appear clinically not involved, e.g. certain conditions that appear purely spinal may have an anterior horn cell component (motor neurone disease/amyotrophic lateral sclerosis [MND/ALS]) and neurophysiology can be diagnostic where imaging has been normal.

SOMATOSENSORY EVOKED POTENTIALS

Somatosensory evoked potentials (SSEPs) test the integrity of the afferent sensory pathways, principally the large nerve fibre dorsal column–medial lemniscal pathway. They are elicited by electrical stimulation of the median and posterior tibial nerves. Evoked peaks from median nerve stimulation are detected in the brachial plexus (Erb’s point), central grey matter of the cervical cord, medial lemniscus, and primary somatosensory cortex. Abnormalities of SSEPs result from single level (e.g. cervical spondylosis) or diffuse (e.g. MS) disease states. SSEPs can be used to monitor the integrity of the spinal cord during spinal surgery.

50 Sagittal diagram of lumbar spine.

LUMBAR PUNCTURE

Lumbar puncture (LP) is a useful test for measuring CSF pressure and obtaining CSF samples, important in the diagnosis of inflammatory disorders (oligoclonal bands [OCBs] in MS) and infections (borreliosis, syphilis, viral infections, spinal tuberculosis) (50). If, however, a mass lesion is suspected, particularly if it is high in the vertebral canal, LP, without provision for prior spinal imaging and ‘on-site’ surgical decompression, may be contraindicated. Some patients with compressive cord lesions become abruptly worse after LP due to shifting of the cord, and they may show signs of a complete cord syndrome. If the lesion is cervical, respiratory paralysis may occur suddenly.

If spinal MRI is performed after LP, bleeding or leakage of CSF into the epidural space may result in meningeal enhancement after contrast, erroneously interpreted as part of the disease process under investigation. It is therefore normally preferable to obtain imaging prior to CSF examination.

INVESTIGATING THE PERIPHERAL NERVOUS SYSTEM (NERVE, NEUROMUSCULAR JUNCTION, AND MUSCLE)

The investigation of disorders of the peripheral nervous system relies primarily on neurophysiology, with selected use of more invasive nerve and muscle biopsy.

NEUROPHYSIOLOGY

The use of nerve conduction studies to investigate peripheral nerve function, and EMG to investigate muscle disease, allows extensive investigation of the peripheral nervous system noninvasively.

Nerve conduction studies

By administering an electrical stimulus to a peripheral nerve, it is possible to measure signal conduction in the nerve, and thus deduce how the nerve is functioning. Motor and sensory nerves may be studied by recording the distal latency (latency from stimulus to recording electrodes), evoked response amplitude, and conduction velocity (51). Normal conduction velocity values vary depending on age and body temperature. Significant delay indicates impairment in nerve conduction, as is seen in demyelinating neuropathies such as Guillain–Barré syndrome or multifocal neuropathy, and in nerve entrapments.

F waves and H waves

While neurophysiology is mainly directed at disorders of nerve and muscle, it is possible to obtain indirect evidence of spinal or nerve root impairment by means of F and H waves (52). F waves require the motor nerve to be stimulated antidromically, i.e. from distal to proximal. The stimulus then travels proximally up the motor nerve axon, and then returns orthodromically (i.e. proximal to distal) back down to the initial stimulation point.

H reflexes differ in that the initial nerve stimulated is a sensory nerve. The signal in conducted proximally up to where the sensory nerve synapses with the motor nerve. The signal then returns down the motor axon and is recorded peripherally.

These F-response and H-reflex studies can be of use in peripheral neuropathies, particularly when the pathological process is too proximal to be detectable by conventional nerve conduction studies. However, in conditions such as radiculopathy, while responses may be abnormal, EMG would also be abnormal, such that F and H studies would not usually add any additional diagnostic value to EMG.

