LOCALISED NEUROLOGICAL DISEASE AND ITS MANAGEMENT C. PERIPHERAL NERVE AND MUSCLE

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SECTION IV LOCALISED NEUROLOGICAL DISEASE AND ITS MANAGEMENT C. PERIPHERAL NERVE AND MUSCLE

THE POLYNEUROPATHIES – FUNCTIONAL ANATOMY

The function of the peripheral nervous system is to carry impulses to and from the central nervous system. These impulses regulate motor, sensory and autonomic activities.

The peripheral nervous system is comprised of structures that lie outside the pial membrane of the brain stem and spinal cord and can be divided into cranial, spinal and autonomic components.

SPINAL PERIPHERAL NERVOUS SYSTEM

Entry to and exit from the central nervous system is achieved by paired spinal nerve roots (30 in all).

These dorsal and ventral roots lie in the spinal subarachnoid space and come together at the intervertebral foramen to form the spinal nerve.

The dorsal roots contains sensory fibres, arising from specialised sensory receptors in the periphery.

The dorsal root ganglia are collections of sensory cell bodies with axons extending peripherally as well as a central process which passes into the spinal cord in the region of the posterior horn of grey matter and makes appropriate central connections.

Sensation can be divided into:

These different forms of sensation are carried from the periphery by axons with specific characteristics. The central connections and pathways vary also (see page 200).

The anterior horns of the spinal cord contain cell bodies whose axons pass to the periphery to innervate skeletal muscle – the alpha motor neurons. Smaller cell bodies also project into the anterior root and innervate the intrafusal muscle fibres of muscle spindles – the gamma motor neurons.

Each alpha motor neuron through its peripheral ramifications will innervate a number of muscle fibres. The number of fibres innervated from a single cell varies from less than 20 in the eye muscles to more than 1000 in the large limb muscles (innervation ratio). The alpha motor neuron with its complement of muscle fibres is termed the motor unit.

Function: TYPE C Function:

The structure of the spinal peripheral nervous system has been considered but the arrangement is also important. Spinal nerves, after emerging from the intervertebral foramen pass into the brachial plexus to supply the upper limbs and the lumbosacral plexus to supply the lower limbs.

The thoracic nerves supply skeletal muscles and subserve sensation of the thorax and abdomen.

The Autonomic Nervous System is described on page 457.

THE POLYNEUROPATHIES – SYMPTOMS

reduced sensitivity to painful stimulus increased sensitivity to any stimulus reduced sensitivity to any stimulus increased sensitivity with increasing threshold to repetitive stimulation pain provoked by a non-painful stimulus

Complex regional pain syndromes (CRPS) were previously termed ‘reflex sympathetic dystrophy’ and ‘causalgia’. These may follow a simple soft tissue injury (CRPS-1) or injury to a large peripheral nerve (CRPS-2). Allodynia and hyperalgesia are associated with local changes in temperature and skin appearance (oedema and discoloration). The pain has a burning, shooting quality. Motor manifestations (weakness or involuntary movements) are common and the pathophysiologic mechanism unknown.

THE POLYNEUROPATHIES – SIGNS

MOTOR EXAMINATION

Muscle wasting. Evident in axonal but absent in demyelinating neuropathies. Oedema of immobile limbs may mask wasting. The 1st dorsal interosseus muscle in the upper limbs and extensor digitorum brevis in the lower limbs are muscles that commonly first show wasting in the neuropathies, but examine all muscle groups. Look for fasciculations – irregular twitches of groups of muscle fibres due to diseased anterior horn cells, these may be induced by exercise or muscle percussion.

Muscle weakness. Weakness is proportional to the number of affected motor neurons. It develops suddenly or slowly and is generally symmetrical, usually starting distally in the lower limbs and spreading to upper limbs in a similar manner before ascending into proximal muscle groups. This pattern of progression is supposedly due to the ‘dying back’ of axons towards their nerve cells – the longest being the most vulnerable.

Some neuropathies, e.g. Guillain-Barré, chronic inflammatory demyelinating polyneuropathy, may affect proximal muscle groups first.

In severe neuropathies, truncal and respiratory muscle involvement occurs. Respiratory muscle weakness may result in death.

THE POLYNEUROPATHIES – CLASSIFICATION

There are several approaches to classification:

The following table based primarily on mode of onset is for reference. Certain neuropathies will be dealt with separately (see pages 439–444).

CAUSE FUNCTIONAL DISTURBANCE PATHOLOGY
ACUTE: days to 4 weeks
Inflammatory (Guillain Barré syndrome) Predominantly motor Demyelinative with perivascular lymphocytic infiltration
Distal or proximal
Autonomic disturbance
Diphtheria—— Cranial nerve onset Demyelinative. No inflammatory infiltration.
Mixed motor/sensory
Porphyria—— Motor (may begin in arm). Axonal
Autonomic disturbance
Minimal sensory loss.

