Introduction

Peripheral neuropathy (PN) represents nerve damage outside the central nervous system. It includes lower motor neurone (LMN) damage as well as sensory neuropathies. Radiculopathy (nerve root damage) is also part of PN.

Ability to differentiate between upper motor neurone (UMN) and LMN damage is fundamental to understanding PN (see Table 11.1). Cranial nerves represent a special type of peripheral nerve emanating from the brainstem. This raises an ‘old chestnut’, namely facial weakness of either UMN or LMN origin.

TABLE 11.1 Differentiation between upper motor neurone and lower motor neurone damage

All too often patients are diagnosed with ‘UMN facial palsy’ and by definition ‘palsy’ is of LMN origin (as is the case with bulbar palsy and pseudo-bulbar palsy, the latter being of UMN origin and hence not really a palsy—thus the use of the term ‘pseudo’). It follows that ‘UMN facial palsy’ is stating ‘UMN LMN weakness’ which is inappropriate. The word ‘palsy’ is a corruption of the French word ‘paralysie’, meaning paralysis but usually implying LMN lesion.

A LMN facial nerve lesion (cranial nerve VII palsy) affects both upper face (forehead) and lower face—hence the saying ‘lower is upper’, while an UMN facial weakness affects only the lower part of the face justifying the statement ‘upper is lower’. To be correct one should refer to either facial palsy, implying LMN deficit, or UMN facial weakness, implying involvement of only lower face.

There are some conditions that may affect both UMN and LMN,¹ such as motor neurone disease (often referred to as amyotrophic lateral sclerosis) or subacute combined degeneration, as may occur with vitamin B₁₂ deficiency.²

What follows in this chapter is a discussion of the nature of nerve damage, causes of PN, diagnosis thereof, a look at some focal neuropathies, nutritionally induced neuropathies and some illnesses associated with PN. The aim is to help general practitioners become more involved in patient care and, hence, better enjoy the therapeutic partnerships with the consultant.

The Nature of Nerve Damage

Nerves transmit their messages via an electrical impulse from the nerve cell (the cell of the neurone) down the length of the ‘arm’ of the cell (the axon) to pass the message either to another nerve or the end target organ. The message is sent via the impulse causing release of the neurotransmitters that produce the desired effect, be it to stimulate or suppress a subsequent response, such as causing a muscle to contract or to stimulate another nerve (see Fig 11.1).

FIGURE 11.1 Schematic of neural transmission.

To speed up transmission the axons of the nerves are usually coated with myelin, which acts in a similar fashion to the plastic coating that insulates and allows unimpeded passage of electrical current down an electrical wire (see Fig 11.1). Along the myelinated nerve fibre the impulse ‘jumps’ between Nodes of Ranvier, breaks in the myelin.

In the same way the wire within the plastic transmits the electrical current, so too does the axon send the neuronal impulse. It follows that damage to the axon (analogous to the electrical wire) will reduce the amount of message able to be transmitted. In neurophysiological terms, axonal damage will reduce the amplitude of the impulse and hence the amplitude of the action potential generated,³ namely the size and strength of the message received rather than the speed of the transmission.

Transmission speed (or conduction time) is facilitated by the myelin sheath. It follows that damage to myelin, as occurs in demyelinating illnesses (such as Landry-Guillain-Barré syndrome, often referred to as GBS—omitting the name Landry, or multifocal motor neuropathy) will slow conduction time.⁴ In the same way damage to the plastic coating of the electrical wire will ultimately allow the wire to corrode and be damaged, so too will established demyelination allow subsequent axonal damage.⁵

From the above it can be seen that nerve damage will occur if there is a disease process that will remove myelin from nerves (demyelination causing slowing of conduction); axonal damage causing less impulse to be transmitted; or a combination of both. Understanding of this will enhance appreciation of the results from neurophysiological testing.

When nerves are damaged it may be akin to a short circuit, to continue the electrical wire analogy. Thus nerves may ‘fire’ spontaneously. This may result in the activation of a single muscle fibre (called fibrillation, see Fig 11.2) or a group of muscle fibres known as a motor unit, which is limited to muscle fibres innervation by a single motor nerve (called fasciculation, see Fig 11.3).

FIGURE 11.2 Fibrillation.

FIGURE 11.3 Fasciculation.

Both fibrillations and fasciculations are recorded using needle electromyography to record spontaneous firing. Fibrillations are not generally visible with the exception of the tongue, which may be wasted and show fibrillations in motor neurone disease. Fasciculations can be seen as small muscle twitches as they involve more than a single muscle fibre. Fasciculations may be completely benign as can occur in marathon runners, or may reflect significant PN and often accompany motor neurone disease.

Causes of Nerve Damage

PN can be the result of either local damage or a more generalised process affecting a wider target (see Box 11.1).

The most common cause of local damage is consequent to pressure, which causes a ‘neurapraxia’ that damages the nerve at the site of the pressure. An example of this process is damage to the lateral popliteal nerve with resultant foot drop. The lateral popliteal nerve activates the tibialis anterior muscle that dorsiflexes the foot.⁶ Similarly, direct damage to the radial nerve as it transverses the humeral groove in the humerus bone will result in wrist drop due to deficits of wrist extension.⁷

Perhaps the best-known local nerve damage is carpal tunnel syndrome (CTS), which affects the median nerve. This causes weakness of the muscles innervated by the nerve plus specific sensory deficit referable to local median nerve damage (see Fig 11.4).

FIGURE 11.4 Sensory innervation of hand.

Only four intrinsic hand muscles are innervated by the median nerve.⁸ The remainder are innervated by the ulnar nerve. There is a mnemonic to remember these four muscles: LOAF (Lateral two lumbricals, Opponens pollicis, Abductor pollicis brevis and Flexor pollicis brevis).

Generalised PN results from a systemic problem that can be the result of a variety of toxins, for example drugs (e.g. vincristine)⁹ or infections, which may be the precipitant for conditions such as motor neuropathy (as may occur with infectious hepatitis)¹⁰. A variety of other environmental toxins, such as lead and other heavy metals,¹¹ can also cause pure motor neuropathy.

Conditions such as Landry-Guillain-Barré syndrome (GBS, also known as idiopathic ascending inflammatory polyradiculopathy)¹² and chronic inflammatory demyelinating neuropathy¹³ (CIDP) represent allergic responses to reputed infective agents causing demyelinating radiculopathy or neuropathy.^12,13

Diagnosis of Nerve Damage