Clinical Disorders Of The Motor System
Disturbances in the motor pathways can produce a range of disorders of movement. These typically involve:
· the basal ganglia, causing abnormal involuntary movement without any effect on power, reflexes or coordination.
· the cerebellum and its connections, causing problems with coordination without any changes in power or reflexes;
· the motor neurones (lower or upper), causing weakness and changes in muscle tone and reflexes;
· the neuromuscular junction (NMJ), causing fatiguable weakness;
· the muscle, causing weakness;
In order to determine the nature and cause of the motor disturbance a full history and examination should be undertaken along with appropriate tests. The majority of patients with isolated motor symptoms have either Parkinson’s or motor neurone disease, although by far the most common clinical scenario is the patient with both motor and sensory abnormalities as a result of strokes or damaged nerves as they emerge or pass along the limb. A typical screen of tests for patients with motor symptoms involves blood tests, nerve conduction studies (NCS), electromyography (EMG) and magnetic resonance imaging (MRI) of brain and spinal cord.
Muscle (see Chapters 20 and 21)
The typical features of a muscle disease are weakness, which may relate to exercise, and, on occasions, muscle pain (myalgia). The age and rate of progression is often helpful in determining the type of muscle disease, e.g. progressive slow weakness without pain from childhood would suggest a degenerative muscular dystrophy (see Chapter 20), while a short history of painful weakness in adulthood would suggest an inflammatory myositis (see Chapter 62). The distribution of weakness is also helpful in defining the likely type of muscle disease, e.g. proximal arm and leg weakness in limb girdle muscular dystrophy. The investigations that are especially useful in muscle disease are blood tests to look at levels of muscle-specific creatine phosphokinase (CPK) – a measure of muscle damage; EMG and muscle biopsy. In some cases genetic testing is of value, especially if the muscle weakness is associated with myotonia and other features of myotonic dystrophy.
Neuromuscular junction (see Chapters 16 and 62) Patients with these disorders present with a history of weakness that gets worse with continued use of the muscle. The most common disorder of the NMJ is myasthenia gravis, which typically presents in early or late adulthood with fatiguable diplopia, ptosis, facial and bulbar weakness and proximal limb weakness. The examination confirms weakness that may be present at rest but clearly gets worse with exercise. Patients can present as a neuro- logical emergency if there is bulbar and respiratory failure. Diagnosis typically relies on history and examination, the presence of acetylcholine receptor (AChR) or muscle-specific kinase (MUSK) antibodies, a positive response to a short-acting acetylcholinesterase inhibitor (Tensilon test) and abnormalities on repetitive stimulation with NCS and EMG. Muscle biopsy is not necessary. In some patients myasthenia gravis is associated with either enlargement (hyperplasia) or a tumour of the thymus gland. Other myasthenic syndromes are rare.
Damage to the peripheral nerves will generally give both sensory and motor symptoms and signs. However, the peripheral motor nerve can be preferentially involved in some neuropathies as well as in conditions such as poliomyelitis and motor neurone disease, which target the actual motor neurone cell body in the ventral horn of the spinal cord and/or brainstem. The typical features of damage to the peripheral motor nerve are weakness, wasting, fasciculation and loss of reflexes – a lower motor neurone (LMN) lesion. Investigation of LMN syndromes involves excluding nerve entrapment as it exits the spinal cord by MRI imaging, along with NCS and EMG – the latter showing features of denervation with spontaneous motor discharges from the muscle that has lost its normal innervation.
The involvement of spinal cord pathways gives a variety of motor syndromes (see Chapters 37 and 54). In rare cases there is involvement of spinal cord interneurones, leading to continuous motor unit activity (CMUA) and stiff man syndrome (see Chapters 35 and 36). Involvement of descending motor pathways from the brain in the spinal cord causes an upper motor neurone (UMN) syndrome of weakness, spasticity, increased reflexes, and clonus and extensor plantars. It is unusual for this pathway to be selectively involved in spinal cord pathology and when it does happen, the patient often also has LMN signs and has a form of motor neurone disease called amyotrophic lateral sclerosis or Lou Gehrig disease. However, if only UMN signs are seen then the patient is said to have primary lateral sclerosis. Structural lesions of the spinal cord typically produce a combination of motor and sensory signs and symptoms. Investigation involves MRI, with cerebrospinal fluid (CSF) examination if an inflammatory aetiology is suspected and in some cases neurophysiological testing with EMG, NCS and central motor conduction time (CMCT).
Damage to supraspinal structures can produce a variety of motor signs and symptoms. Involvement is most commonly seen in cerebrovascular accidents (CVAs) with involvement of all the descending motor pathways from the cortex to the brainstem and spinal cord. This gives rise to contralateral hemiparesis with UMN signs. If the left hemisphere is involved there is typically major disturbance in speech. Occasionally, damage is restricted to the motor cortex, when the patient may present with focal motor seizures such as Jacksonian epilepsy (see Chapters 39 and 61). The mainstay of investigation of supraspinal motor abnormalities is MRI and/ or computed tomography (CT), and CSF examination if an inflammatory aetiology is suspected. In some cases genetic testing is helpful.
Other sites commonly involved in disease processes
This produces either a slowness of movement such as in Parkinson’s disease; an abnormality of limb posture and movement (dystonia) or the development of uncontrollable involuntary movements such as chorea and hemiballismus (see Chapter 42).
This produces incoordination of movement with slurred speech and abnormal eye movements (see Chapter 40). The disease processes that typically affect this part of the CNS are multiple sclerosis, drugs such as anticonvulsants and alcohol along with a series of rare genetic conditions called the spinocerebellar ataxias (SCAs) (see Chapter 63). It can also be involved by tumour growth in which case the situation may be complicated by the development of hydrocephalus through compression of the fourth ventricle and its outflow foramina (see Chapter 5).