Olivopontocerebellar atrophy (OPCA) is a neurodegenerative syndrome characterized by prominent cerebellar and extrapyramidal signs, dysarthria, and dysphagia. Those who study OPCA quickly learn that it is not a single entity, and that its nosology can be confusing. The umbrella term of OPCA includes common sporadic forms and uncommon genetic forms. In the genetic subgroup, all 3 major inheritance patterns (autosomal dominant, autosomal recessive, and X-linked) have been described. The classification scheme for autosomal dominant OPCA overlaps with that of autosomal dominant spinocerebellar atrophies (SCAs) and autosomal dominant cerebellar atrophies (ADCAs). In the sporadic type of OPCA, at least some of the cases are a subset of multiple system atrophy (MSA).
While the classification is seemingly convoluted, there is good reasoning behind the complexity. The study of neurodegenerative ataxias draws from the interplay between clinical observations, neuropathological analysis, and biochemistry and molecular genetics. Historically, however, one had to rely solely on the combination of clinical observation and neuropathology to describe the disorders. Because of this, the recognition of the OPCA as its own entity has evolved over time.
The first named ataxia to emerge as a clinical entity was not an OPCA, but Friedreich ataxia, which Nicolaus Friedreich (1825-1882) managed to separate from numerous other conditions, the most prominent being multiple sclerosis (then called disseminated sclerosis) and neurosyphilis. [1, 2] Thirty years later, Pierre Marie described another grouping of hereditary cerebellar ataxias.  Essentially, he proposed a classification to include all the non-Friedreich ataxia cases and suggested the name heredoataxia cerebelleuse. Some of these cases would now be termed OPCAs.
In 1907, Holmes described a family with a purely cerebellar form of ataxia, and the terms Holmes ataxia and ataxia of Holmes stuck to this category for decades. In 1922, Marie, Foix, and Alajouanine reported a similar family that probably had the same disease.  Thus, both Holmes ataxia and the ataxia of Marie, Foix, and Alajouanine (sometimes called Marie ataxia) are purely cerebellar ataxias. Neither would be considered a type of OPCA.
In 1900, Dejerine and Thomas identified cases that combined purely cerebellar problems with evidence of brainstem pathology.  They coined the term olivopontocerebellar atrophy. The neurologic community accepted this name and collected many cases under this rubric. Gradually, researchers realized that both sporadic and hereditary (mostly autosomal dominant) cases comprised this group, and that, broadly speaking, these cases all had similar neuropathologic features that are described later in this article. Menzel (1890) also had described a similar case. Throughout the years, both Dejerine-Thomas ataxia and Menzel ataxia have been used as terms for certain cases of either hereditary or sporadic OPCA.
Disputes in the clinic, on paper, and in conferences have occurred about the usage of these terms, such as fine distinctions between Menzel ataxia and ataxia of Dejerine-Thomas, but they are mainly now of historical interest only. OPCA type 1 (OPCA-I) is synonymous with SCA type 1 (SCA-1) and is sometimes referred to as Menzel-type ataxia. Dejerine-Thomas ataxia might be used for any of the 6 major phenotypic OPCAs, which are better defined below. However, the authors recommend against applying either of these terms to any new case of ataxia. These terms are mentioned here only so the reader may understand where they came from if they are encountered in other literature.
In 1954, Greenfield proposed a new clinicopathological classification of OPCA.  This was revised by Harding in 1982, based on anatomical, pathological, and biochemical features.  As applied to the purely autosomal dominant ataxias, the classification is as follows:
Type 1 ADCA (ADCA-1) - Ataxia and noncerebellar findings (eg, pyramidal or extrapyramidal dysfunction and ophthalmoplegia)
Type 2 ADCA (ADCA-2) - Similar to ADCA-1 but includes retinal degeneration
Type 3 ADCA (ADCA-3) - Includes relatively pure cerebellar dysfunction
In the ADCA grouping, the OPCAs are found in ADCA-1 and ADCA-2.
