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                                                                                                           MYOCLONUS

 

Arvind Goutham1,Sujith Ovallath2

 

1-Junior Resident 2-Professor and head

James Parkinson’s Movement Disorder Research Centre.

Dept .Of Neurology,Kannur Medical College.

 

 

 

Introduction:

   Myoclonus, one of the hyperkinetic disorders, occurs as a brief, shock like movement, due to abrupt muscle contractions or a short loss in muscle tone, resulting in movement of a limb at one of its joints.

Myoclonus may be commonly confused with various jerks/ twitches and other hyperkinetic disorders, which do not cause a change in the position of the body parts at the site of their occurrence. Myoclonus-like hyperkinetic movements include:

 

·      Myokymia: seen in peripheral nerve hyperexcitability disorders

·      Fasciculations: seen in motor neuron diseases

·      Tics: Tourette syndrome, dystonic tics

·      Chorea: seen in Rheumatic fever( Syndenham), Benign hereditary chorea

·      Ballism( severe form of chorea): seen in nonketotic diabetic ketoacidosis

·      Dystonic tremors

Non organic /psychogenic movement disorders.The similar-appearing hyperkinetic disorders can be distinguished from myoclonus by parameters such as, the variations in the muscles involved, muscle recruitment order, the durations and amplitudes in burst (recorded in an electromyogram).

 

    Myoclonus may be broadly described in two types:

Ø POSITIVE MYOCLONUS: due to muscle contractions

Ø NEGATIVE MYOCLONUS: due to lapses in muscle tone

Negative Myoclonus includes:

·      Asterixis: flapping tremors seen in renal/ hepatic encephalopathy

·      Axial muscles: causing wobbling gaits/ sudden falls

·      Epileptic: brief lapse in the resting muscle tone, synchronous with an

                  Epileptic EEG spike

 

The electrophysiological studies available to aid the diagnosis of myoclonus are

·      Multi channel surface EMG

·      EEG

·      Somatosensory Evoked Potential

·      EEg-EMG Backaveraging

CLASSIFICATION OF MYOCLONUS:

Myoclonus can be classified based on the following criteria:

A] FREQUENCY OF OCCURRENCE:

v Isolated: occurring at different time intervals

v Repitive: occurring after a specific time interval, further classified into

i)               Rhythmic: often mistaken for a tremor Ex: spinal segmental    myoclonus

ii)             Arrhythmic: polyminimyoclonus seen in neurodegenerative disorders, observed as fine finger jerks in outstretched hands

 

B] INITIATING FACTOR:

v Spontaneous Myoclous: occurs at rest/ without a stimulus

v Action Myoclonus: occurs on provocation by an external stimulus(auditory/ tactile)

 

C] SITE OF ORIGIN:

v CENTRAL: includes Cortical and Subcortical Myoclonus

v SPINAL: includes Segmental and Propriospinal

 

CORTICAL MYOCLONUS:

·      Arises from the hyperexcitable sensory and motor cortices

·      most common form of myoclonus

·      multifocal in origin

·      commonly affects the face and distal limbs, due to large area of representation in the cortex

·      provoked by tactile stimulus

·      often occurs due to cerebellar cortical disease which results in loss of granule cells

       Causes of Cortical Myoclonus include:

Ø Degenerative disorders: Corticobasal degeneration

                                            Alzheimer’s disease

                                            Multi System Atrophy

                                            Huntington Disease( Westphal type)

                                            Panthothenate-Kinase related

                                                 Neurodegeneration

                                            Lewy body Dementia

               

Ø Metabolic:  Hepatic/ Renal failure

                     Hyponatremia

                     Hypoglycemia

                     Nonketotic Hyperglycemia

                     Carboxylase enzyme defects (Biotin deficiency)

                    

Ø Infectious: Herpes simplex Encephalitis

                    Subacute Sclerosing Panencephalitis

                    Whipple Disease

                    Creutzfeld-Jacob disease

 

Ø Epileptic: Progressive myoclonic encephalopathies

                  Epilepsia partialis continua

                  Angelman Syndrome

 

Ø Autiommune: Coeliac disease

                          Hashimoto Encephalopathy

                          Stiff Person Syndrome

                          Anti NMDAR encephalitis

                          Progressive Encephalomyelitis with Rigidity and

                             Myoclonus(PERM)

Myoclonus in Parkinson disease may occur as a result of the medications like levodopa and amantidine.

