Nervous System

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Chapter 14 Nervous System

Nervous System – Anatomy and Physiology 526, 527
Neuronal Damage 528
Glial Reactions 529
Increased Intracranial Pressure 530, 531
Cerebral Oedema 532
Increased Intracranial Pressure 533
Circulatory Disturbances 534
Cerebral Infarction 535, 536
Brain Damage due to Cardiac Arrest 536
Cerebral Infarction 537
Cerebral Haemorrhage 538
Subarachnoid Haemorrhage 539
Head Injury 540–543
Ageing and Dementia 544
Alzheimer’s Disease 545
Dementia 546
Infections 547
Bacterial Infection 548, 549
Virus Infections 550–554
Prion Diseases 555
Miscellaneous Infections and Infestations 556, 557
Demyelinating Diseases 558, 559
Parkinson’s Disease 560
Miscellaneous Disorders 561, 562
Diseases of the Spinal Cord 563, 564
Disorders of Motor Pathways 565
Motor Neurone Disease 566
Mixed Motor and Sensory Disorders 567
Sensory Disorders 567
The Peripheral Nerves/The Neuropathies 568
The Neuropathies 569
Hydrocephalus 570–572
Developmental Abnormalities 573
Tumours of the Nervous System 574
Secondary Brain Tumours 575
Primary Brain Tumours 576
Tumours of the Nervous System 577–579
Tumours of Peripheral Nerves 579, 580
Cerebrospinal Fluid 581
The Eye 582–584

Nervous System – Anatomy and Physiology

Considerations of anatomy and physiology have important applications to diseases of the central nervous system (CNS), particularly their effects and spread.

The anatomy of the various coverings is important.

The skull and vertebrae form a rigid compartment protecting the delicate CNS tissues.

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This rigidity has serious disadvantages when pressure inside the skull increases, e.g. an expanding lesion soon takes up the small reserves of space available and the delicate brain tissues are progressively compressed, with very serious results.

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Meninges and Cerebrospinal Fluid (CSF)

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Diseases (particularly infections) at this site are usually widespread over the whole brain and cord surfaces, e.g. meningitis. Impediment to the flow of CSF causes serious effects hydrocephalus.

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The pia is invaginated into the brain substance along with the small penetrating vessels.

The dura, arachnoid and pia act as barriers which selectively separate the CSF and the blood from the CNS tissues.

It is important to understand that, although the CSF is very similar in composition to the extracellular fluid of the brain, changes in the CSF only very indirectly reflect changes in the CNS in disease.

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Thus the pia and the membrane formed by the foot processes of the astrocytes separate types of tissue derived from two embryological layers.

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Neuronal Damage

NEURONES are sensitive to damage by a wide variety of agents including anoxia, hypoglycaemia, virus infections and intracellular metabolic disturbances (e.g. associated with vitamin B deficiencies).

There are two main types, depending on the rapidity of the changes.

1. Rapid NECROSIS – associated with acute failure of function.

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2. Slow ATROPHIC CHANGES – associated with gradual loss of function.

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The process of ageing involves cumulative atrophy and disappearance of neurones; in some individuals the process is speeded up resulting in presenile dementia.

Note: There is no regeneration of destroyed neurones.

A large group of disorders of cerebral function seen in psychiatric practice have (as yet) no morphological evidence of nerve cell damage. They are caused by disturbances of poorly understood biochemical control mechanisms within the brain.

In addition to the primary degenerations described above, neurones are subject to secondary degeneration in certain circumstances.

1. Retrograde degeneration – when the main axon is damaged there is degeneration of the neurone as well as the classical distal degeneration of the axon.

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2. Trans-synaptic degeneration – in closely integrated neurone systems, neurone loss may be followed by degeneration of associated neurones across synapses.

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Glial Reactions

The glial cells react vigorously in many diseases of the CNS.

1. The NEUROGLIAL cells have supportive and nutritive functions.

(a) The astrocytes with their numerous fibrillary processes give structural support. They are less susceptible to damage than neurones.

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This process is called gliosis. It is a feature of many diseases and is analogous to scar tissue. These cells and fibres contain Glial Fibrillary Acidic Protein (GFAP), recognition of which, using antibodies, is useful in histological sections.

Note: Collagenous scar tissue is only formed in the CNS when mesodermal structures such as large blood vessels are damaged.