Harding was well aware that this was essentially a phenotypic grouping that lumped a number of different genetic diseases into 3 classes. However, the system was valuable for further genetic and other scientific work, in which Harding herself has been a significant contributor.
Working on a somewhat separate but related track, in 1970, Konigsmark and Weiner attempted to bring some order to the heterogeneity found among the OPCAs.  The proposed classification was based on clinical, genetic, and anatomic factors, as follows:
OPCA-I (Menzel-type OPCA) - Autosomal dominant
OPCA type 2 (OPCA-II or Fickler-Winkler type OPCA) - Autosomal dominant
OPCA type 3 (OPCA-III or OPCA with retinal degeneration) - Autosomal recessive
OPCA type 4 (OPCA-IV or Schut-Haymaker type OPCA) - Autosomal dominant
OPCA type 5 (OPCA-V or OPCA with dementia and extrapyramidal signs - Likely autosomal dominant
OPCA type X (OPCA-X) - X-linked OPCAs (added to classification at later date)
These are detailed in Table 1 in Causes.
In 1974, Skre studied the hereditary ataxia diseases in western Norway and chose to consider all these disorders as members of a comprehensive group of diseases termed spinocerebellar ataxias.  This classification then evolved in the classification of SCAs. According to Paulson and Ammache in 2001, SCAs include all well-understood types of dominant OPCA and many other dominant ataxias.  Geneticists sometimes state that the OPCA classification has been replaced by the SCA classification. This does not mean that every currently defined SCA is also an OPCA. The SCAs that could typically be considered to be an OPCA are SCA types 1, 2, 3, 7, and possibly 17.
In addition to these major forms, which might be called the traditional or classic OPCAs, some extremely rare diseases also involve degeneration of the same, or very similar, anatomical regions. These are mainly infantile or childhood diseases. They are not what neurologists (even pediatric neurologists) usually call OPCAs. However, occasionally in the literature they are called infantile OPCAs and thus they are included in Table 2 in Causes.
Table 3 in Causes lists a large number of the known SCAs (no table of such diseases is ever totally up-to-date for long), and those that can be reasonably identified as OPCAs are noted.
Finally, the sporadic OPCAs are considered. According to current knowledge, sporadic cases can be classified into the 3 following categories, which may be modified later based on further research findings:
Type 1 - A subtype; essentially the presentation of MSA
Type 2 - Sporadic cases that are not part of an MSA, as presently understood
Type 3 - De novo mutations that are actually genetic cases (but authorities do not realize they are genetic)
A separate but related question is whether the sporadic diseases are simply multigenetic, with the genetics being presently too complex to recognize as such.
A large percentage of the sporadic OPCAs are a subset of MSA. Some authorities have claimed that all sporadic OPCAs will progress to include significant autonomic and parkinsonian features and thus evolve into full-blown MSA if the patient lives long enough. According to this view, MSA typically starts as an ataxic OPCA form, an autonomic form (Shy-Drager syndrome), or a parkinsonian form (striatonigral degeneration). Motor neuron degeneration and dementia also eventually occur.
However, a large and careful study by Gilman et al published in 2000 showed that of the cases they selected for analysis, only 25% of the sporadic OPCAs converted to full-blown MSA within 5 years.  Nevertheless, all the sporadic OPCAs, Shy-Drager syndrome, striatonigral degeneration, and full-blown MSAs appear on the molecular level to be alpha-synucleinopathies; that is, they involve abnormalities of the protein alpha-synuclein. In addition, Jellinger reports in 2003 that the molecular pathology involves alpha-synuclein–positive glial (and less abundant neuronal) cytoplasmic inclusions in MSA and in all the purported subtypes.  These inclusions are also different from the alpha-synucleinopathic inclusions (eg, Lewey bodies), which are seen in other diseases.
The genetic OPCAs are all more pure in the sense that they do not evolve to an MSA picture. Many of the genetic forms are considered SCAs. Some genetic forms have additional characteristics such as retinal involvement, extrapyramidal degeneration, spinal cord degeneration, dystonia, dementia, and other neurological abnormalities dependent mainly on the genetic subtype but even showing variability within the same subtype. The genetic OPCAs are generally not alpha-synucleinopathies.