Minipolymyoclonus, low amplitude irregular jerks observed in one or many fingers on keeping the limbs outstretched is characteristically seen in Multi System Atrophy.

 

Progressive myoclonic encephalopathies are manifested by a range of hyperkinetic disorders including seizures, myoclonus along with ataxia, hallucinations, cognitive impairment. These features are common in children born of consanguineous marriage. PME may often be confused with juvenile myoclonic epilepsy. The age of onset in PME may be as early as infantile. Ex: Potassium channel tetramerization mutations resulting in myoclonus with mental retardation and motor detetrioration.

                

 Diagnosis of cortical myoclonus:      

·      Polymyography: Short Burst duration <100ms, muscle recruitment

                               sequence

·      Somatosensory Evoked potentials: detection of a giant potential

·      Electroencephalogram: P27 peak has an amplitude of >5mV

                                           N35 peak has an amplitude of >10mV

                                           Presence of C-Reflex: longer duration of

                                                the mediated polysynaptic reflex

·      EMG-EEG readings superimposition: EEG cortical spikes preceding the

                                          spikes on EMG with a short latency

·      Long Latency reflex and Cutaneous Reflex: Exaggerated

·      Transcranial Magnetic stimulation: Decreased intracortical and

                                           Transcortical inhibition

  

 SUBCORTICAL MYOCLONUS:

 

Causes of Subcortical Myoclonus include:

Ø Toxins/ Poisons: Bismuth, Methyl Bromide, DDT, Alcohol, Toulene

Ø Drug induced: Levodopa, Amntidine, Nifedepine, Lithium, Verapamil,

                           Lamotrigine, TCAs, Acyclovir, Isoniazid, Fentanyl

       

 

  It may be further divided into:

v Myoclonus- Dystonia

v Brainstem Myoclonus

 

 

Myoclonus Dystonia:

·      usually sets in under 20yrs of age

·      myoclonus involving upper trunk and proximal arms

·      commonly  associated with dystonia which manifests as cervical

   dystonia/ writer’s cramp

·   autosomal dominant inheritance with maternal imprinting of the mutation involving the epsilon-sarcoglycan gene on chromosome7 (SGCE, DYT11)

·      late onset myoclonus dystonia seen in Silver Russel syndrome due to maternal uniparental disomy of chromosome 7

 

Brainstem/ Reticular Myoclonus:

 Causes: Posthypoxic (Lance Adams Syndrome)

                Friedrich Ataxia

                Ataxia-Telengectasia

·  electrical spikes beginning in brainstem with simultaneous sequential recruitment in rostral and caudal directions

·  provoked by an auditory/ tactile stimulus

·  observed predominantly in the proximal muscles of bilateral arms

·  EMG: Burst Duration >100ms

 

Lance Adams Syndrome/ Posthypoxic Myoclonus: action/ intention provoked myoclonus, observed to develop within days to weeks after improvement in the consciousness of the patients recovering from cardiac arrest or respiratory failure. Bronchial asthma causing acute hypoxia with hypercapnia is a common precipitating factor.

 

Levodopa causes multifocal, action induced/ spontaneous myoclonus, observed in association with dyskinesia.

Amantidine causes myoclonus observed in limbs and orofacial region. Drug induced myoclonus may result in speech arrests and stuttering if the causative drug is not discontinued. Amantidine induced myoclonus is worse in patients with renal dysfunction as the drug is largely excreted in urine and is barely filtered off by hemodialysis. Amantidine may further worsen myoclonus by escalating the availability of serotonin in presynaptic neurons.

