Pathological processes in neurology

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Pathological processes in neurology

It is useful to appreciate the ways in which the nervous system malfunctions. This is important clinically because the time course of any pathological process is described by the patient in the history. It also provides a framework for understanding different types of disease, particularly rarer diseases that you might not otherwise know about.

Broadly, pathological processes can be divided into (Fig. 1):

Systemic – abnormalities in the nervous system secondary to systemic disease.

Intrinsic – arising within the nervous system.

Vascular – resulting from disturbances in the blood supply to the nervous system.

Extrinsic – affecting the nervous system from outside.

Fig. 1 Four types of pathological processes.

Hint: remember this as SIVE. There is some overlap between these categories.

Systemic

These changes are usually due to metabolic, toxic, nutritional, immunological or endocrine disorders (Table 1).

Table 1 Systemic processes

Disorder	Clinical syndrome
Metabolic	Hypo- or hyperglycaemia, hyponatraemia, hypoxia, hypercarbia, hyper- or hypocalcaemia Renal failure Liver failure Wilson’s disease

}Metabolic encephalopathy
Extrapyramidal syndrome Toxic

Drugs

Heavy metals

Organic solvents

Nutritional

Vitamin B12 deficiency

Thiamine deficiency

Neuropathy, myelopathy Neuropathy, Wernicke–Korsakoff encephalopathy Endocrine

Hypothyroidism

Hyperthyroidism

Addison’s disease

}Myopathy

Vascular

The nervous system can be affected by vascular disease in two ways:

1. Haemorrhage – can be within the substance of the brain, in the subarachnoid space, in the subdural compartment or the extradural space.

2. Infarction – can be caused by an embolus, thrombosis or arterial disease such as vasculitis.

With the exception of subdural haematoma, these are characterized by a sudden onset.

Extrinsic

In these situations the nervous system itself is normal but is being disrupted by external factors. Management of these conditions depends on correcting the external factor and is therefore usually neurosurgical.

Every level of the nervous system can be affected. Some of the processes are summarized in Table 2.

Table 2 Extrinsic pathological processes classified according to the level of the nervous system affected

Level of nervous system	Pathological process
Brain	Trauma Hydrocephalus Extrinsic tumour (e.g. meningioma) Abscess

Spinal cord

Trauma

Cervical spondylitic compression

Abscess

Extrinsic tumour (e.g. meningioma, neuroma)

Nerve root Lumbar and cervical disc disease Plexus

Trauma

Tumour infiltration

Radiation damage

Peripheral nerves Compression, e.g. median at wrist, ulnar at elbow

Intrinsic

These processes can be divided into:

This can be remembered as MIND PIG.

Metabolic

Metabolic diseases that affect the nervous system often have systemic features and so this category overlaps with systemic metabolic problems.

There are a large number of different conditions, usually recessively inherited, that affect the nervous system and usually present in childhood. Commonly there is a combination of developmental delay or regression, intellectual impairment, ataxia and seizures. Specific metabolic disturbances have particular features. All are rare, the most common being phenylketonuria and Hartnup’s disease.

Several metabolic conditions can present in adult life; many of these affect muscle, for example McArdle’s disease (myophosphorylase deficiency). Mitochondrial diseases are being increasingly recognized as the cause of neurological disease in adults; features include seizures, deafness, episodic lactic acidosis and myopathy.

Infectious

The most common infections in the developed world are viral and bacterial meningitis and viral encephalitis. Leprosy, cerebral malaria and tuberculous meningitis are common infections in other parts of the world.

Less common infections include Lyme disease, human immunodeficiency virus infection and its complications, human T-cell leukaemia virus 1 myelopathy and Creutzfeldt–Jakob disease. These are discussed on pages 98–101.

Neoplastic

Neoplasia can affect the nervous system in three ways:

1. Primary tumours.

2. Secondary tumours.

3. Paraneoplastic manifestations.

Paraneoplastic syndromes are non-metastatic syndromes related to cancer. These syndromes are thought to be immunologically mediated and include a limbic encephalitis, cerebellar degenerations and a myasthenic syndrome called Lambert–Eaton myaesthenic syndrome.

Degenerative diseases of the nervous system (Table 3) are progressive; some are familial but most are of uncertain aetiology. In some, a genetic cause has been found, for example Huntington’s disease. All levels of the nervous system can be affected. These conditions are grouped together for convenience. When the underlying pathophysiology of these conditions is better understood they may be further reclassified as genetic or metabolic diseases.

Table 3 Some degenerative diseases of the nervous system