Fragile X-associated tremor/ataxia syndrome
Fragile X-associated tremor/ataxia syndrome (FXTAS) is a progressive degenerative movement disorder resulting from a fragile X “premutation”, defined as 55-200 CGG repeats in the 5’-untranslated region of the FMR1 gene 1. The premutation can expand in subsequent generations (during oogenesis) to a full mutation causing fragile X syndrome.
The FMR1 premutation occurs in approximately 1/800 males and 1/250 females 1. Penetrance of FXTAS is age-dependent, affecting 40-45% of male and 8-16% of female premutation carriers (PMC) over the age of 50 years2.
Typically, therefore, the affected individual will be an elderly male. If they have had a daughter, then his grandchild may have a known diagnosis of fragile X, as during oogenesis the mutation will have undergone mutation, allowing for the full phenotype of fragile X to be expressed 7.
The classical presentation is a kinetic tremor and cerebellar ataxia occurring over the age of 50 1. Other clinical manifestations include cognitive decline, dementia, psychiatric disorders, peripheral neuropathy and autonomic dysfunction 2. Affected females typically experience less severe disease.
The pathogenesis of FXTAS is not yet fully understood 2. Neuronal toxicity in FXTAS is thought to result from the formation of pathognomonic eosinophilic and ubiquitin-positive intranuclear inclusions in neurones and astrocytes in the cerebrum, thalamus, basal ganglia and inferior olivary, dentate and cranial nerve XII nuclei, as well as in the spinal cord and autonomic ganglia.
Neurodegeneration in the cerebellum is characterised by marked Purkinje cell loss, gliosis and axonal swelling. Furthermore, elevated FMR1 mRNA levels seen in premutation carriers have been implicated by way of a neurotoxic gain of function effect.
The adrenals, thyroid, Leydig cells, pancreas, gastrointestinal tract, kidneys and heart are also involved.
FXTAS diagnostic criteria
Diagnostic criteria have been developed that rely on clinical and radiological features 3. The combination allows patients to be grouped into one of three diagnostic certainty categories:
- definite: one major clinical and one major radiological; or one major clinical and intranuclear inclusions
- probable: two major clinical; or one minor clinical and one major radiological
- possible: one major clinical and one minor radiological
- intention tremor
- gait ataxia
- moderate to severe short-term memory deficiency
- executive function deficit
- MRI white matter lesions in middle cerebellar peduncle
- MRI white matter lesions in splenium of the corpus callosum
- MRI white matter lesions in cerebrum
- moderate to severe generalised atrophy
The revised FXTAS diagnostic criteria include two major radiological features 3.
The best recognised feature of FXTAS is the MCP sign, which refers to T2 hyperintensity in the middle cerebellar peduncles, and is present in 60% of affected males and 13% of affected females 4. The second major radiological feature is T2 hyperintensity in the splenium of the corpus callosum, which is found as frequently as the MCP sign and also occurs more commonly in males 2.
Other imaging features:
- white matter T2 hyperintensities in the pons, insula and periventricular region 2
- T2 hyperintensity in the afferent projections of the middle and superior cerebellar peduncles has been reported in asymptomatic premutation carriers and may be the earliest neuroanatomical marker heralding the onset of FXTAS 3
- generalised brain and cerebellar atrophy
- can also occur in asymptomatic premutation carriers
- brainstem volume has been found to be significantly smaller in asymptomatic premutation carriers than in healthy controls 4
diffusion tensor imaging (DTI)
- marked reduction of fractional anisotropy in the middle and superior cerebellar peduncles, cerebral peduncles, fornix and stria terminalis in male premutation carriers with FXTAS
- axial and radial diffusivity values in the middle cerebellar peduncle and cerebral peduncle have been shown to be increased in asymptomatic premutation carriers, without alteration in fractional anisotropy 5
Treatment and prognosis
The rate of progression of FXTAS is variable, and life expectancy ranges from 5 to 25 years following the onset of symptoms 2. The presence of the MCP sign in patients with FXTAS is correlated with more severe cognitive deficits and a longer history of symptoms 2. Currently, there are no disease-modifying treatments for FXTAS.
For the MCP sign consider 6:
- multiple system atrophy of cerebellar type (MSA-C)
- recessive ataxia
- acquired hepatocerebral degeneration
For T2 hyperintensity in the splenium of the corpus callosum consider:
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- 2. Muzar Z, Lozano R. Current research, diagnosis, and treatment of fragile X-associated tremor/ataxia syndrome. Intractable Rare Dis Res. 2014;3 (4): 101-9. doi:10.5582/irdr.2014.01029 - Free text at pubmed - Pubmed citation
- 3. Hall DA, Birch RC, Anheim M et-al. Emerging topics in FXTAS. J Neurodev Disord. 2014;6 (1): 31. doi:10.1186/1866-1955-6-31 - Free text at pubmed - Pubmed citation
- 4.Leehey MA. Fragile X-associated tremor/ataxia syndrome: clinical phenotype, diagnosis, and treatment. J. Investig. Med. 2011;57 (8): 830-6. Free text at pubmed - Pubmed citation
- 5. Hashimoto R, Srivastava S, Tassone F et-al. Diffusion tensor imaging in male premutation carriers of the fragile X mental retardation gene. Mov. Disord. 2011;26 (7): 1329-36. doi:10.1002/mds.23646 - Free text at pubmed - Pubmed citation
- 6. Brunberg JA, Jacquemont S, Hagerman RJ et-al. Fragile X premutation carriers: characteristic MR imaging findings of adult male patients with progressive cerebellar and cognitive dysfunction. AJNR Am J Neuroradiol. 2003;23 (10): 1757-66. Pubmed citation
- 7. Robbins and Cotran Pathologic Basis of Disease Professional Edition, 9e (Robbins Pathology). Saunders. ISBN:0323266169. Read it at Google Books - Find it at Amazon