 

SPINAL MYOCLONUS:

·      manifested as a result of damage to the spinal cord/peripheral nervous system ex: lesions of brachial plexus- Erb’s palsy, Klumpke’s palsy, polyradiculopathies, spinal cord trauma

·      Focal/ segmental, usually unilateral, irregular myoclonus

·      Seen commonly in the arms and trunk

·      EMG: Burst Duration >100ms

             PROPRIOSPINAL MYOCLONUS: a fraction of patients with spinal

                 myoclonus, develop worsening of the movement on lying down with a

                 few preceding sensations, particularly during switching between awake

                 and sleep states.

                 A few patients with the above mentioned features have been found

    to have a psychological cause for the myoclonus, as normal patients

    have been seen to demonstrate the EMG pattern of propriospinal

    myoclonus.

BEREITSCHAFTSPOTENTIAL/ Pre Movement Potential: a slow negative EEG potential that precedes the EMG spike onset by 1-2.5sec. This potential is useful in detecting cases with a psychological cause as this potential points out the conscious preparation of the patient which causes the movement.

    

D] ETIOLOGICAL CLASSIFICATION OF MYOCLONUS:

 

Primary/ Idiopathic:

Ø Physiological: hypneic jerks, sneezing, hiccups

Ø Exaggerated startle: Hyperkplexia

Ø Epileptic: Benign Rolandic Epilepsy

                  Juvenile Myoclonic Epilepsy

                  Angelman Syndrome

 

          Secondary:

Ø Neurodegenerative disorders

Ø Infections

Ø Metabolic causes

Ø Dementias

Ø Autoimmune disorders

Ø Toxins/ Poisons

Ø Drug induced

 

E] PROGRESSION:

 

     Non Progressive: Physiologic

                                     Hyperekplexia

                                     Epileptic

                                     Posthypoxic/ Lance Adams Syndrome

                                     Drug Induced

                                     Metabolic causes

                                     Myoclonus Dystonia

                                     Ataxia Telengectasia

                                     Myoclonic Epilepsy with Ragged Red Fibres(MERRF)

                                    Opsoclonus-Myoclonus syndrome

                                    Mitochondrial Encephalopathy, Lactic acidosis and Stroke Like

                                         Episodes(MELAS)

 

   Progressive:

·      Rapid Onset: Renal Failure

                        Dialysis Disequilibrium Syndrome

                        Serotonin Syndrome

                        Psychogenic

·      Moderately progressive: Corticobasal Degeneration

                                             Lewy body Dementia

                                             Alzheimers Disease

                                             Multi System Atrophy

·      Severe/ Rapidly Progressive: Creutzfelt- Jakob Disease

                                                    Progressive Myoclonic Encephalopathies

                                                                Subacute Sclerosing Panencephalitis

                                                                HSV, HTLV-1 Encephalitis

                                                                NMDAR Encephalitis

                                                                Coeliac Disease

                                                                Bismuth Encephalopathy

 

TREATMENT:

Tratment of muoclonus differs  dependin on etiology 

v Identification and gradual withdrawal of  offending drugs like amantidine, levodopa in cases of drug induced myoclonus.

v Correction of metabolic abnormalities which has precipitated the myoclonus Eg; Hyponatremia,Acidosis.

Drug therapy

v Cortical Myoclonus:

Levetiracetam 1g-3g per day is effective in most of the cases

    Valproate 1.5g/day with CoEnzyme Q and L-Carnitine to prevent

     secondary carnitine deficiency

              

Cortical myoclonus also respond to Piracetam 8-20 gm/day,Primidone(5---100mg/day.

Clonazepam 2-15 mg/day.Gives excellent response in myoclonus.

 

Zonisamide

Topiramate

                Sodium Oxybate: effective in resting, action components of

                                                ethanol responsive myoclonus

 

v Subcortical Myoclonus:

   Clonazepam has been demonstrated to have maximum efficacy. Benzodiazapenes provide short term relief. Patients soon develop tolerance and discontinue them due to the sedative side effects.

 

Levertiracetam and clonezepam have been shown to provide relief in distal action and reflex myoclonus in upper limbs.

 

v Progressive Myoclonic Encephalopathies:

-  Lisuride: Dopaminergic + Sertonergic agonist

-  L-Serotonin + Carbidopa

-  Thyroid Releasing Hormone

- N –acetyl cystine .

-  

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