Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.68660
Source of Support: None, Conflict of Interest: None
Progressive myoclonic epilepsy (PME) is a disease complex and is characterized by the development of relentlessly progressive myoclonus, cognitive impairment, ataxia, and other neurologic deficits. It encompasses different diagnostic entities and the common causes include Lafora body disease, neuronal ceroid lipofuscinoses, Unverricht-Lundborg disease, myoclonic epilepsy with ragged-red fiber (MERRF) syndrome, sialidoses, dentato-rubro-pallidal atrophy, storage diseases, and some of the inborn errors of metabolism, among others. Recent advances in this area have clarified molecular genetic basis, biological basis, and natural history, and also provided a rational approach to the diagnosis. Most of the large studies related to PME are from south India from a single center, National Institute of Mental Health and Neurological Sciences (NIMHANS), Bangalore. However, there are a few case reports and small series about Lafora body disease, neuronal ceroid lipofuscinoses and MERRF from India. We review the clinical and research experience of a cohort of PME patients evaluated at NIMHANS over the last two decades, especially the phenotypic, electrophysiologic, pathologic, and genetic aspects.
Keywords: Lafora body disease, myoclonic epilepsy with ragged-red fiber, neuronal ceroid lipofuscinoses, progressive myoclonic epilepsy, Unverricht-Lundborg disease
Progressive myoclonic epilepsy (PME) is a syndrome complex characterized by progressive myoclonus, cognitive impairment, ataxia, and other neurologic deficits.  It encompasses several diagnostic entities and often causes diagnostic problems leading to nosological confusion. Over the last two decades, considerable developments have occurred in the field of PMEs and they have been recognized as a group of syndromes with specific etiologies. ,, Genetic tests had further enhanced the understanding of the disease process. The most important causes of PME include:Unverricht-Lundborg disease (ULD), myoclonic epilepsy with ragged-red fiber (MERRF) syndrome, Lafora body disease (LBD), neuronal ceroid lipofuscinoses (NCL), and sialidoses.  Further, recent advances have clarified the clinical features and facilitated a rational diagnostic approach. ,,,,,,, Understanding of the molecular genetics may help in determining the biological basis and also the natural history of these disorders.
There are a few case reports and case series on various PMEs from India. ,,,,, However, most of the larger studies are from a single center, National Institute of Mental Health and Neurological Sciences (NIMHANS), Bangalore, in south India. ,,, The research interest in one of the PMEs, LBD has started in early 1990s and the genetic analyses of patients with LBD have been carried out in collaboration with IIT, Kanpur. ,
A total of 147 patients with PME have been evaluated at NIMHANS, Bangalore, India, till date, they include: LBD: 54; NCL: 65; ULD: 08; MERRF: 10; and Tay-Sachs disease More Details (TSD): 10 [Table 1]. We have earlier described a cohort 97 histopathologically confirmed patients with PME [Table 2]. This cohort included 63 males and 34 females; mean age at onset of illness 10.7 ± 8.2 years (median: 11 years, range: 6 months to 48 years); and duration of illness at presentation 2.5 ± 2.2 years. History of consanguineous parentage was evident in 60 patients (61.8%). Most of the patients presented with the classical triad: myoclonus, cognitive decline, and neurologic deficits.
The characteristics of LBD include: generalized tonic-clonic seizures (GTCS), resting and action myoclonus, ataxia, dementia, polyspike and wave discharges in the electroencephalogram (EEG) basophilic cytoplasmic inclusion bodies in portions of brain, liver, and skin, as well as the duct cells of the sweat glands. The disease has autosomal recessive inheritance with the age of onset between 5 and 20 years. Most often patients with LBD present at 13 or 14 years of age with few exceptions.  Death usually occurs within 10 years of onset. Seizures, myoclonus, or learning disability may be the first symptom in the majority. ,, Myoclonus is said to be more often fragmentary, asymmetric, arrhythmic, and progressively disabling. ,,, Presence of optic atrophy and retinal degeneration has been documented but normal retina is usually noted. Ataxia is often missed because of severe myoclonus. The characteristic EEG pattern is slowing of background activity with recurrent epileptiform discharges: spikes/polyspikes, with or without slow waves.  LBD is caused by mutation in the PME 2 gene (EPM2A) on chromosome 6q and EPM2B gene. 
