11th August 2024

Overview of movement disorders

This section will explain what a movement disorder is, and the forms that it can take. Movement disorders affect the nervous system, classically the basal ganglia or cerebellum, resulting in abnormal voluntary movements or causing involuntary movements. It is important to note that not all disorders causing abnormalities of movement are part of a movement disorder syndrome; examples include hemiplegia in stroke, tonic-clonic seizures in epilepsy and neuromuscular disorders. There are a few treatable movement disorders – Parkinson’s disease, Wilson’s disease and dopa-responsive dystonia – and there should be a high index of suspicion for these.

Movement disorders have historically been, and still are, broadly divided into two categories: hypokinetic (reduced movement) or hyperkinetic (excess movement). Even though a degree of overlap is now recognised, with many disease processes not falling neatly into one category, they are still widely used today. This original classification was based on clinical syndromes with suspected underlying basal ganglia dysfunction. However with longer life spans allowing diseases to progress, the development of modern imaging techniques and new investigations we now have a much greater understanding of these conditions and the widespread effects they have on the CNS, outside of the classical movement disorder presentation from which they were originally described and categorised.

Tremor

Tremor is the rhythmic shaking of a part of the body due to involuntary muscle contractions. It can be divided into two main forms, resting or action, the latter occurring during voluntary movement. Action tremors can be divided into sub-categories including intention, postural, and isometric amongst others. Everyone has a physiological tremor, most commonly a fine shaking of the hands which is often not visible to the naked eye due to its low amplitude and frequency. In some individuals it is more prominent, including in those with hyperthyroidism, hypoglycaemia and alcohol withdrawal in which the normal physiological tremor is enhanced.

Essential tremor

This is the most common type of tremor and most common movement disorder worldwide. It can be a mix of a number of different tremor types, including kinetic, postural and sometimes rest. The tremor is mostly symmetrical and progresses over time. It commonly involves the hands and forearms and can also involve the voice, head and jaw. Incidence increases with age, although in those with a family history it can appear in adolescence. Stress and caffeine can make essential tremors more prominent while alcohol can lessen them. It is a clinical diagnosis although sometimes investigations are used to rule out other pathologies such as hyperthyroidism or Wilson’s disease.

For most the tremor does not cause any problems and no treatment is needed. As it progresses it may start to interfere with daily activities and cause social distress for which propranolol or primidone can be used. Second-line agents include gabapentin and topiramate. In very severe cases deep brain stimulation (DBS) or thalamotomy, either using MRI-guided focused ultrasound or gamma knife radiation, are possible.

Myoclonus

Sudden, brief, involuntary jerking or twitching movements, most commonly non-rhythmic. It can be described as positive or negative with the former being a sudden muscle contraction and the latter being a sudden loss of muscle tone. Myoclonic jerks occur physiologically including as hiccups or as limb twitches when a person drifts off to sleep. However, myoclonus can also be pathological and can occur in many different forms in isolation and as part of other neurological disorders, e.g. post-anoxic brain injury, genetic and neurodegenerative disease. Of note it can sometimes be the only manifestation of epilepsy and thus can indicate underlying seizure activity. Asterixis is a form of negative myoclonus which can occur due to metabolic disturbance such as hypercapnia (carbon dioxide retention flap), uraemia and liver failure (liver flap).