Progressive Myoclonic Epilepsies (PME) are a rare group of inherited neurodegenerative diseases characterized by

myoclonus Myoclonus is a brief, involuntary, irregular (lacking rhythm) twitching of a muscle or a group of muscles, different from clonus, which is rhythmic or regular. Myoclonus (myo "muscle", clonic "jerk") describes a medical sign and, generally, ...

, resistance to treatment, and neurological deterioration. The cause of PME depends largely on the type of PME. Most PMEs are caused by

autosomal dominant In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...

or recessive and mitochondrial mutations. The location of the mutation also affects the inheritance and treatment of PME. Diagnosing PME is difficult due to their

genetic heterogeneity Genetic heterogeneity occurs through the production of single or similar phenotypes through different genetic mechanisms. There are two types of genetic heterogeneity: allelic heterogeneity, which occurs when a similar phenotype is produced by diff ...

and the lack of a genetic mutation identified in some patients. The prognosis depends largely on the worsening symptoms and failure to respond to treatment. There is no current cure for PME and treatment focuses on managing myoclonus and

seizure An epileptic seizure, informally known as a seizure, is a period of symptoms due to abnormally excessive or synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much of the body with lo ...

s through antiepileptic medication (AED). The age of onset depends on the specific PME but PME can affect people of all ages. In Unverricht-Lundborg disease (ULD) the age of onset is between 6–15 years, while in Adult Neuronal ceroid lipofuscinoses (Adult NCL) the age of onset can be as late as 30. Symptoms often include action or stimuli induced

neuropathy Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or o ...

, cognitive decline, and spike and wave or no cerebral discharges. The prognosis of those diagnosed with PME is poor. The person often becomes reliant on a wheelchair, enters a vegetative state due to myoclonus, and has a shortened life expectancy.

Signs and symptoms

The most common symptom of PME is myoclonus. The myoclonus can be fragmented or multifocal and can be triggered by posture, actions, and external stimuli such as light, sound, and touch. The type of myoclonus differs between the types of PME. Other symptoms of PME include generalized, tonic clonic, tonic, and atypical absence seizures. In Lafora's disease the seizures are

occipital The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the c ...

and the person experiences transient blindness as well as visual

hallucination A hallucination is a perception in the absence of an external stimulus that has the qualities of a real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. Hallucination is a combinati ...

s. The person may also have atypical absences and atonic and complex partial seizures. In Myoclonus epilepsy with ragged-red fibers ( MERRF) the person experiences generalized epilepsy along with myoclonus, weakness, and

dementia Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...

. As PME progresses neurological ability decreases and can lead to

myopathy In medicine, myopathy is a disease of the muscle in which the muscle fibers do not function properly. This results in muscular weakness. ''Myopathy'' means muscle disease (Greek : myo- ''muscle'' + patheia '' -pathy'' : ''suffering''). This m ...

, cognitive decline, cerebellar ataxia, and

. The different symptoms in each of the PME and between individuals makes diagnosis difficult. Therefore, diagnosis of PME is dependent on failure to respond to antiepileptic drugs and therapy but diagnosis of specific PME depends on genetic testing, EEG (electroencephalography),

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...

measurements and more.

Diagnosis

Diagnosis of PME is based on the individual's signs and symptoms as well as failure to respond to antiepileptic drugs and therapy. Further diagnosis support includes EEG results, genetic testing, enzyme testing, and skin and muscle biopsies. Gaucher's disease can be diagnosed through enzyme testing as it is a metabolic disease. Lafora's disease can be diagnosed using skin biopsies. While Action myoclonus renal failure (AMRF) syndrome can only be diagnosed using genetic test. Using EEG's as a form of diagnosis can prove difficult as patients differ in their neurophysiology. In Lafora's disease EEGs can show slowing background activity or focal discharges as well as epileptiform discharges. In ULD EEGs show generalized epileptiform discharges and in MERRF patients show background slowing. Therefore, diagnosis is best made using a combination of different tools like signs and symptoms, age of onset, EEG, gene testing, enzyme measurements, and

biopsy A biopsy is a medical test commonly performed by a surgeon, interventional radiologist, or an interventional cardiologist. The process involves extraction of sample cells or tissues for examination to determine the presence or extent of a disea ...