In our cohort, there was a slight male dominance (M:F» 24:14). History of consanguineous parentage was note in 73.7% of patients. The mean age at onset of illness was 14.4 ± 3.9 years (range: 10-35 years, median: 14 years) and the mean duration of illness was 2.8 ± 2.1 years. GTCS was the presenting symptom in 71% of patients. Myoclonus with or without generalized seizures and progressive cognitive decline were universally present in all the patients. Seizures are often refractory to antiepileptic medications. Occipital seizures with visualization of flashes of light (ictal phenomena) were reported in a third of the patients, while behavioral changes were evident in almost one-fourth of the patients. In our cohort, progression from a presymptomatic stage to only electrophysiologic abnormalities (EEG changes or giant somatosensory-evoked potential [SSEP] potential), and finally to a clinically obvious stage have been documented. 
Scalp EEG was done in 37 patients and the findings included varying degrees of slowing of background activity in 97.4% of patients [Figure 1]a. Generalized epileptiform discharges in 84.2% of patients, while focal discharges were present in ten patients. One of the patient had multifocal epileptiform activity. Photosensitivity with fast frequency stimulus was observed in 25% of patients, significantly less as compared to the western series..  Presymptomatic EEG abnormalities were detected in 3 families.  Progressive worsening in the background activity was observed in 4 patients. Interestingly, even presymptomatic individuals were found to have EEG changes, first time documented from our center. 
Giant SSEP (14-175 μV) was demonstrated in 24 of the 31 patients studied [Figure 1]b, while visual-evoked potential (VEP) studies revealed a prolonged P100 in four of the 31 patients studied and absent waveform in eight. Giant VEP potentials have also been documented for the first time from our center , [Figure 1]b. Brainstem auditory-evoked potential studies did not reveal any abnormality. Electrophysiologic features of neuropathy were present in only one patient of LBD. 
Computed tomography (CT) (n=32) and magnetic resonance imaging (MRI) (n=4) of brain revealed diffuse cortical atrophy with no obvious parenchymal changes similar to the observations in the earlier studies. However, MR spectroscopic abnormalities have been noted in patients with LBD with no structural MRI abnormalities: reduction in the N -acetylaspartate (NAA):creatine ratio and altered NAA:choline, and choline:creatine ratios in frontal cortex, cerebellum, and basal ganglia. ,
In two patients brain biopsy established the diagnosis by the presence of neuronal intracytoplasmic basophilic, round to oval bodies, which were periodic acid-Schiff (PAS)-positive and diastase-resistant [Figure 1]c. Brain biopsy is rarely performed now a days as the diagnosis can easily be established by axillary skin biopsy. The typical inclusions were described initially by Busard et al., , and later by Berkovic et al..  Similar was the experience at our center. The axillary skin biopsies (n=35) revealed characteristic oval to round PAS-positive, diastase-resistant Lafora body inclusions in the sweat glands [Figure 1]d. The Lafora bodies were positive for Lugol's iodine and ubiquitin immunostaining [Figure 1]d, inset. Histochemically, Lafora bodies are polyglucosan and its accumulation could be an error of carbohydrate metabolism. Busard et al. had demonstrated normal pyruvate metabolism in the body fluids and brain. ,
High degree of consanguineous parentage in south India might be responsible for high clustering of the disease in this region. The official name of this gene is "epilepsy, progressive myoclonus type 2A, Lafora disease (laforin)." EPM2A is the gene's official symbol. The EPM2A gene located at 6q24 provides instructions for making a protein called laforin and the initial paper related to it by Minassian et al., included cases from our center as well. Subsequently, EPM2B (malin) has been discovered. Although laforin protein is active in cells throughout the body, it appears to play a critical role in the survival of nerve cells (neurons) in the brain. Studies suggest that laforin has multiple functions within the cells. To carry out these functions, laforin interacts with several other proteins, including malin (which is produced from the NHLRC1 gene). These proteins are part of complex networks that transmit chemical signals and break down unneeded or abnormal proteins. Additionally, laforin may act as a tumor suppressor protein, which means that it keeps the cells from growing and dividing in an uncontrolled way. Laforin and malin probably play a critical role in regulating the production of a complex sugar called glycogen. Glycogen is a major source of stored energy in the body. The body stores this sugar in the liver and muscles, breaking it down when it is needed for fuel. Researchers believe that laforin and malin may prevent a potentially damaging buildup of glycogen in tissues that do not normally store this molecule, such as those of the nervous system. Recently, Rao et al. from National Brain Research Center (NBRC), New Delhi, had suggested that malin is unstable, and the aggregate-prone protein and co-chaperone CHIP can modulate its stability and therefore cause cell death.