(b) The oligodendrocytes – small cells with short processes – have a nutritive function in respect of neurones and especially myelin. This reaction is best seen when neurones are damaged.

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2. The MICROGLIAL cells are members of the mononuclear-phagocytic system. Reaction is best seen when there is necrosis of tissues.

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Microglial cells are well seen in and around infarcts; the activated cells which have ingested lipids are strikingly different from the small inactive microglia.

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These cells have been given a variety of names: e.g. ‘gitter’ cells, or lipophages (lipid phagocytes).

Increased Intracranial Pressure

INCREASED INTRACRANIAL PRESSURE (ICP) occurs in two main circumstances:

1. Due to the presence of an expanding lesion
2. Due to obstruction of the free flow of the CSF – this causes hydrocephalus and is dealt with on page 570.

Intracranial Expanding Lesions

These lesions may occur within the brain substance or in the meninges. Important examples are:

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The situation is often aggravated by cerebral OEDEMA.

The severity of the effects is modified by two important factors:

(1) the size of the lesion and
(2) the rapidity of expansion.

There are three stages in the progress of increased intracranial pressure (ICP).

(1) The stage of Compensation

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(2) The stage of Decompensation At this stage there are herniations and distortions of the brain with their associated complications including:

– reduction in level of consciousness
– dilatation of pupil ipsilateral to mass lesion and papilloedema
– bradycardia with raised blood pressure (‘Cushing’ effect)
– Cheyne-Stokes’ respiration

(3) A vicious circle is established leading to the stage of vasomotor paralysis.

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Effects

Distortions and Dislocations of the Brain Substance

These are to some extent dependent on the site of the initiating lesion; the effects of a unilateral expanding lesion are illustrated.

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Note: The sudden removal of even small amounts of CSF by lumbar puncture may precipitate medullary ‘coning’ with fatal results due to damage to the ‘vital centres’.

Cerebral Oedema

Swelling of the brain, of which oedema is the major component, is an important complication of many brain diseases because the enlargement either initiates or aggravates increased intracranial pressure.

The process may be localised or generalised depending on the type of initiating disorder.

Localised conditions Generalised conditions
Examples: Examples: Intoxications
Infarcts, and local ischaemia Metabolic disturbances, e.g. hypoglycaemia
Haematomas (due to vessel rupture and injury) Generalised hypoxia
Tumours Severe head trauma
  Malignant hypertension

The pathological mechanism is as follows:

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Notes

(a) Components (1) and (2) overlap.
(b) The oedema fluid tends to spread in the white matter.
(c) The severity of oedema formation is very variable and unpredictable clinically.
(d) In clinical practice, therapy has two aspects:

1. Treatment of the initiating disorder by any appropriate means.
2. Minimising the formation of oedema by the use of

(i) osmotic agents, e.g. urea or mannitol
(ii) steroids.

Increased Intracranial Pressure

Secondary Complications

1. Vascular damage

(a) Compression of the central retinal vein causes papilloedema, an important clinical sign of raised ICP.

Note: Axonal flow in the optic nerve is reduced, contributing to swelling of the optic disc.

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(b) Stretching and compression of blood vessels may cause haemorrhage and infarction quite remote from the initiating lesion – secondary midbrain and calcarine infarction and haemorrhage are common.

2. Intracranial nerve damage

Oculomotor (III) and abducens (VI) nerves are particularly prone to damage, giving rise to paralysis of ocular movements in varying combinations.

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The VIth nerve is specially vulnerable due to its long subarachnoid course. It is often the nerve on the side opposite the lesion which is stretched giving rise to paradoxical signs.

3. Obstruction of flow of CSF

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4. Changes in the skull bones

Long continued ICP causes bone erosion and thinning visible on X-ray.

(a) Erosion of posterior clinoid processes of sphenoid bone.
(b) In children, before the skull is fully ossified, the inner table is thinned at the sites of convolutional pressure giving a striking X-ray appearance.

Circulatory Disturbances

(1) Hypoxia and ischaemia and (2) intracranial haemorrhage are the important and common mechanisms causing brain damage.

Acute Hypoxic Disorders

When the cardiac output falls, an autoregulatory vascular control mechanism protects the cerebral blood supply the arterial BP must be kept above 50 mmHg.

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There are no reserves of O2 or glucose in the brain, therefore a constant delivery via arterial blood is necessary.