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different d ...

and

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...

Differential diagnosis

The main component setting PME apart from other forms of

epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...

is progressive deterioration and resistance to treatment. Therefore, in the early stages of PME the symptoms and EEG may appear like Generalized epilepsy, Juvenile myoclonic epilepsy, benign childhood myoclonic epilepsy, and

Huntington's disease Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...

. It is crucial for ensure initial treatment is appropriate to measure how the condition progresses. Incorrect treatment can also result in wrong PME diagnosis.

Management

The is no cure for PME. Efforts are instead placed in managing the symptoms, specifically the

and

s as these can cause major harm to the individual. However, treating the symptoms with antiepileptic drugs can be difficult because PME individuals can become resistant. Some antiepileptic drugs used in treatment are

valproic acid Valproate (VPA) and its valproic acid, sodium valproate, and valproate semisodium forms are medications primarily used to treat epilepsy and bipolar disorder and prevent migraine headaches. They are useful for the prevention of seizures in those ...

benzodiazepine Benzodiazepines (BZD, BDZ, BZs), sometimes called "benzos", are a class of depressant drugs whose core chemical structure is the fusion of a benzene ring and a diazepine ring. They are prescribed to treat conditions such as anxiety disorders, ...

s, phehobarbital,

piracetam Piracetam is a drug marketed as a treatment for myoclonus. It is also used as a cognitive enhancer to improve memory, attention, and learning. Evidence to support its use is unclear, with some studies showing modest benefits in specific popula ...

, zonisamide,

clonazepam Clonazepam, sold under the brand names Klonopin and Rivotril, is a medication used to prevent and treat seizures, panic disorder, anxiety disorders, and the movement disorder known as akathisia. It is a tranquilizer of the benzodiazepine ...

, and

levetiracetam Levetiracetam, sold under the brand name Keppra among others, is a medication used to treat epilepsy. It is used for partial-onset, myoclonic, or tonic–clonic seizures and is taken either by mouth as an immediate or extended release form ...

. It is important to note that some antiepileptic drugs can worsen the symptoms, like

vigabatrin Vigabatrin, brand name Sabril, is a medication used to treat epilepsy. It became available as a generic medication in 2019. It works by inhibiting the breakdown of γ-aminobutyric acid (GABA). It is also known as γ-vinyl-GABA, and is a struc ...

, carbamazepine, phenytoin, and

gabapentin Gabapentin, sold under the brand name Neurontin among others, is an anticonvulsant medication primarily used to treat partial seizures and neuropathic pain. It is a first-line medication for the treatment of neuropathic pain caused by diabeti ...

Clonazepam Clonazepam, sold under the brand names Klonopin and Rivotril, is a medication used to prevent and treat seizures, panic disorder, anxiety disorders, and the movement disorder known as akathisia. It is a tranquilizer of the benzodiazepine ...

is currently the only drug approved by the

FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food s ...

for

monotherapy Combination therapy or polytherapy is therapy that uses more than one medication or modality. Typically, the term refers to using multiple therapies to treat a ''single'' disease, and often all the therapies are pharmaceutical (although it can al ...

treatment of myoclonic seizures. Other treatments that have been used in PME patients are

deep brain stimulation Deep brain stimulation (DBS) is a neurosurgical procedure involving the placement of a medical device called a neurostimulator, which sends electrical impulses, through implanted electrodes, to specific targets in the brain (the brain nucleu ...

vagus nerve stimulation Vagus nerve stimulation (VNS) is a medical treatment that involves delivering electrical impulses to the vagus nerve. It is used as an add-on treatment for certain types of intractable epilepsy and treatment-resistant depression. Medical use VN ...

