Clinical Description
Action myoclonus – renal failure (AMRF) syndrome typically comprises a continuum of two major (and ultimately fatal) manifestations: progressive myoclonic epilepsy (PME) and renal failure (Table 2); however, in some instances, renal failure is not observed. Thus, progressive myoclonus epilepsy without renal failure caused by biallelic SCARB2 pathogenic variants is considered to be one end of the spectrum of AMRF [Badhwar et al 2004, Dibbens et al 2009, Rubboli et al 2011, Guerrero-López et al 2012, Zeigler et al 2014].
The age of onset varies, even within the same family.
Neurologic manifestations can appear before (in 1/3 of the cases), simultaneously, or after the renal manifestations. In juvenile AMRF onset is usually in the late teens or early twenties [
Andermann et al 1986,
Badhwar et al 2004].
In some persons renal manifestations occur early (late childhood or early teens) and neurologic involvement much later (late 20s or early 30s) [
Badhwar et al 2004,
Hopfner et al 2011].
In three persons of Japanese heritage who did not develop renal failure, neurologic manifestations appeared in the fifth or sixth decade [
Higashiyama et al 2013,
Fu et al 2014].
The neurologic and renal manifestations progress independently. Of note, the neurologic manifestations are not the result of a metabolic encephalopathy due to renal failure and are not improved by dialysis or by renal transplantation [Andermann et al 1986, Badhwar et al 2004].
Even in the same family, the number and order of appearance of the clinical manifestations can vary. Neurologic manifestations may occur first or in isolation in some family members and renal manifestations may be first or isolated in other family members [Badhwar et al 2004]. In some families, all affected individuals have neurologic manifestations and none develop renal failure [Dibbens et al 2009, Rubboli et al 2011, Guerrero-López et al 2012, Fu et al 2014, Perandones et al 2014, Zeigler et al 2014]; however, some affected family members have proteinuria [Dibbens et al 2009, Guerrero-López et al 2012] or reduced creatinine clearance [Zeigler et al 2014].
The disease progresses relentlessly with neurologic deterioration (especially increasing severity of myoclonus) and renal failure leading to death within seven to 15 years after onset.
Table 2.
Clinical Manifestations of AMRF
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Disease Onset | Major Manifestations | CNS Involvement | Renal Disease | Other Possible Manifestations |
---|
Juvenile
| PME and RF | Tremor Progressive action myoclonus Myoclonus at rest Ataxia, dysarthria Severe epileptic photosensitivity GTCS (rare at onset; relatively infrequent even in later stages)
|
|
|
PME | Tremor Progressive action myoclonus Myoclonus at rest Ataxia, dysarthria Severe epileptic photosensitivity GTCS (infrequent at onset)
|
|
|
RF | Neurologic manifestations as listed above may develop late in disease course. |
| |
Late | PME |
| None reported | |
DCM = dilated cardiomyopathy; GP = glomerulopathy; GTCS = generalized tonic-clonic seizures; PME = progressive myoclonus epilepsy; PNP = peripheral neuropathy; RF = renal failure; SNHL = sensorineural hearing loss; TP = tubulopathy
Neurologic Disease
Fine tremor. The disease begins with bilateral fine tremor of the fingers that is noted at rest and increased by delicate movement such as writing, by intention of movement, and by maintaining an attitude in horizontal extension [Andermann et al 1986, Badhwar et al 2004, Vadlamudi et al 2006]. The tremor can be relieved by alcohol [Andermann et al 1986, Andermann 2011, Guerrero-López et al 2012]. It becomes progressively worse until it is masked by myoclonic jerks [Badhwar et al 2004, Vadlamudi et al 2006].
Action myoclonus. The fine tremor is followed by jerking movements first of the upper and then of the lower extremities. Referred to as action myoclonus, these jerking movements are typically triggered by movements or intended movements. They are asynchronous and of variable severity.
With time, myoclonic jerks involve the proximal limbs; their amplitude and number increases by movements of the limbs, typically by walking down stairs. Action myoclonus can also involve the trunk. Attempts at speaking and executed speech can induce myoclonus of the bulbar musculature, contributing to the dysarthria. There is no palatal myoclonus.
Action myoclonus, which is also reflex-sensitive to touch over the distal extremities, can be exacerbated by anxiety, excitement, stress, and fatigue [Badhwar et al 2004, Zeigler et al 2014] and by auditory stimuli [Perandones et al 2012]. Some patients exhibit occasional myoclonus in response to startle [Badhwar et al 2004]. Of note, myoclonic jerks were significantly less frequent during pregnancy in one patient [Amrom et al 2017].
