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.
Peripheral nerves
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.
Spinal cord
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).
Brain
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
Basal ganglia
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).
Cerebellum
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).
