Movement disorders appear in a myriad of diseases and are broadly distinguished into hyperkinetic and hypokinetic. Chorea, ballism, myoclonus, dystonia, tremor, tics and dyskinesia are some of the most common types of movement disorder. The diagnosis is based on clinical criteria and the underlying cause is determined together with findings obtained on imaging studies such as PET, MRI or CT. Treatment depends on the etiology, as well as type of disorder.
The clinical presentation of patients suffering from movement disorders is diverse. Basically, any involuntary movement should be considered as a disorder and based on its frequency, rhythmicity, magnitude and type, a more specific diagnosis can be made :
In addition to various forms of hyperkinetic disorders mentioned above, hypokinesia and its associated forms may also be seen in many patient, such as reduced facial expression and muscle rigidity, whereas gait and balance disturbances are hallmarks of ataxia. For these reasons, it is important to identify the exact type of aberration before pursuing the underlying cause.
Distinguishing between numerous types of movement disorders can be achieved only through a patient and thorough physical examination, together with patient history that may provide vital information regarding their onset and severity (for ex. stress can be a precipitating factor in FMDs). When the initial diagnosis is made, imaging studies are of great benefit in identifying the exact lesion that provoked such symptoms. CT, SPECT, MRI and FDG-PET are all shown to be very efficient in patient evaluation and they are often used in combination to solidify the obtained results  . Lesions in the thalamus, the lenticular nucleus are seen in patients who present with chorea . , whereas subthalamic nuclei and other subcortical areas are affected in the setting of ballism . However, pathological findings may be encountered in virtually any part of the central nervous system involved in movement, including the motor cortex, basal ganglia and the spinal cord. Electromyography (EMG), nerve conduction studies and electroencepalography (EEG) may serve as additional studies.
Depending on the type of disorder, several therapeutic strategies exist. Botulinum toxin, which impairs the release of acetylcholine from the presynaptic cleft, is efficient against dystonia, whereas baclofen, anticholinergic drugs and benzodiazepines serve as alternatives . In the setting of chorea and ballism, dopamine receptor blockers, such as haloperidol, perphenazine and fluphenazine and other neuroleptic agents are used as first-line therapy . Atypical neuroleptics (olanzapine), clozapine, risperidone, as well as catecholamine depletors - tetrabenazine and reserpine, are viable therapeutic agents as well  , but the appearance of adverse effects (depression, parkinsonism and hypotension) mandate their careful use. A different approach through GABA agonists, such as clonazepam or valproic acid can provide good outcomes against chorea, but also against myoclonus . Hypokinetic disorders, on the other hand, may be treated with levodopa and other dopamine-enhancing agents. As a last resort, surgical deep brain stimulation, which was primarily performed in patients who suffered from levodopa-induced dyskinesias, may be attempted . If a clinical suspicion toward drug-induced movement disorder is made, discontinuation of the drug is advocated. Symptomatic therapy and adequate support is often necessary, as the quality of life may be mildly or significantly impaired.
The prognosis significantly depends on the underlying cause, but since the majority of conditions that trigger movement disorders are chronic and incurable, the success of pharmacological therapy and severity of symptoms at the time of diagnosis are factors that determine their outcomes. A progressive course is observed in patients suffering from Huntington's and Parkinson's disease and many disorders have a significant impact on daily activities, which is why they must be identified and treated as soon as possible.
Diseases that induce movement disorders possess a wide clinical spectrum, as they may be of genetic, autoimmune, infectious , neurodegenerative or iatrogenic origin. Huntington's disease and paroxysmal dyskinesias are both induced by autosomal dominant mutations, whereas stiff-person syndrome and PERM have a presumable autoimmune etiology, in which several receptor families (glioma-inactivated, glycine and leucine-rich) are targeted by autoantibodies . Chorea is also seen in rheumatic fever, an infection caused by Streptococcus pyogenes as a result of molecular mimicry and a subsequent immune response . Tremor, rigidity, hypokinesia and bradykinesia are most frequently caused by Parkinson's disease, a progressive and complex neurodegenerative condition , while drug-induced (neuroleptics, antiemetics and substances that induce/inhibit dopaminergic effects) and paraneoplastic causes of movement disorder have been well-documented  . Psychogenic or functional movement disorders are triggered by significant emotional trauma or stress . Cerebrovascular insults, such as ischemic or subarachnoid hemorrhage (post-stroke movement disorders) also present an important cause of chorea, ballism, tremor, dyskinesia and myoclonus .
The incidence and prevalence rates of movement disorders are reflected through the appearance of underlying diseases in the population. Parkinson's disease is affecting about 1% of individuals aged 65-70 years and 3% of individuals over 80 years , while other studies suggest that approximately 4% of patients who suffer from stroke . A prevalence rate of 2.7 per 100,000 individuals was determined for Huntington's diseases . Some large-scale studies have, in fact, evaluated the presence of movement disorders in various age groups and showed that tremor is present in up to 4% of adults over 40 years and 14% in individuals over 65 years of age . Additionally, tics have been diagnosed in more than 20% of adolescents and school-age children . On the basis of these findings, increasing age is considered as a significant risk factor for the development of various movement disorders . , but some conditions appear almost exclusively during childhood, such as Tourette syndrome and Sydenham's chorea seen in rheumatic fever .
