Performing
electromyography (EMG)
The clinical diagnosis of ALS is supplemented by additional diagnostic tests and tests of exclusion. The aim of these investigations is to rule out other diseases that may have similarities to ALS.
The individual symptoms of ALS also occur in other neurological diseases. However, this only applies to individual elements of ALS but not to the overall condition. These diseases are much rarer than ALS and can be distinguished by further investigation. Distinction from ALS is particularly important because “ALS like“ diseases can usually be better treated.
A few diseases that must be considered when establishing the diagnosis include, for example, mechanical damage of the spinal cord (e.g. cervical myelopathy), muscle diseases (e.g. inclusion body myopathy), diseases of the peripheral nerves (motor polyneuropathy), certain forms of multiple sclerosis, neurological diseases due to tumours (paraneoplastic syndromes) and very rare metabolic diseases. The doctor trained in neurology is familiar with these diseases and will arrange appropriate investigations at the slightest evidence of these diseases. Depending on the query, a variety of diagnostic methods such as blood tests, analysis of the cerebrospinal fluid or imaging procedures may be performed.
Electromyography (EMG) is a very important diagnostic method in diseases of the nerves and muscle. ALS is characterised by degeneration of the motor nerve cells, which only subsequently leads to impairment of muscle function (Fig. 2 1 3).
The disorder of the motor neurons leads to characteristic findings during the EMG (Fig. 1). The EMG examination consists of a needle test in which individual muscle groups are punctured with a sterile electrode and the electrical activity of the corresponding muscles is recorded. A full examination involves a needle test of several groups of muscles in the upper and lower limbs and occasionally in the face also (e.g. muscles used in chewing). The electrode employed is very thin so that the pinpricks (insertions) are well tolerated by the majority of patients.
Fig. 1
Diagram of the changes in nerves and muscles in ALS.
In a healthy person (Fig. 1.1) motor control of several muscle fibres (circles) is taken over by one nerve cell (rectangle). The combination of nerve cell, nerve fibre and muscle fibre is called a motor unit. The areas supplied by different motor units overlap ("chequered pattern").
In ALS (Fig. 1.2) degeneration of motor nerve cells occurs (yellow neuron). At first only individual nerve cells are affected. The as yet intact neurons take over the function of the degenerated neurons by sending out nerve fibres to additional muscle fibres (sprouting). This process is an attempted compensation mechanism by the nervous system to correct the loss of motor neurons and maintain the nerve supply of the muscle fibres. The result of sprouting is that the motor units increase in size (increase in the blue muscle fibres); this change can be established by a muscle test using electromyography (EMG) or by muscle biopsy.
In conjunction with the EMG investigation, a further electrophysiological investigation is often performed. This is electroneurography (ENG), which is used to confirm or rule out diseases of the peripheral nerves (motor neuropathies). Selected peripheral nerves in the arms and legs are stimulated with precisely dosed electrical impulses and their reaction to these electrical stimuli is examined. With transcranial magnetic stimulation (TMS) changes in certain conduction pathways in ALS can be detected. In TMS, controlled magnetic impulses are delivered through a hand sized coil held on the patient’s head and these lead to a response in the arm or leg. Analysis of this non invasive method is used to assess a nerve tract (pyramidal tract), which functions abnormally in ALS. Taking a sample from a muscle (muscle biopsy, Fig. 2 4 5) or from a peripheral nerve (nerve biopsy) is required in only a few cases.
Fig. 2:
Morphological changes in motor nerve cells and muscles in ALS. The cause of the ALS symptoms is the loss of motor nerve cells (motoneurons) in the brain and spinal cord.
Fig 2.1:
Light microscopic appearance of a motoneuron at the stage of degeneration. The number of motoneurons in the spinal cord (anterior horn) becomes smaller. Support cells of the nervous system take the place of the former nerve cells.
Fig 2.2:
The motor nerve fibres are derived from the cellular processes of thousands of motoneurons; these leave the spinal cord as the anterior nerve roots and are connected to the muscle fibres in the form of motor nerves. As a result of degeneration of the motor nerve cells in the spinal cord, there is wasting of the anterior roots (Fig 2.3) and transformation of the muscle (Fig 2.4). The changes in the skeletal musculature develop following loss of the neurons and the resulting compensation mechanism of nerve sprouting (see Fig. 1). On microscopic examination of the muscle (Fig. 2.4) a chequered pattern can be seen, consisting of preserved muscle fibres (supplied by intact motor neurons) and muscle fibres of very low calibre (associated with degenerated motoneurons). The ALS induced changes in the motor nerves and skeletal musculature can be detected by the electrophysiological investigations of electroneurography (ENG) and electromyography (EMG).
Muscle biopsy is performed when a muscle disease is suspected (e.g. inclusion body myopathy). When muscle tissue is examined under the microscope, it is possible to distinguish between muscle disease and ALS induced changes in the musculature. Nerve biopsy, e.g. from a small cutaneous nerve on the calf (sural nerve), is ordered when there is evidence of certain diseases of peripheral nerves (neuropathy). Both procedures are performed rarely and after explanation by the doctor, in the outpatient department or in the course of inpatient investigations.