All known types of prion disease are rare; in fact, most people don’t even know that prion diseases other than Creutzfeldt-Jakob (CJD) exist. Out of this rare group of diseases, CJD is seen most often. But while the other forms of prion disease are extremely unusual, they have a dramatic impact on the lives of the afflicted.
Kuru, the first prion disease to be discovered in humans, was endemic in Papua New Guinea among tribes who engaged in ritual cannibalism. The prions were likely transmitted by eating the brains of infected people, but when these practices were stopped, the rate of new cases of kuru fell dramatically.
The name kuru means “shivering,” as the disease causes tremors. Kuru also causes clumsiness (ataxia) and difficulty walking, and eventually the patient develops involuntary movements like fasciculations and chorea. Dementia appears in later stages of the disease, and death usually results from pneumonia within 9 to 24 months of initial symptoms.
There aren’t many tests that are helpful for diagnosing kuru. Upon autopsy, the brain will show a build-up of protein, especially in the cerebellum.
Gerstmann-Sträussler-Scheinker syndrome (GSS) is a very rare prion disease. About 1 to 10 new cases per 100 million people occur every year. GSS is inherited and spreads in an autosomal dominant fashion, meaning that if someone has the disease, at least one of his or her parents probably has it as well.
Symptoms of GSS typically start in middle age. Early signs can include ataxia, sensory changes, and leg weakness. Dementia may or may not occur, depending on the affected family and on the individual. Symptoms usually worsen for about five years before death.
Like kuru, the diagnosis of GSS is made through a patient's history and physical exam. Because the disease is genetic, it may be possible to demonstrate the genetic mutation. The findings upon autopsy are similar to those with kuru.
Fatal Familial Insomnia
Fatal familial insomnia (FFI) is one of the more rapidly-progressing prion diseases. The entire disease, which usually affects people in midlife, usually only lasts about 13 months—from the first symptoms until death.
As the name suggests, FFI disrupts sleep, leading to a blurring of normal sleep patterns. People may exist in a kind of dream-like state while awake, with confusion, poor concentration, and hallucinations. Eventually, problems with movement, such as ataxia, develop. FFI is unique among prion diseases in its ability to cause dysautonomia, resulting in changes in heart rate and body temperature.
FFI has pathological findings that are different from many other prion diseases. Spongiform degeneration is less prominent, and the worst degeneration is in the thalamus.
Other Forms of Prion Disease
Proteinase-sensitive prionopathy is a very rare form of prion disease first described in 2008. It differs from CJD in that there are no distinguishing MRI or electroencephalography (EEG) features, but like CJD, it causes a rapidly progressive dementia. Fewer than 30 cases have been documented. Other cases of unique prion diseases have also been documented, but have not been found to exist in more than a few people each.
One of the more frightening notions about prion diseases is that they may not be as rare as we believe. There’s a growing theory that many neurodegenerative diseases, like Parkinson’s disease and Alzheimer’s, are caused by mutated proteins that cause other proteins to also deform in a prion-like fashion. In this case, the so-called “rare” prion disorders may come to be recognized as everyday and commonplace. A more disturbing notion is that, because prion diseases take a long time to incubate before they become symptomatic, many more of us may be carrying these abnormal proteins than are commonly realized.
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