Under physiological conditions, a complex network involving the motor cortex, thalamus, the basal ganglia - substantia nigra, striatum, putamen, globus pallidus and the subthalamic nuclei, as well as several other structures are responsible for movement . At rest, signals from the primary motor cortex reach the striatum, followed by inhibition of the pallidum and consequently, the thalamus. When movement is planned, the substantia nigra and the subthalamic nucleus are stimulated by the activity of the striatum and their excitation will promote the activity of the motor cortex and enable the execution of movement . These actions are mediated through the release of glutamate, dopamine and GABA. The pathogenesis of virtually all movement disorders in some form interfere with the activity of these neurotransmitters in the basal ganglia and the motor cortex , whereas loss of neuronal circuitry in degenerative diseases such as Parkinson's and Huntington's disease directly reduce the capacity for signaling conduction. The role of cannabinoid receptors in motor activity has been extensively researched and there is growing evidence that their stimulation greatly influence the regulation of GABA, glumtate and dopamine uptake .
Under-recognition has been reported as a major problem when it comes to movement disorders . Studies have determined a prevalence rate of almost 30% in older adults . , suggesting that more needs to be done in terms of early recognition and diagnosis. The role of neurological examination needs to be emphasized in all levels of medical practice in order to identify these disorders while being in their milder stages, as therapy shows much better results compared to patients suffering from advanced disease.
Movement disorders are interpreted as neurologic syndromes characterized by either excess or paucity or both voluntary and involuntary movement that is unrelated to weakness, spasticity or paralysis . On the basis of this definition, they are categorized into two main groups:
Hypokinetic disorders are most commonly associated with Parkinson's disease, a neurodegenerative condition affecting about 1% of individuals between 65-70 years of age , while numerous conditions may induce hyperkinetic movements     . :
Movement disorders may also have a paraneoplastic or infectious origin (an autoimmune response following group A beta-hemolytic streptococcal infection will result in Sydenham's chorea) , implying that the differential diagnosis of movement disorders is broad and requires an extensive and detailed workup. The single most important part, however, is a meticulous neurological examination that can reveal the exact type of movement disorder and guide the physician toward the underlying cause. Several imaging studies, including computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET) using fludeoxyglucose (18F), or FDG-PET, single photon emission tomography (SPECT) are highly useful in assessing the exact site of brain lesions that are responsible for the appearance of symptoms  . Since the basal ganglia are the most important structures when it comes to movement, pathological findings in these areas will most likely be found. Treatment depends on the underlying cause, but involves several drug groups that either attempt to suppress the excitatory activity of dopaminergic pathways (such as haloperidol, perphenazine and fluphenazine), enhancing deficient GABAergic activity seen in myoclonus  or deplete the stores of catecholamies through use of tetrabenazine . Deep brain stimulation, particularly in patients suffering from Parkinson's disease, has shown marked success and is becoming more frequently used for many movement disorders . The prognosis depends on several factors, most important being the underlying etiology, severity of symptoms at the time of diagnosis and responsiveness to therapy.
The term movement disorder encompasses a wide range of movements that are considered as abnormal. Based on a broad classification, they may be placed into two categories: hyperkinetic (in which unwanted and excessive movements are seen) and hypokinetic (where a reduced level of activity is observed). Examples of hyperkinetic disorders include chorea, ballism, myoclonus, tics, tremor and many other, while hypokinesia, rigidity and bradykinesia are main forms of hypokinetic movement disorders. The list of diseases that can cause these symptoms is quite long, but the most important are Parkinson's disease, Huntington's disease and cerebrovascular disease (such as stroke or subarachnoid hemorrhage), while various autoimmune, infectious, iatrogenic (drug-induced) and paraneoplastic conditions have been described as well. Movement is carried out primarily through the activity of dopamine and glutamate, the the main excitatory, and GABA, the main inhibitory neurotransmitters in basal ganglia and other parts of the brain. Conditions that affect movement can cause brain lesions, impair the activity and levels of neurotransmitters, or cause progressive loss of neurons that carry these signals (which is the case in Parkinson's disease). For these reasons, physicians must determine the exact type of movement disorder during physical examination, the single most important part of the diagnostic workup. Since some diseases are transmitted by an autosomal dominant pattern of inheritance (meaning that parents are also experiencing the same symptoms), a detailed patient history may provide vital information. The vast majority of diseases originate in the nervous system, which makes imaging studies such as computed tomography (CT scan), magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT) and positron emission tomography (PET scan) highly useful in determining the exact site of the lesion and the underlying cause. Evaluation of muscle and nerve activity through electromyography and nerve conduction studies (EMG and NCS, respectively), as well as electroencephalography (EEG) can provide additional clues. When it comes to treatment, various drugs can be used and the choice depends on the type of movement disorder. The goal of therapy is to correct the levels of neurotransmitters, either by reducing or enhancing their activity, but the overall prognosis is poor. The majority of movement disorders are caused by progressive conditions that are most often incurable and over time, the quality of life becomes severely impaired due to the constant presence of movement disorders. Many studies have determined that these disorders are surprisingly common in the general population (almost 30% of older adults have shown to suffer from tremor or some other form), which is why the focus of prevention is turned to early identification and screening, primarily because treatment efficacy is much higher if patients are diagnosed in milder stage of disease.