Nearly 100 distinct mutations have been discovered in the 2 genes in over 200 independent LD families. Nearly half of them are missense mutations, and the deletion mutations account for one-quarter. Defects in at least 3 genes underlie LBD, of which 2 have been isolated and their mutations characterized: The EPM2A gene (MIM# 607566) encoding laforin ,,, and the NHLRC1 gene (MIM# 608072) encoding malin.  Laforin is a protein phosphatase, which is ubiquitinated by malin before degradation. ,,,, Aberrant functions of laforin and/or malin, which eventually affect the posttranslational modification of target proteins, are likely to underlie the onset and progression of LBD. , Almost all the work related to the genetics of LBD in India has been carried out by Ganesh's team at IIT, Kanpur, on patients from our center [Table 3].
NCL represents one of the common progressive neurodegenerative disorders during childhood. The term "NCL" was coined by Zeman et al.32 to distinguish the familial cerebromacular degeneration clinically and pathologically distinctive from gangliosidoses. The disease group has an autosomal recessive pattern of inheritance and is commonly characterized by progressive myoclonus with visual failure and accumulation of an autofluorescent lipopigment in the neurons and glial elements. , The first report of NCL from India was a patient with juvenile onset NCL by Gulati et al., subsequently we reported 11 patients initially  and later 40 patients. 
Our cohort included 28 males and 12 females; mean age at onset of symptom 5.9 ± 9.1years (median: 4 years); mean age at presentation 6.6 ± 5.5 years (median 5 years, range 6 months to 48 years), and mean duration of illness 2.5 ± 1.4 years (range 45 days to 7 years). Five patients (12.5%) had positive family history with autosomal recessive pattern of Mendelian inheritance. Consanguineous parentage was noted in 25 (62.5%) patients. Based on the age of onset of illness, patients could be categorized into four clinical forms: infantile NCL - 8 (mean age: 1.5 ± 0.5 years), late infantile NCL - 19 (mean age: 3.5 ± 0.5 years), juvenile NCL - 11 (mean age: 6.5 ± 1.8 years), and adult NCL - 2 (mean age: 44.0 ± 5.6 years). Juvenile NCL is the commonest in most reported series. ,, The two Kuf's adult variant cases in our series could be categorized into type "B" with predominant behavioral changes, dementia, ataxia, and rigidity.  The oldest patient of NCL reported in the literature was aged 63 years.  Generally the infantile and late infantile NCL have a rapid course and children usually die within 5-10 and 12 years, respectively. In juvenile NCL death occurs by 15-25 years, whereas the adult group has a variable course. ,,, The presenting clinical features in infantile NCL, late infantile NCL, and juvenile NCL group were regression of milestones, seizures, myoclonus, chorea, visual loss, and ataxia. Two patients with adult variant of NCL presented with abnormal behavior and extrapyramidal features. One of them in addition had pyramidal signs, ataxia, and dementia. ,
Visual impairment at onset was noted in ten patients (25%) and subsequently 26 (65%) of them developed visual abnormalities, which gives diagnostic clue in patients with early onset PME syndrome. Two patients of INCL had retinal degeneration and one had optic atrophy. Optic atrophy and retinal degeneration was evident in 11 and three patients of LINCL, respectively. The patients in juvenile group had ophthalmic abnormalities in the form of primary optic atrophy (n=5), macular degeneration (n=3) and retinitis pigmentosa (n=1). The adult forms did not have any ophthalmic abnormality. Zeman et al. (1970) have found pigmentary changes in 25, macular degeneration in 12, and optic atrophy in 5 patients giving clue to the diagnosis when associated with regression of milestones and myoclonus in pediatric patients. 