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Neurones are very susceptible to hypoxia (and hypoglycaemia); with complete O2 deprivation neuronal necrosis occurs in 5–7 minutes (at normal temperatures).

The following flow diagram illustrates the factors which influence availability of O2 and the conditions giving rise to hypoxia.

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Cerebral Infarction

This condition, the commoner of the two main types of stroke (the other is spontaneous intracerebral haemorrhage), is caused by failure of the supply of oxygen (and glucose) to maintain the viability of the tissues in the territory of a cerebral arterial branch. This is not always due to simple local arterial occlusion, and very often a component of central circulatory deficiency is contributory. The lesion is essentially necrosis of all the tissues in the affected territory.

Mechanism

Precipitating condition → Perfusion failure → infarction (ischaemic necrosis)

LOCAL ARTERIAL DISEASE (particularly atheroma) and its complications, are the most common.

1. Arterial occlusions

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2. Arterial stenosis

atheroma – the widespread loss of arterial lumen potentiates cerebral perfusion deficiency in two ways: (1) by distributing the normal arterial flow and (2) by prejudicing anastomotic communications.

Atheromatous stenosis alone is not usually a cause of infarction, but when central circulatory deficiency is added, infarction is common, e.g. this may vary from the slight fall in BP during sleep to the severe hypotension of shock or myocardial infarction.

Other rarer causes of arterial stenosis are dissecting aneurysm and arteritis. Arterial spasm is even rarer.

Cerebral infarction can now be treated by administering ‘clot busting’ drugs. To avoid permanent damage these need to be given within 3 hours of the onset of symptoms. Tissue plasminogen activator which converts plasminogen to plasmin is most frequently used. Cerebral bleeding is a potential side effect. Alternatively thrombus can be surgically removed from the carotid artery. The success of these treatments is variable.

Sites

While infarcts may occur anywhere in the brain, depending on the vagaries of the precipitating arterial lesions, certain sites are more commonly affected.

1. In cases of local arterial occlusion, internal structures supplied by ‘end’ arterial branches are particularly vulnerable. The cortex is often protected in variable degree by anastomoses of other cerebral arteries. This is illustrated in the territory of the middle cerebral artery.

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2. Boundary zones (see p.177)

The cortex in particular is damaged in boundary zone infarction. In these cases, central circulatory deficiency is an important component, e.g. hypotension.

Brain Damage Due to Cardiac Arrest

This is characterised by widely distributed selective neuronal necrosis – the neurones are more susceptible to hypoxia than the supporting cells.

Affected areas – total cortical necrosis  
  or  
  most sensitive zones – hippocampus
    – layers III, V, VI of cortex
    – within sulci
    – Purkinje cells of cerebellum.

Note:

(a) These changes do not become apparent unless the patient survives at least 12 hours after the arrest.
(b) Similar types of neuronal damage can be seen in severe acute hypoglycaemia, carbon monoxide or barbiturate poisoning.

Cerebral Infarction

The diagram below illustrates the evolution of an infarct, e.g. in the territory of the middle cerebral artery. Up to 24 hours there is virtually no visible change.

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Cerebral Haemorrhage

Spontaneous intracranial bleeding is the second main type of stroke. In the great majority of cases there is localised arterial disease aggravated by hypertension. A small number are associated with cerebral tumours, systemic bleeding diathesis or arteriovenous malformations.

In most hypertensives over middle age, microaneurysms are found in the very small cerebral arteries. It is believed that rupture of one of these aneurysms is the immediate cause of intracerebral haemorrhage.

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Note: Intracerebral bleeding may track irregularly and often reaches the subarachnoid space and ventricles.

Subarachnoid Haemorrhage

This is commonly but not exclusively the result of rupture of a ‘berry’ aneurysm at or near the circle of Willis. The basic abnormality is a congenital weakness of the elastic tissues in the arterial wall; only rarely is an aneurysm present at birth, and while subarachnoid haemorrhage does occur in young people, the incidence increases with age. Hypertension is an important contributing factor.

Sites Often multiple, near arterial junctions.

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Not all aneurysms rupture; they are found incidentally at autopsy.

Massive haemorrhage may be preceded by one or more small leaks – marked by headache without functional loss.

Progress

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Note: The aneurysm may rupture directly into the brain and mimic an intracerebral haemorrhage.