SUBACUTE – occasionally CHRONIC: months and years
ASYMETRICAL and MULTIFOCAL
Infections
Leprosy Sensory neuropathy, often multifocal; associated depigmentation Spectrum from paucibacillary (few organisms with intense inflammation) to multibacillary (many organisms with little inflammation)
HIV Range of associated neuropathies  
Vasculitic disorders
Polyarteritis nodosa; Usually presents with mononeuritis multiplex or an asymmetrical sensorimotor neuropathy. Vasculitis with Wallerian degeneration in distal nerves
Wegner’s granulomatosis;
Churg-Strauss syndrome
  Often painful  
Non-systemic vasculitis As above without systemic features  

SUBACUTE and CHRONIC: months and years
SYMMETRICAL
Metabolic and endocrine disorders
Diabetes Most commonly distal sensorimotor  
But wide range of other forms (see later)
Ureamia Distal sensorimotor Axonal degeneration
Hypothyroidism Distal sensorimotor  
Acromegaly Distal sensorimotor  

CAUSE FUNCTIONAL DISTURBANCE PATHOLOGY
Nutritional deficiencies
Vitamin B1 (thiamine) Predominantly sensory, with burning feet. Axonal degeneration with segmental demyelination
Includes alcoholic neuropathy Weakness may develop.  
  Autonomic involvement common but mild  
B12 deficiency Predominantly sensory; may be associated spinal cord involvement  
Malignant disease
Paraneoplastic Sensory or sensorimotor Axonal; may be associated antibodies (anti-Hu)
Infiltrative Multifocal, often a polyradiculopathy More common with lymphoma
Paraprotein associated
Monoclonal gammopathy (IgG, IgA, IgM) Sensorimotor neuropathy Axonal with segmental demyelination
Chronic inflammatory demyelinating polyneuropathy (CIDP) (see later)
Amyloid
Primary, secondary or familial Sensorimotor neuropathy often with autonomic involvement Thickened nerves with amyloid deposition and small fibre neuropathy
  May present as multiple mononeuropathies  
Inherited neuropathies
Charcot-Marie-Tooth disease (see below)
Refsum’s disease Phytanic acid storage disorder. Sensorimotor neuropathy with ichthyosis, retinitis pigmentosa and deafness  
Drug induced
Wide range of drugs induce neuropathies including:
Antibiotics Metronidazole; ethambutol; isoniazid; nitrofurantoin; dapsone
Oncology drugs Adriamycin; cisplatin; taxanes; vincristine
HIV drugs Didanosine; stavudine; zalcitabine
Others Amiodarone; gold; phenytoin  
Toxin induced
Solvents
Heavy metals Lead; arsenic; thallium  

Chronic idiopathic axonal neuropathy:

In patients about 20% of patients with a chronic neuropathy no cause is identified. Follow up of cohorts of such patients has found that while their symptoms slowly progress they do not develop significant disability.

INVESTIGATION OF NEUROPATHY

Investigation of a neuropathy will be led by the history and the pattern of the neuropathy. In many patients the diagnosis will be relatively straightforward, for example a typical distal symmetrical neuropathy in a patient with diabetes or with a history of alcoholism. Where the aetiology is known and the neuropathy mild and typical there is often no need for further investigation. However, in many patients the diagnosis is not clear and then the investigations will be led by the pattern of the neuropathy. Unlike the situation for chronic neuropathies (see previous page) the cause of acute or subacute neuropathy can usually be defined.

For a patient with a distal symmetrical sensorimotor neuropathy:

Initial investigations:

Further investigations (depending on clinical history):

Asymmetrical or multifocal neuropathies are much less common and there are usually clues in the history to direct investigation towards what is most frequently an underlying inflammatory disease, for example vasculitis or a specific inflammatory neuropathy. Inflammatory markers and autoantibodies may be helpful. In such patients nerve conduction studies and nerve biopsy may lead to diagnosis.

THE POLYNEUROPATHIES – SPECIFIC TYPES

GUILLAIN BARRÉ SYNDROME (ACUTE INFLAMMATORY DEMYELINATING POLYNEUROPATHY)

Incidence: 2 per 100 000 population per year. Characteristically it occurs 1–3 weeks after a viral or other infection or immunisation.

THE POLYNEUROPATHIES – SPECIFIC TYPES – INHERITED NEUROPATHIES

PLEXUS SYNDROMES AND MONONEUROPATHIES

Disease of a single peripheral or cranial nerve is termed mononeuropathy. When many single nerves are damaged one by one, this is described as mononeuritis multiplex. Damage to the brachial or lumbosacral plexus may produce widespread limb weakness which does not conform to the distribution of any one peripheral nerve. A knowledge of the anatomy and muscle innervation of the plexuses and peripheral nerves is essential to localise the site of the lesion and thus deduce the possible causes.

Certain systemic illnesses are associated with the development of mononeuropathy or mononeuritis multiplex:

Entrapment mononeuropathies result from damage to a nerve where it passes through a tight space such as the median nerve under the flexor retinaculum of the wrist. These are often related to conditions such as acromegaly, myxoedema and pregnancy, in which soft tissue swelling occurs. A familial tendency to entrapment neuropathy has been described.

Cranial nerve mononeuropathies have been dealt with separately.

BRACHIAL PLEXUS SYNDROMES

THORACIC OUTLET SYNDROME