Action myoclonus represents the most disabling manifestation: it prevents affected individuals from being able to feed themselves and, thus, they become malnourished unless they receive assistance with feeding or are fed by artificial means. In the final stages, they may become bedridden or wheelchair bound. Swallowing difficulties can lead to aspiration pneumonia and death [Andermann et al 1986, Badhwar et al 2004, Vadlamudi et al 2006].
Myoclonus at rest. Subtle myoclonic movements of the eyelids, jaws, and perioral musculature appear at rest and while speaking. Ocular dysmetria can occur later in the disease course.
Clonic-tonic-clonic seizures, which can be diurnal or nocturnal, begin with generalized clonic jerking with preserved consciousness and proceed to unconsciousness with tonic-clonic features. They occur infrequently, starting with one per annum initially [Badhwar et al 2004]. TV viewing or other light stimulation may trigger generalized myoclonic seizures or tonic-clonic seizures [Rubboli et al 2011]. Photosensitivity can become so severe that affected individuals choose to live in almost complete darkness [Rubboli et al 2011].
Ataxia and dysarthria, common findings, can be distinguished from myoclonic jerks by the presence of the cerebellar abnormalities of pendular reflexes, abnormal rebound, and hypermetric ocular saccades.
Progressive myoclonus ataxia. Some patients develop significant progressive ataxia before or after the appearance of myoclonus and, thus, have been reported to have "progressive myoclonus ataxia."
A woman age 29 years had a history of clumsiness in the lower limbs, mild gait instability, and difficulties in riding a bicycle beginning at age 21 years, two years before the onset of progressive myoclonus at age 23 years. Renal failure was evident at age 25 years, and bilateral severe SNHL was diagnosed at age 27 years [Perandones et al 2012, Perandones et al 2014].
A woman age 22 years developed postural hand tremor exacerbated by fine voluntary movements and stress. The tremor slowly worsened with multifocal spontaneous and stimulus-sensitive myoclonic jerks. No other seizures were reported, and the EEG did not show any epileptic activity (of note, the patient was taking several anti-seizure medications). Ataxia was first reported when she was wheelchair bound at age 27 years. The co-occurrence of tremor, myoclonus, and ataxia in the same patient – without generalized tonic-clonic seizures – increases the complexity of the clinical picture of this disorder [Guerrero-López et al 2012].
Peripheral neuropathy. In some families, a sensorimotor peripheral neuropathy (most often predominantly demyelinating or more rarely axonal) may be present.
Some affected individuals may be diagnosed with a predominantly demyelinating peripheral neuropathy before the onset of renal failure [Badhwar et al 2004, Costello et al 2009, Dibbens et al 2011, Hopfner et al 2011]. In the German Family I reported by Badhwar et al [2004], one family member with AMRF who was asymptomatic for polyneuropathy was initially found to have a predominantly axonal neuropathy by nerve conduction studies; several years later, he and his two affected sibs were reported to have predominantly demyelinating polyneuropathy [Hopfner et al 2011].
Mild generalized muscle atrophy was observed in one individual who exhibited mild generalized reduced tone and no fasciculations [Zeigler et al 2014].
Renal Disease
Mild proteinuria may progress to nephrotic syndrome and ultimately to renal failure (Table 2).
Dialysis and renal transplantation can prolong survival, but do not improve the neurologic features.
Mental Status
Cognitive function. Unlike individuals with most other types of progressive myoclonus epilepsy, the majority of individuals with AMRF syndrome remain mentally alert. However, dementia has been documented in two unrelated individuals of Japanese ancestry, one with the juvenile-onset form, and the other with the late-onset form, who have different SCARB2 pathogenic variants [Fu et al 2014].
Psychological complications. Individuals with AMRF may exhibit somatic concerns or depressed mood, or may in exceptional cases commit suicide [Amrom et al 2017].
Other Findings
Cardiac disease. In a German family, echocardiography revealed dilated cardiomyopathy in two of three affected sibs at ages 14 and 21 years [Hopfner et al 2011]. In addition, these sibs had sensorimotor peripheral neuropathy.
Hearing loss / deafness. Frank or subclinical sensorineural hearing loss (SNHL) can be part of the spectrum of AMRF syndrome. Three individuals with AMRF had adult-onset SNHL, which was mild in one individual from one family [Rubboli et al 2011] and severe and asymmetric in an individual from a second family [Perandones et al 2012], in which a sister had preclinical hearing loss [Perandones et al 2014].
