posterior to the PLL. Disruption of the anterior column tends to make the spine unstable in extension, and posterior column damage favours instability in flexion (Fig. 15.2).

Radiological measurements can also be used to assess spinal instability, although there is poor correlation between radiographic abnormality and neurological symptoms and signs. he most commonly used descriptors of instability are:

• Translation:

° C1-2: anterior atlanto-dental interval (ADI) >5 mm, posterior ADI <13 mm

° C2-T1: >3.5 mm between points on adjacent vertebrae

• Angulation:

° >11° between vertebrae.

Table 15.1 Glossaryofspinalterms

Spondyl(o)

Word element [Gr], vertebra; vertebral column. The term spondylosis is used to describe degenerative changes, commonly osteophytic projections encroaching on spinal or root canals, and spondylolisthesis to describe loss of vertebral alignment.

Myelopathy

Any functional disturbance and/or pathological change in the spinal cord, such as transverse myelopathy (extending across the spinal cord), central cord syndrome, anterior cord syndrome, posterior cord syndrome, Browne-Sequard syndrome.

Radiculopathy

Any functional disturbance and/or pathological change in a spinal nerve root.

Spinal stenosis

Reduction in the calibre of the spinal canal and hence the space available for the cord; due to disc protrusions, osteophytes, tumours and instability.

Instability

Instability of the spine is a spectrum of clinical situations, ranging from complete disruption to slowly worsening deformity. It is not easy to define, and the anaesthetic implications range from considerable to slight.

Subluxation

A significant structural displacement, visible on static imaging, such as subluxation of the atlantoaxial joint caused by rheumatoid arthritis, Down’s syndrome or infection of the occipito-atlanto-axial complex. Can be anterior, posterior or vertical in direction. Rotatory subluxation of the atlas on the axis can occur in infection (Grisel’s disease).

Lateral radiograph showing extensive spinal fixation.

Fig. 15.1. Lateral radiograph showing extensive spinal fixation.

CTscan showing disruption of the anterior column at C4/5/6.

Fig. 15.2. CTscan showing disruption of the anterior column at C4/5/6.

Spine and spinal cord anatomy relevant to anaesthesia

Craniocervical junction

The anatomy of the spine will be familiar to most anaesthetists, but the special characteristics of the occipito-atlanto-axial complex deserve particular study (Fig. 15.3).

Diagram of the craniocervical junction or occipito-atla nto-axia complex.

Fig. 15.3. Diagram of the craniocervical junction or occipito-atla nto-axia complex.

The head weighs about 6 kg so the ligaments and joints of the craniocervical junction have to be powerful. he transverse portion of the cruciate ligament of the axis is said to be as strong as the cruciate ligament of the knee.

Airway management is influenced by craniocervi-cal movement, as extension is required for both basic life support and direct laryngoscopy. In addition, mouth opening is limited if craniocervical extension is impaired. For example, persistent airway obstruction has been reported after fixation of the craniocervical junction in excess flexion. he range of motion between complete flexion and extension is normally about 24°, but in clinical practice it is very difficult to identify reduced craniocervical movement because of compensatory movement at lower levels. Curiously, a test of mouth opening such as the Mallampati score may be a better indicator of craniocervical rigidity than observation of craniocervical movement.

Spinal cord

In most individuals, the spinal cord descends to the level of L1/2 but in 2-3% as far as L2/3. he spinal cord is intolerant of retraction and, because of the varying anatomical structures that surround the spine, the implications of this (in terms of the simplicity of the surgical approach) on the most common indication for spine surgery, disc protrusion, are considerably different at different spinal levels. Lumbar disc protrusions can be approached posteriorly because the cauda equina can be retracted to allow extraction of disc fragments. However, at thoracic levels, the approach must be lateral, either through a thoracotomy or costo-transversectomy (removing the head of a rib), whereas at cervical levels the approach is usually anterior, although the oesophagus and great vessels are at risk. Lesions above C2 may require a mandibular or maxillary split for access.

The spinal cord derives its blood supply from anterior and posterior longitudinal arteries arising from the vertebral arteries, and radicular arteries arising from the aorta. he main vessels form a plexus around the cord, from which perforating vessels enter the cord. Spinal cord blood flow is believed to be governed by the same mechanisms that apply to cerebral blood flow. here are characteristic patterns of neurological impairment due to ischaemia, the commonest ofwhich are the syndromes of central and anterior cord ischaemia (Fig. 15.4). Anterior cord syndrome is an incomplete spinal cord injury (SCI) due to ischaemia of the anterior regions of the cord. here is motor paralysis below the level of the lesion because of interruption of the corticospinal tract, and loss of pain and temperature sensation at and below the level because of interruption of the spinothalamic tract. Proprioreception is retained. Central cord syndrome also results in incomplete SCI but with a disproportionately greater loss of motor power in the arms than the legs, as the axons supplying the legs are peripheral in the descending tracts. Sensory loss is variable below the lesion.

Vessel damage may occur at several sites during spine surgery. here is often a relatively large radicular vessel, known as the artery of Adamkiewicz, and damage to it results in significant risk of severe neurological dysfunction. Although the vertebral arteries are well protected, they are vulnerable to damage in cervical traum a and surgery as their course lies within the bone. The aorta and inferior vena cava lie in close approximation to the anterior of the spine and are vulnerable to damage duringlumbar surgery (Fig. 15.5).

Diagram of the characteristic ischaemic areas in anterior and central cord syndromes.

Fig. 15.4. Diagram of the characteristic ischaemic areas in anterior and central cord syndromes.

Spinal cord injury

In summary, it can be complete or incomplete, with loss of motor, sensory and autonomic function below the level of the lesion in complete injuries. Te commonest cause of incomplete SCI is central cord syndrome. Anterior and Brown-Sequard syndromes are well recognized, as is the cauda equina syndrome, which occurs due to damage to the neuraxis below the conus of the spinal cord.Motor power after SCI can be classified according to the Medical Research Council (MRC) grading scale (Table 15.2) or the American Spinal Injury Association (ASIA) neurological classification of SCI.

Table 15.2 Medical Research Council (MRC) motor power grades

Grade 5

Normal movement

Grade 4

Movement against resistance, but weaker than the other side

Grade 3

Movement against gravity, but not against resistance

Grade 2

Movement only with gravity eliminated

Grade 1

Palpable contraction but no visible movement

Grade 0

No movement

 MRI scan showing the proximity of the aorta and inferior vena cava to the spine (arrow). Surgeons can perforate the great vessels during discectomy, but bleeding into the disc space does not occur, probably because the disc annulus and the anterior longitudinal ligament seal the path.

Fig. 15.5. MRI scan showing the proximity of the aorta and inferior vena cava to the spine (arrow). Surgeons can perforate the great vessels during discectomy, but bleeding into the disc space does not occur, probably because the disc annulus and the anterior longitudinal ligament seal the path.

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