Paroxysmal movement disorder of the Soft Coated Wheaten Terrier, phenotypically close to human paroxysmal non-kinesigenic dyskinesia (PNKD). The episodes combine hyperkinesia and dystonia lasting from minutes to several hours, with preserved consciousness, not induced by intense exercise and sometimes triggered by excitement. It is inherited in an autosomal recessive manner and is associated with a missense mutation in the PIGN gene, involved in GPI anchor biosynthesis.

Affected breed: Soft Coated Wheaten Terrier. No reliable large-scale carrier frequencies are published for the general population; the disease is rare but known in the breed.

- Episodes of hyperkinesia and dystonia lasting from several minutes to hours
- More than 10 episodes a day in some dogs
- Preserved consciousness during the episode
- No relation to intense exercise or fasting
- Triggering by excitement in some cases
- Onset in young adults

PxD of the Soft Coated Wheaten Terrier was characterised as a distinct clinical entity among canine paroxysmal dyskinesias, with a phenotype in young adults that segregates in a recessive manner. In 2016, Kolicheski and co-workers compared whole genomes of two affected dogs with 100 controls and found a unique homozygous missense variant, PIGN:c.398C>T (p.Thr133Ile). Functional assays in PIGN-deficient HEK293 cells showed a partial reduction of CD59 antigen on the surface. The genetic test is now offered for breed management.

- Test breeding animals with the PIGN test before mating
- Do not mate two carriers: 25% risk of affected homozygotes
- A carrier may be mated to a clear animal; offspring intended for breeding must be tested
- Exclude affected homozygous animals from breeding
- Consider acetazolamide as a clinical management tool, without it replacing genetic prevention

Differential diagnosis with other canine paroxysmal dyskinesias (PED of the Sheltie, episodic falling of the CKCS due to BCAN, focal epilepsy) and with toxic/metabolic myoclonus. Video of the episode is key to classifying the phenotype. Partial response to acetazolamide has been described in some cases. In humans, PIGN mutations cause MCAHS1 syndrome, suggesting comparable pathogenic pathways.

1. Kolicheski AL et al. (2017). A homozygous PIGN missense mutation in Soft-Coated Wheaten Terriers with a canine paroxysmal dyskinesia. Neurogenetics 18(1):39-47. PMID: 27891564
2. OMIA:002084-9615. Dyskinesia, paroxysmal, PIGN-related in Canis lupus familiaris (dog).