Electrophysiology and imaging
Scalp EEG (n=37) showed varying degrees of diffuse slowing of background activity in 94.6% and generalized epileptiform discharges in 81.1% of patients [Figure 1]a. Slow frequency photic stimulation-evoked response in five (22.7%) of the 22 patients studied [Figure 2]a. These observations were similar to the observations reported in the literature. ,, Giant SSEP was record in 7 of the 25 patients studied [Figure 2]b and evaluation of VEP study revealed a prolonged P100 in two, absent waveform in seven. Nerve conduction showed axonal neuropathy in three of the ten patients studied. CT (n=35)/MRI (n=3) scans revealed diffuse atrophy of variable degrees, similar to findings in the literature. ,
The cornerstone in the diagnosis of PMEs is histopathologic and ultrastructural examination of the various tissues, such as brain, skin, muscle, and liver.  During the initial periods, diagnosis was based on brain biopsy but it has been conclusively observed that extracerebral tissues, such as lymphocytes, axillary skin, muscle, rectal mucosa, and peripheral nerves may also reveal abnormalities on electron microscopy studies to establish the diagnosis. , Ultrastructural studies of skin biopsy have yielded consistent results. Histopathologic examination of brain biopsy (n=12) revealed that the brunt of the disease is in the gray matter. The cortical mantle was atrophic and showed neuronal depletion and reactive astrocytosis. The preserved neurons were normal in size and on H & E stains no identifiable inclusions could be seen. PAS stain for polysaccharide and Luxal fast blue stain for myelin revealed intense granular staining of the entire neuronal cytoplasm indicating the carbohydrate and lipid nature of the stored material, similar to lipofuscin pigment [Figure 2]c. Examination of unstained sections under fluorescent microscope showed yellow autofluorescence of the substance [Figure 2]d. Skin and muscle were normal on light microscopy. However, ultrastructural studies of brain (n=5) and skin (n=28) showed characteristic curvilinear inclusions [Figure 2]e in several cell types: neurons, occasional astrocytes, sweat glands, vascular endothelial and smooth muscle cells, fibroblasts, and occasional Schwann cells. In a single muscle biopsy, the inclusions were seen in the myofibers. Lamellar and electron dense bodies were infrequent. Clearly, the presence of curvilinear inclusions in multiple cell types makes the latter a feasible choice for diagnostic biopsy at our center. Inclusions are also present in the peripheral blood lymphocytes and their morphology has been found to correlate with the clinical course and genetic analysis based on which NCL is further categorized as (1) infantile NCL-granular bodies/GRODs, (2) late infantile NCL-curvilinear bodies, (3) juvenile NCL-finger print bodies, and (4) adult onset NCL with varied forms and combination of inclusions. ,, . Overlapping of the morphologic features of inclusions in different subtypes can pose a problem in the diagnosis by ultrastructural studies, although they may reflect evolution of cytologic pathology. ,,,
The details of genetics in NCL are mentioned in the [Table 4].  All the mutations known to cause NCL are listed in the NCL mutation database. More than 150 mutations are known. By 2008, at least 8 genes that cause NCL in children had been identified. They are the CLN10/CTSD, CLN1, CLN2, CLN3, CLN5, CLN6, CLN7, CLN8 genes and possibly also the CLCN6 gene. It is known as to what protein is encoded by each gene and for some it is known as to what function the protein is involved in, but for others this is not yet clear. The CLN10/CTSD gene causes the earliest onset of NCL, which affects babies at birth or even before they are born as well as milder cases that start in late infancy or even in the teenage years. The CLN1 gene causes infantile NCL, and also milder cases that start in late infancy, at a juvenile age, and even in adulthood. CLN2, CLN5, CLN6, CLN7, and CLN8 all cause NCL that almost always starts in late infancy. CLN2 is referred to as the classic late infantile gene and CLN5, CLN6, CLN7, and CLN8 are referred to as variant late infantile genes because the disease course is slightly different to that caused by CLN2. These 4 types of variant LINCL are virtually indistinguishable clinically so are sometimes called Finnish, Czech, or Turkish variant LINCL, respectively, because they were first described in families from these countries. CLN3 causes NCL that begins at a juvenile age. Recently 2 patients with late onset (teenage and adulthood) were described with single mutations in CLCN6. There are no genetic studies of NCL from India. ,,,,,,
Myoclonic epilepsy with ragged-red fibers (MERRF) is a disorder that affects many parts of the body, particularly muscles and the nervous system. In most cases, the signs and symptoms of this disorder appear during childhood or adolescence. The features of MERRF vary widely among affected individuals, even among members of the same family. MERRF is characterized by myoclonus, myopathy, and spasticity. The characteristic histological feature is feature abnormal muscle cells are called ragged-red fibers. Other features of MERRF include seizures, ataxia, peripheral neuropathy and dementia. These patients may also develop deafness and optic atrophy. Affected individuals sometimes have short stature and heart abnormalities, cardiomyopathy. Less commonly, people with MERRF develop lipomas. From India cases with classical phenotype have been described by Mehndiratta et al. and Sundaram et al.
The clinical features of MERRF in our cohort were similar to those reported in the literature: six males and four females and mean age at onset 14.6 ± 5.8 years (range: 8-26 years). One patient had a family history of similar illness and three patients had a history of consanguineous parentage. Eight patients had myoclonus and GTCS. Cognitive decline and ataxia were evident in seven patients. One patient had in addition, abnormal behavior. EEG was abnormal in six of the nine patients and demonstrated slowing of background activity with/without generalized epileptiform discharges. Electrodiagnostic studies showed neuropathy in one patient and myopathic and neurogenic in one each. CT/MRI scan showed diffuse cerebral atrophy. One patient had giant SSEP (27 μV). Histochemical stains of muscle biopsy showed subsarcolemmal accumulation of reaction product for nicotinamide adenine dinucleotide/succinate dehydrogenase (NADH/SDH) enzymes and classical ragged-red fibers by modified gomori trichrome (MGT) stain [Figure 3]a. Electron microscopic studies confirmed the aggregation of abnormal forms of mitochondria subsarcolemmally [Figure 3]b.