Blink reflex

In this test, the supraorbital nerve is stimulated, and activity in orbicularis oculi is recorded. An abnormal blink reflex can be helpful in showing evidence of a subtle trigeminal or facial nerve lesion.

52 Diagram to show F and H reflexes in nerve conduction studies. a: in F waves, the signal travels from distal to proximal up the motor nerve, and then from proximal to distal down the same motor nerve; b: in H waves, the signal travels from distal to proximal up the sensory nerve, and then proximal to distal down the motor nerve.

Electromyography

In EMG, a fine bore needle is inserted directly into muscle. The needle comprises a central recording electrode, and the outer casing acts as a reference. The potential difference between the two provides a measure of spontaneous muscle activity.

Normal muscle is electrically silent. When muscle is contracting normally, there appear motor unit potentials. As more of these are recruited, an interference pattern arises, which is the summation of many motor unit potentials (53). Abnormalities detected during EMG include the following: spontaneous rest activity, motor unit potential abnormalities, interference pattern abnormalities, and other phenomena, e.g. myotonia.

53 Diagram to show normal interference picture in nerve conduction.

Spontaneous rest activity

Fibrillation potentials are due to single muscle fibres contracting, and indicate active denervation, as occurs in neurogenic disorders such as neuropathy. Sharp positive spikes may be seen in chronically denervated muscle, such as in motor neurone disease, but also occur in acute myopathy (54).

Motor unit potential abnormalities

In neuropathy, collateral reinnervation causes potentials of large amplitude and long duration (55). This is in contrast to myopathies and muscular dystrophies, where potentials are of small amplitude and short duration (56).

Abnormalities of the interference pattern

Neuropathy leads to a loss of motor units under voluntary control, hence a reduction in interference. By contrast, in myopathy, recruitment of motor units and the interference pattern are normal.

Special phenomena

In myotonia, voluntary movement results in high frequency repetitive discharges. The amplitude and frequency of the potentials fluctuate, resulting in the typical ‘dive bomber’ sound on audio monitor (57).

54

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100 V

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10 ms

54 Diagram illustrating positive sharp waves in nerve conduction.

56

200 V

20 ms

55

200 V

20 ms

55 Diagram illustrating polyphasic, large amplitude, long duration motor unit potentials, commonly seen in neuropathy.

57

200 V

20 ms

56 Diagram illustrating polyphasic, small amplitude, short duration potentials seen in myopathies and muscular dystrophies.

57 Diagram illustrating myotonic discharge on electromyography as seen in myotonia.

Repetitive nerve stimulation/jitter

The neuromuscular junction may be specifically investigated using these techniques. In normal subjects, repetitive stimulation of a motor nerve results in a constant muscle potential, and decrement in the amplitude only occurs when the rate of stimulation is >30 Hz. Abnormalities on repetitive stimulation indicate neuromuscular junction dysfunction. In myasthenia, there occurs a decremental response at stimulus rates as low as 3–5 Hz (58). In Eaton–Lambert syndrome, an incremental response occurs with rapid stimulation, i.e. at 20–50 Hz.

Single fibre EMG

A further way of investigating the neuromuscular junction is using single fibre EMG. This requires a very fine needle, which records from approximately

1–3 muscle fibres from a single motor unit, rather than the 20 or so motor units sampled when using a standard EMG needle. There is normally a degree of variability in the action potentials recorded from different muscle fibres in a single motor unit, on account of variation in neuromuscular transmission. In myasthenia, there is increased jitter due to greater variability in neuromuscular transmission (59). This can be helpful in supporting the diagnosis in those cases of myasthenia where repetitive stimulation may have been normal.

NEUROPATHOLOGY

Nerve biopsy

Although nerve conduction studies often give sufficient information to result in the diagnosis of a peripheral nerve disorder, it is occasionally necessary

58 Electromyograph in myasthenia showing decremental response on repetitive stimulation.

59 Diagram illustrating jitter on single fibre electromyography. The gap between the two muscle action potentials is variable, i.e. jitter.