Thus in the same family, the SNHL can be severe or subclinical or absent [Badhwar et al 2004, Perandones et al 2012, Perandones et al 2014].
Common causes of death in AMRF. Sudden death may occur during or after a generalized epileptic seizure due to aspiration, severe myoclonus and unmanageable saliva, or an undetermined cause. Death can also occur due to aspiration pneumonia, renal failure, or rejection of a renal transplant [Andermann et al 1986, Badhwar et al 2004, Vadlamudi et al 2006, Rubboli et al 2011].
Late-Onset AMRF
Disease onset in the fifth or sixth decade has been reported in two Japanese families.
Higashiyama et al [2013] reported one family with two sibs with AMRF without renal failure. The sister presented with myoclonic jerks at age 43 years; her older brother presented with gait difficulties at age 52 years. Both were homozygous for the SCARB2 pathogenic variant c.1385_1390del6insATGCATGCACC.
Fu et al [2014] reported single affected individuals from two other Japanese families, one of whom (Patient 1) had late-onset disease. Patient 1 presented with onset of difficulties going up and down the stairs at age 45 years. Of note, he was homozygous for the same pathogenic variant as the two sibs reported by Higashiyama et al [2013]. Although the two families with the late-onset form are likely related as they originate from the same rural area, the precise relationship is unknown [Hiroshi Doi, 2014, personal communication; Hitoshi Takahashi, June 2014, personal communication]. Of note, Patient 2 reported by Fu et al [2014] presented at age 20 years, was unrelated to Patient 1, and was homozygous for a different SCARB2 pathogenic variant.
Specialized Studies
EEG findings [Andermann et al 1986, Badhwar et al 2004]. Background activity may be normal in some patients or show diffuse slowing at 6.5 to 7.5 Hz. Relatively low-voltage spike and spike-wave discharges, rather infrequent, bilaterally synchronous and generalized or confined to the central vertex or both occipital regions, increased by hyperventilation and intermittent photic stimulation, may be present.
Some of the brief spike potentials are difficult to distinguish from muscle potentials except that they are seen at the vertex where there is no muscle artifact. The electromyogram myoclonic potentials are sometimes associated with cerebral potentials and at other times occur independently, suggesting a subcortical origin with a secondary corticoreticular generalization.
Intermittent photic stimulation may produce whole-body myoclonus with multiple spikes in the EEG record associated with slow waves [Andermann et al 1986]. These generalized spike-polyspike-wave bursts can outlast the duration of light stimulation [Rubboli et al 2011].
Myoclonic seizures can be triggered by eye closure and resolve by eye opening [Rubboli et al 2011].
Overnight sleep recording can show fast spikes over the vertex spreading to bilateral frontocentral regions during rapid eye movement (REM) sleep.
Follow up over the course of the disease shows a preserved alpha background activity at disease onset, with rare generalized or focal epileptiform discharges. Over the years, irregular slower theta and delta waves progressively intermix with the alpha waves, and the epileptic activity becomes more frequent [Rubboli et al 2011].
MRI findings. Brain MRI may be normal or show mild diffuse cerebral and cerebellar atrophy [Andermann et al 1986, Badhwar et al 2004, Perandones et al 2012].
Electromyography findings. Nerve conduction analysis can show slowed nerve conduction velocities and prolonged F-waves, consistent with a mixed, mainly demyelinating polyneuropathy [Dibbens et al 2011, Hopfner et al 2011, Rubboli et al 2011].
In one patient, concentric needle electromyography (EMG) was suggestive of chronic anterior horn involvement [Zeigler et al 2014].
Brain histologic findings. A constant and pathognomonic finding is the presence of small and large autofluorescent pigment granules up to 10 µm in size in astrocytes and in certain cells in the meninges. The pigment granules are more prominent in laminae I and II of the cerebral cortex, the globus pallidus and putamen, and the Bergmann astrocytes in the cerebellar cortex; they are not seen in the thalamus, brain stem nuclei, dentate nuclei of the cerebellum, or spinal cord gray matter. The granules are both separate from as well as adjacent to glial cell nuclei, suggesting that at least some were within astrocytes [Andermann et al 1986, Badhwar et al 2004, Berkovic et al 2008].
Neurons contain normal amounts of lipofuscin and no pigment granules [Andermann et al 1986, Badhwar et al 2004]. In two affected individuals of Japanese ancestry extraneuronal brown pigment deposition, exclusively in astrocytic cytoplasm and surrounded by a membrane, was widely scattered throughout the brain [Fu et al 2014].
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