, and diet but they have not been shown to improve seizures.

Prognosis

The prognosis of PME is ultimately dependent on the type of PME. In Lafora body disease the neurological deterioration progresses until resulting in a vegetative state and death within 10 years of diagnosis. Due to research and advances in antiepileptic medication, individuals with ULD can live up to 60 years of age. Nevertheless, severe myoclonus can lead to injury by falling and becoming reliant on a wheelchair.

Research

Because PME is so rare it is hard to do studies specifically double blind studies used to test different antiepileptic drugs. The wide range of symptoms including the differing EEG makes studying the effects of the AEDs difficult. In ULD,

oligonucleotide Oligonucleotides are short DNA or RNA molecules, oligomers, that have a wide range of applications in genetic testing, research, and forensics. Commonly made in the laboratory by solid-phase chemical synthesis, these small bits of nucleic acids ...

therapeutic strategies have been used to replace gene effects while in Sialidosis

replacement therapy has been studied in mouse models. In Lafora's disease

metformin Metformin, sold under the brand name Glucophage, among others, is the main first-line medication for the treatment of type 2 diabetes, particularly in people who are overweight. It is also used in the treatment of polycystic ovary syndrome. ...

has been approved for treatment by the

European commission The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body ...

. In MERRF bacterial proteins have been identified in treatment in

mitochondrial disease Mitochondrial disease is a group of disorders caused by mitochondrial dysfunction. Mitochondria are the organelles that generate energy for the cell and are found in every cell of the human body except red blood cells. They convert the energy of ...

s but further studies are needed.

History

The first instance where myoclonus and its relationship to epilepsy was in 1822 by Prichard. Lundborg was the first to name progressive myoclonus epilepsy in 1903 due to his study of several Swedish families as well as research done by Heinrich Unverricht in 1891. However, ULD was not recognized as a disease until a century later due to the rarity of the disease. In 1911, Lafora identified Lafora bodies but believed to be part of ULD. Lafora's genetics was not described until 1995.

Specific disorders

Several conditions can cause progressive myoclonic epilepsy. * Unverricht-Lundborg disease (Baltic myoclonus) * Myoclonus epilepsy and ragged red fibres (MERRF syndrome) *

Lafora disease Lafora disease is a rare, adult-onset and autosomal recessive genetic disorder which results in myoclonus epilepsy and usually results in death several years after the onset of symptoms. The disease is characterized by the accumulation of inclu ...

* Neuronal ceroid lipofuscinoses * Sialidosis * Dentatorubropallidoluysian atrophy (DRPLA) * Noninfantile neuronopathic form of Gaucher disease * Tetrahydrobiopterin deficiencies * Alpers disease * Juvenile Huntington disease * Niemann-Pick disease type C * North Sea progressive myoclonus epilepsy (NSPME)

Unverricht-Lundborg disease

This disease manifests between six and sixteen years and is most prevalent in Scandinavia and the Baltic countries. Myoclonus gradually becomes worse and less susceptible to medication. Cognitive decline is slow and sometimes mild. Patients typically do not live beyond middle-age, but there are exceptions.

Phenytoin Phenytoin (PHT), sold under the brand name Dilantin among others, is an anti-seizure medication. It is useful for the prevention of tonic-clonic seizures (also known as grand mal seizures) and focal seizures, but not absence seizures. The intr ...

, an old and commonly used anticonvulsant, is known to seriously exacerbate the condition. It has autosomal recessive inheritance, and is caused by a mutation in the cystatin B (EPM1) gene on chromosome 21q22.3, which was discovered in 1996. It has been described as the least severe type of PME.

Myoclonus epilepsy and ragged red fibres (MERRF syndrome)

Onset of this disease may be at any time and the severity and progression are varied. Tonic-clonic seizures and dementia are less apparent than with other forms of PME. The cause is a mitochondrial DNA mutation, so most familial cases are transmitted from the mother. A skeletal muscle biopsy will show ragged red fibres, hence the name.