Clinical distinction of these entities is based on the dominant feature, which may be cerebellar ataxia (observed in OPCAs, SCAs, and MSA), parkinsonism (observed in MSA, striatonigral degeneration, and Shy-Drager syndrome), or autonomic failure (observed in MSA and Shy-Drager syndrome). Whatever the subtype, the term OPCA indicates a form of progressive ataxia distinguished by pontine flattening and cerebellar atrophy on brain imaging studies and at autopsy.
When faced with an adult having progressive ataxia suggestive of OPCA, the role of the clinician includes (1) excluding readily treatable alternative diagnoses, (2) discussing the value of genetic testing with patients in whom such testing is informative, (3) managing symptoms, and (4) advising the patient and family regarding the natural history and the need to plan for the future. No definitive therapy exists for OPCA. 
The OPCAs are progressive neurodegenerative conditions. Genetic and pathological evidence suggest that abnormalities of alpha-synuclein (αSYN) are important in the pathogenesis of these disorders. Many specific genes have been identified for the genetic forms, although how the genetic abnormalities lead to the specific αSYN abnormalities or to the specific clinical findings remains uncertain. Recently developed models using transgenic mice possessing the genes for human αSYN suggest that MSA involves dysfunction of the ubiquitin-proteasome system causing proteolytic stress that disrupts the oligodendroglial/myelintrophic support. Oligodendroglia in such models, as well as in pathological specimens of the human disease, have cytoplasmic inclusions of fibrillar αSYN.  There is also evidence for involvement of the ubiquitin-binding protein p62/sequestosome-1 in some cases of OPCA. 
On the gross level, brains show some common characteristics in all cases of OPCA. The pons is diminutive, especially in the area of the basis pontis. Degeneration of the cerebellum occurs, especially in the white matter. This white matter loss is probably due to the dying back of axons from degenerating neurons rather than a primary attack on the myelinated tracts. Loss of Purkinje cells is common. Major neuronal loss occurs in the inferior olivary, arcuate, and pontine nuclei. Dentate nuclei are well preserved. The middle cerebellar peduncles are also atrophic, possibly secondary to degeneration of the basal pontine gray matter. The substantia nigra of the midbrain shows evidence of tissue loss. Cellularly, one sees neuronal degeneration in the arcuate, pontine, inferior olivary, pontobulbar nuclei, and the cerebellar cortex.
Additional areas of degeneration probably account for the difference in subtypes. In sporadic OPCA, oligodendroglial and neuronal intracytoplasmic and intranuclear inclusions characteristic of MSA are frequently seen. Many of these are accumulations of alpha-synuclein. In autosomal dominant OPCA, spinal cord lesions, especially in the posterior columns, spinocerebellar tracts, and anterior gray horn cells, are more common. The cerebellar features may be less prominent. However, so many variations of both the sporadic and genetic forms are described that one can find cases that appear to be exceptions to these generalizations.
The prevalence of OPCA is 3-5 cases per 100,000 individuals; this may represent approximately 5-6% of patients diagnosed with atypical Parkinson disease.
The OPCAs are progressive neurodegenerative disorders that have no definitive treatment. Eventually, many patients become wheelchair bound. Severe dysarthria, anarthria, and dysphagia are not uncommon as the disease progresses.
Morbidity increases significantly, including falls and aspiration pneumonia.
Enteral feeding becomes necessary for many patients.
Death commonly results from aspiration pneumonia.
The duration of familial OPCA is approximately 15 years. The duration of sporadic OPCA is approximately 6 years.
No apparent racial preference is observed in OPCA. This is unlike Machado-Joseph disease, which has a predominance in certain Azorean, Indian, and Italian families.
A male preponderance is observed in familial cases of OPCA, with a male-to-female ratio of 2:1. However, no such distinction is seen in sporadic cases.
The mean age of onset of sporadic OPCA is 53 years. The mean age of onset of familial OPCA is 28 years (excluding the infantile forms in Table 2 in Causes).
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