CSF Findings

1–24 hours – blood stained; blood content constant in sequential samples (distinguishes blood derived from a traumatic tap)

– centrifuge supernatant – may be pink due to haemolysis.

24 hours onwards – supernatant shows xanthochromia (yellow colour due to presence of blood degradation products).

Other causes of cerebral haemorrhage of either type are vascular malformations and coagulation disorders.

Head Injury

Head injuries of varying severity are common nowadays, particularly as a consequence of road traffic accidents. Immediate damage is caused by two main mechanisms which overcome the protection of the vulnerable cerebral tissues provided by the skull and the CSF ‘water cushion’.

1. Direct blows to the head (e.g. missile head injury)

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2. Non-missile head injury

Since the head is usually freely moveable on the neck, the sudden application of forces derived from acceleration, deceleration and, particularly, rotation of the head often causes serious brain injury.

(a) Skull fractures

(i) Skull damage

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Acceleration/deceleration injury

(ii) Brain damage

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(b) Rotation

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Note:

It will be appreciated that more serious cerebral damage is the result of interaction of complex physical forces and anatomical features. An understanding of these mechanisms explains why serious cerebral injury is not uncommon in the absence of damage to the scalp or fracture of the skull, and also why brain damage may be remote from the site of impact: so-called ‘contre-coup’ injury is sustained when the brain tissue opposite the site of impact is contused.

Delayed Complications

In addition to damage sustained immediately at the time of impact, certain serious complications may supervene over the next hours or few days.

1. Haemorrhages

(a) Extradural haematoma

This type of haemorrhage classically occurs as a complication of linear fracture of the skull vault when the middle meningeal artery is torn.

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The classic clinical association is a direct blow to the head from which recovery is rapid. After a lucid interval of varying duration up to several hours, signs of increased intracranial pressure supervene. This chain of events is explained by the time taken for the haemorrhage to accumulate by stripping the dura from the skull.

(b) Subdural haematoma. This may occur at any site and is often extensive because of the loose attachment of the dura and arachnoid membranes. It is usually due to rupture of small bridging veins.

These acute subdural haematomas are often associated with subarachnoid haemorrhage and cerebral contusions.

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(c) Intracerebral haematomas occur in association with cortical contusions particularly in the temporal and frontal lobes (burst lobe); but also at random deep within the hemispheres due to shearing at the time of impact. Large haematomas are uncommon.

2.

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3. External leakage of CSF (and blood) from the ear and nose may complicate fractures of the skull base. This complication may be of long duration and is always a potential entry for infection.
4. Local infection may complicate compound fractures and progress to meningitis.

Late Complications

1. Epilepsy

Head injury is an important cause of epilepsy. The risk is highest in severe missile head injury and may be related to ischaemic brain damage.

2. Chronic subdural haematoma

A thick layer of fluid and partially clotted blood gradually accumulates between the dura and arachnoid membranes which show considerable reactive thickening.

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The precise cause is not known; most cases occur in alcoholics or in elderly people already suffering from cerebral atrophy, and it is possible that the small bridging veins are unduly stretched and become more susceptible to damage.

The clinical signs are usually insidious in onset and progressive, and in many cases there is a history of either no or only very trivial injury.

Ageing and Dementia

Normal Brain Ageing

With normal ageing the brain becomes atrophic, but the morphological changes described below are not necessarily accompanied by loss of intellect.

Changes in Old Age

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There is compensatory enlargement of the lateral ventricles.

Dementia

Dementia is defined as ‘an acquired progressive global impairment of intellect, memory and personality, without impairment of consciousness’. Around 5% of the population over 65 years are affected and the proportion rises to over 20% of these over 80 years.

Main causes are:

1. Alzheimer’s diseases 70%
2. Multi-infarct dementia 10–15%
3. Lewy body dementia 10–20%

Rare causes include:

1. Genetic disorders (Huntington’s disease, Pick’s disease)
2. Infections (Creutzfeld-Jacob disease, see p.555, AIDS)

Alzheimer’s Disease

This disease accounts for around 70% of cases of dementia. While typically a disease of the elderly, especially females, it is also seen in patients under 60 years, in whom there is often a family history. Almost all patients with Down’s syndrome who survive to 50 years develop Alzheimer’s (suggesting that chromosome 21 is important).

Pathology

The changes of Alzheimer’s resemble those of normal ageing, but are greatly exaggerated in the temporal and parietal lobes and in the hippocampus.

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