Lafora body disease

This disease typically begins between six and nineteen years after apparently normal development and generally results in death within ten years. It is characterised by the presence of Lafora bodies (polyglucosan inclusions) in neurons and other body tissue. The generalized seizures are usually well controlled by anticonvulsants, but the myoclonus soon proves refractory to treatment. Within a couple of years, a wheelchair is required for locomotion and within five to ten years, the person is confined to bed and is often tube fed.

Valproic acid Valproate (VPA) and its valproic acid, sodium valproate, and valproate semisodium forms are medications primarily used to treat epilepsy and bipolar disorder and prevent migraine headaches. They are useful for the prevention of seizures in those ...

and zonisamide are first choice anticonvulsants, and the

ketogenic diet The ketogenic diet is a high-fat, adequate-protein, low-carbohydrate dietary therapy that in conventional medicine is used mainly to treat hard-to-control (refractory) epilepsy in children. The diet forces the body to burn fats rather than ...

may be helpful. An autosomal-recessive genetic defect is responsible, which has been tracked down to two genes. The EPM2A gene on chromosome 6q24 was discovered in 1998 and encodes for the protein laforin. It is responsible for 80% of cases. The EPM2B gene on chromosome 6p22.3 was discovered in 2003 and encodes for the protein malin. There may be a third gene of unknown locus.

Neuronal ceroid lipofuscinoses

There are various forms of these disorders, each with their own genetic cause and geographical variation, which lead to accumulation of lipopigments (

lipofuscin Lipofuscin is the name given to fine yellow-brown pigment granules composed of lipid-containing residues of lysosomal digestion. It is considered to be one of the aging or "wear-and-tear" pigments, found in the liver, kidney, heart muscle, r ...

) in the body's tissues and are inherited in an autosomal-recessive fashion. Onset and symptoms vary with the particular form, but death usually occurs within five to fifteen years.

Type I sialidosis

This is an autosomal recessive disorder in which the body is deficient in α-

neuraminidase Exo-α-sialidase (EC 3.2.1.18, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: : Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glyc ...

Myoclonic epilepsy and ataxia due to potassium (K+) channel mutation (MEAK)

MEAK is a form of progressive myoclonus epilepsy that typically begins between the ages of 3 and 15 years (the average of onset is 10 years). The first symptoms may include ataxia and myoclonus (unsteadiness and difficulty coordinating movements), along with generalized tonic-clonic ("grand mal") seizures. Individuals with MEAK typically do not experience developmental delays. The symptoms are progressive, and individuals with MEAK often need to use a wheelchair by their late teenage years because of movement difficulties and myoclonus. Many individuals with MEAK report temporary improvement of symptoms when they have a high fever. Seizures may become less frequent in adulthood, but other neurological complications, including myoclonus, ataxia and tremor, may worsen. Myoclonic epilepsy and ataxia due to potassium (K+) channel mutation (MEAK) is caused by a specific pathogenic variant ("mutation") in KCNC1 (G>A; p.Arg320His). KCNC1-related developmental and epileptic encephalopathy is associated with other pathogenic variants in KCNC1. In most individuals with KCNC1-related disorders, the pathogenic KCNC1 variant occurred spontaneously (de novo) and was not inherited from either parent.

Epidemiology

PME accounts for less than 1% of epilepsy cases at specialist centres. The incidence and

prevalence In epidemiology, prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number ...

of PME is unknown, but there are considerable geography and ethnic variations amongst the specific genetic disorders. One cause, Unverricht Lundborg Disease, has an incidence of at least 1:20,000 in Finland.

References

External links

GeneReview/NCBI/NIH/UW entry on Progressive Myoclonus Epilepsy with Ataxia
{{DEFAULTSORT:Progressive Myoclonic Epilepsies Epilepsy types