Ross & Wilson Anatomy and Physiology in Health and Illness (83 page)

•
one sided, causing pressure damage to a nerve root

•
midline, compressing the spinal cord, the anterior spinal artery and possibly bilateral nerve roots.

The outcome depends upon the size of the hernia and the length of time the pressure is applied. Small herniations cause local pain due to pressure on the nerve endings in the posterior longitudinal ligament.

Large herniations may cause:

•
unilateral or bilateral paralysis

•
acute or chronic pain perceived to originate from the area supplied by the compressed sensory nerve, e.g. in the leg or foot

•
compression of the anterior spinal artery, causing ischaemia and possibly necrosis of the spinal cord

•
local muscle spasm due to pressure on motor nerves.

Syringomyelia

This dilation (syrinx) of the central canal of the spinal cord occurs most commonly in the cervical region and is associated with congenital abnormality of the distal end of the fourth ventricle. As the central canal dilates, pressure causes progressive damage to sensory and motor neurones.

Early effects include
dissociated anaesthesia
, i.e. insensibility to heat and pain, due to compression of the sensory fibres that cross the cord immediately they enter. In the long term there is destruction of motor and sensory tracts, leading to spastic paralysis and loss of sensation and reflexes.

Tumours and displaced fragments of fractured vertebrae

These may affect the spinal cord and nerve roots at any level. The pressure damage initially causes pain and later, if the pressure is not relieved, there may be loss of sensation and paralysis. The areas affected depend on the site of pressure.

Diseases of peripheral nerves

Learning outcomes

After studying this section you should be able to:

compare and contrast the causes and effects of polyneuropathies and mononeuropathies

describe the effects of Guillain–Barré syndrome and Bell’s palsy.

Peripheral neuropathy

This is a group of diseases of peripheral nerves not associated with inflammation. They are classified as:

•
polyneuropathy: several nerves are affected

•
mononeuropathy: a single nerve is usually affected.

Polyneuropathy

Damage to a number of nerves and their myelin sheaths occurs in association with other disorders, e.g.:

•
nutritional deficiencies, e.g. vitamins B
₁
, B
₆
, B
₁₂

•
metabolic disorders, e.g. diabetes mellitus, renal failure, hepatic failure, carcinoma

•
toxic reactions to, e.g., alcohol, lead, mercury, aniline dyes and some drugs, such as phenytoin, isoniazid

•
infections, e.g. leprosy.

The long nerves are usually affected first, e.g. those supplying the feet and legs. The outcome depends upon the cause of the neuropathy and the extent of the damage.

Mononeuropathy

Usually only one nerve is damaged and the most common cause is ischaemia due to pressure. The resultant dysfunction depends on the site and extent of the injury. Examples include:

•
pressure applied to cranial nerves in cranial bone foramina due to distortion of the brain by increased ICP

•
compression of a nerve in a confined space caused by surrounding inflammation and oedema, e.g. the median nerve in carpal tunnel syndrome (see
p. 424
)

•
external pressure on a nerve, e.g. an unconscious person lying with an arm hanging over the side of a bed or trolley

•
compression of the axillary (circumflex) nerve by ill-fitting crutches

•
trapping of a nerve between the broken ends of a bone

•
ischaemia due to thrombosis of blood vessels supplying a nerve.

Guillain–Barré syndrome

Also known as acute inflammatory polyneuropathy, this is sudden, acute, progressive, bilateral ascending muscular weakness or paralysis. It begins in the lower limbs and spreads to the arms, trunk and cranial nerves. It usually occurs 1 to 3 weeks after an upper respiratory tract infection. There is widespread inflammation accompanied by some demyelination of spinal, peripheral and cranial nerves and the spinal ganglia. Paralysis may affect all the limbs and the respiratory muscles. Patients who survive the acute phase usually recover completely in weeks or months.

Bell’s palsy

Compression of a facial nerve in the temporal bone foramen causes paralysis of facial muscles with drooping and loss of facial expression on the affected side. The immediate cause is inflammation and oedema of the nerve. The underlying cause is unknown although viruses may be involved. The onset may be sudden or develop over several hours. Distortion of the features is due to muscle tone on the unaffected side, the affected side being expressionless. Recovery is usually complete within a few months although the condition is sometimes permanent.

Developmental abnormalities of the nervous system

Learning outcomes

After studying this section you should be able to:

describe developmental abnormalities of the nervous system

relate their effects to abnormal body function.

Spina bifida

This is a congenital malformation of the embryonic neural tube and spinal cord (
Fig. 7.53
). The vertebral (neural) arches are absent and the dura mater is abnormal, most commonly in the lumbosacral region. The causes are not known, although the condition is associated with dietary deficiency of folic acid at the time of conception. These neural tube defects may be of genetic origin or due to environmental factors, e.g. irradiation, or maternal infection (rubella) at a critical stage in development of the fetal vertebrae and spinal cord. The effects depend on the extent of the abnormality.