Paroxysmal kinesigenic dyskinesia (PKD), also called paroxysmal kinesigenic choreoathetosis (PKC), is a rare hyperkinetic movement disorder of the paroxysmal dyskinesias characterized by attacks (paroxysms) of involuntary movements, which are triggered by sudden voluntary movements. The number of attacks can increase during puberty and decrease in a person's 20s to 30s. Involuntary movements can take many forms such as ballism, chorea or dystonia and usually only affect one side of the body or one limb in particular. There are two types of PKD, primary and secondary. Primary PKD can be further broken down into familial and sporadic. Familial PKD, which means the individual has a family history of the disorder, is more common, but sporadic cases are also seen. Secondary PKD can be caused by many other medical conditions such as multiple sclerosis (MS), stroke, pseudohypoparathyroidism, hypocalcemia, hypoglycemia, hyperglycemia, central nervous system trauma, or peripheral nervous system trauma. PKD has also been linked with infantile convulsions and choreoathetosis (ICCA) syndrome, in which patients have afebrile seizures during infancy (benign familial infantile epilepsy) and then develop paroxysmal choreoathetosis later in life. This phenomenon is actually quite common, with about 42% of individuals with PKD reporting a history of afebrile seizures as a child. PKD affects about 1 in 150,000 people. It accounts for 86.8% of all the types of paroxysmal dyskinesias and occurs more often in males than females.
Genetics Paroxysmal kinesigenic dyskinesias are often inherited in an autosomal dominant fashion and several genes have now been identified where mutations can cause this disease. The genes typically code for proteins known to be involved in synaptic transmission, ion channels or ion transporters. The first gene to be identified was the PRRT2 gene on chromosome 16, found in 2011 to be the cause of the disease in some patients. The mutations in this gene included a nonsense mutation identified in the genome of one family and an insertion mutation identified in the genome of another family. Researchers found PRRT2 mutations in 10 of 29 sporadic cases affected with PKD, thus suggests PRRT2 is the gene mutated in a subset of PKD and PKD is genetically heterogeneous. Later reports have identified the genes SCN8A, CHRNA4, and SLC16A2 as further causes of PKD.
Pathophysiology The pathophysiology of PKD is not fully explained. A few mechanisms have been suggested thus far:
GABA dysregulation Abnormal breakdown of dopamine in the basal ganglia Dysfunction of the substantia nigra A form of epilepsy Multiple methods are being used to study the potential brain abnormalities of individuals with PKD compared with “normal” individuals. These methods include SPECT studies, fMRI studies, and diffusion tensor imaging. The main problem with many of the studies concerned with the pathophysiology of the disorder is the small sample size. Because the studies normally only include about 7-10 patients with PKD, the results cannot be generalized to the entire population of patients. However, the studies do bring up possibilities for further study.
SPECT studies In a study by Joo et al., the researchers performed interictal studies, meaning they scanned the patient's brain between attacks to find an underlying abnormality, rather than ictal scans, which look at the abnormalities that present themselves during an attack. The researchers found interictally decreased cerebral blood flow in the posterior parts of the bilateral caudate nucleus. However, the literature does state that although this could be a cause of PKD, it could also be a result of PKD. Another SPECT study showed an increase in the cerebral blood flow in the left posterior thalamus in a PKD patient during an attack. The researchers also subtracted the ictal from the postictal scans, and saw increased blood flow in the thalamus. They ultimately suggested that hyperactive blood flow in this area could be causing the pathophysiology of PKD. This study, however, was only performed on one patient, and would need to be replicated many more times in order to be generalized to the population of PKD patients. Other SPECT studies have been cited showing hyperactivity in the basal ganglia.
fMRI studies In a study by Zhou et al., the researchers performed fMRI studies on PKD patients, and analyzed the differences between the amplitude low frequency fluctuations (ALFF) of the patients. They found that the left postcentral gyrus and the bilateral putamen had increased ALFF in PKD patients. The researchers concluded that the hyperactivity in these regions suggested that there is a dysfunction in the basal ganglia-thalamo-cortical circuit in PKD. This circuit is part of the motor control circuit in the brain, making it a reasonable place for abnormality in a movement disorder, but again, researchers are still unsure of the role these differences they found play in the disease pathology.
Diffusion tensor imaging Diffusion tensor imaging (DTI) displays physical alterations in the brain that may not be seen on regular MRI. In one study researchers found that some of the patients had abnormalities in their thalamus. However, this does not prove that all patients have abnormalities in their thalamus. Other cases are cited, including a patient who developed a similar paroxysmal dyskinesia after a thalamic infarction, implicating that an abnormality in the thalamus of individuals could contribute to PKD. It is not fully known, however, what role a thalamic abnormality plays in the disease pathophysiology.
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