MERRF is inherited in a mitochondrial pattern, also known as maternal inheritance. This pattern of inheritance applies to genes contained in mtDNA. Mutations in the MT-TK, MT-TL1, MT-TH, and MT-TS1 genes cause MERRF. Mutations in the MT-TK gene are the most common cause of MERRF, occurring in more than 80% percent of the cases. Less frequently, mutations in the MT-TL1, MT-TH, and MT-TS1 genes have been reported to cause the signs and symptoms of MERRF. Individuals with mutations in these genes often have features of other mitochondrial disorders as well. These genes are contained in mitochondrial DNA (mtDNA). Mitochondria are structures within cells that use oxygen to convert the energy from food into a form that cells can use. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. The genes associated with MERRF provide instructions for making molecules called transfer RNAs, which are chemical cousins of DNA. These molecules help assemble protein building blocks called amino acids into full-length, functioning proteins within mitochondria. Mutations that cause MERRF impair the ability of mitochondria to make proteins, use oxygen, and produce energy. These mutations particularly affect organs and tissues with high energy requirements, such as the brain and muscles. Researchers have not determined how changes in mtDNA lead to the specific signs and symptoms of MERRF. They continue to investigate the effects of mitochondrial gene mutations in various tissues. ,,
ULD is a rare inherited form of epilepsy, more commonly reported from Scandinavian countries. Affected individuals usually begin showing signs and symptoms of the disorder between the ages of 6 and 15 years. The characteristic feature is myoclonus with increase in frequency and severity over time and stimulus sensitive. Within 5-10 years of onset of the disease the myoclonic jerk may become severe enough to interfere with walking and other activities daily living. GTCS is the other seizure type. After several years of progression, the frequency of seizures may stabilize or decrease. Eventually these patients develop ataxia, depression, and mild decline in intellectual functioning. Patients with ULD typically live into adulthood and the life expectancy may be normal.  From India there no reports of genetically proven cases of ILD. However, we have reported the probable cases.
Nine patients of probable ULD, five females and four females, were studied. The diagnosis of ULD was after detailed evaluation including skin and/or muscle biopsy. A history of consanguineous parentage was reported in four patients. Their mean age at onset was 13.8 ± 9.5 years (range: 5-32 years) and the mean duration of illness 4.1 ± 4.05 years. The presenting features were myoclonus and ataxia with no evidence of cognitive dysfunction. Neuroimaging revealed diffuse atrophy of cerebrum, brainstem, and cerebellum. EEG (n=9) showed generalized epileptiform discharges. Somatosensory evoked potential (SSEP) done in seven patients revealed giant potentials (32-60 μV) in four.
Mutations in the CSTB gene cause ULD. The CSTB gene provides instructions for making a protein called cystatin B. This protein reduces the activity of enzymes called cathepsins. Cathepsins help break down certain proteins in the lysosomes (compartments in the cell that digest and recycle materials). While the specific function of cystatin B is unclear, it may help protect the cells' proteins from cathepsins that leak out of the lysosomes. One region of the CSTB gene has a particular repeating sequence of 12 DNA building blocks (nucleotides). This sequence is normally repeated 2 or 3 times within the gene and is called a dodecamer repeat. Most people with this disorder have more than 30 repeats of the dodecamer sequence in both copies of the CSTB gene. The increased number of dodecamer repeats in the CSTB gene seems to interfere with the production of the cystatin B protein. Levels of cystatin B in affected individuals are only 5%-10% of normal, and cathepsin levels are significantly increased. These changes are believed to cause the signs and symptoms of ULD, but it is unclear how a reduction in the amount of cystatin B leads to the features of this disorder. ,
Most of the patients with PME have onset of the disease within the first two decades of life and have an autosomal recessive pattern of inheritance. A general approach to patients with PME syndrome is mentioned in [Figure 4]. Recent advances in the field of genetics of PME would differentiate these groups. This might lead to the development of drugs for the management of these fatal forms of disease in future.
We are grateful to Drs. A. Mahadevan, T, Yasha, and S.K. Shankar, Department of Neuropathology, NIMHANS, Bangalore, India, and Dr. S. Ganesh, IIT, Kanpur, for their association with the research work on PME at various stages. Not the least, we are very thankful to our patients who consented for the research work.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4]