As with other movement disorders, myoclonus may be classified in several ways including phenomenologically, anatomicophysiologically (see Ch§pterJ6 ), and etiologically. The etiological divisions are physiological, essential, epileptic, and symptomatic myoclonus. Physiological forms include benign hiccups and hypnic jerks, which are sudden movements experienced by many normal individuals when falling asleep. These two phenomena rarely require evaluation or treatment. 'soi Because epileptic myoclonus and the vast array of symptomatic causes of myoclonus are described in the appropriate etiological chapters in this book, this section reviews only the hereditodegenerative (essential) forms of myoclonus.
Pathogenesis and Pathophysiology.Using the term paramyoclonus multiplex, Friedreich presumably was first to describe a patient with essential myoclonus. Although myoclonus has been clinically well defined, there has been no biochemical investigation that has explained the pathophysiological mechanism underlying essential myoclonus. Cerebral blood flow studies utilizing SPECT and xenon-133 or [technetium-99m]-d,l-hexamethylpropylene-amineoxime have revealed focal asymmetries in cerebral blood flow patterns consisting of relative hypoperfusion of the left cerebral hemisphere combined with an associated crossed cerebellar diaschisis.[81i This pattern could result from small lesions in the brain stem or basal ganglia, with secondary deafferentation of the ipsilateral frontal lobe and contralateral cerebellum, suggesting that essential myoclonus has a subcortical origin. Electrophysiological studies also suggest a subcortical origin for this form of myoclonus.
Epidemiology and Risk Factors. Essential myoclonus is a rare disorder; however, the overall prevalence in the population is unknown.
Clinical Features and Associated Disorders. Since its initial description, the diagnostic criteria for this form of myoclonus have evolved to the present state ( ..Table 34-7 ). The jerks in essential myoclonus are brief (50 to 200 msec) and may be generalized, multifocal, segmental, or unilateral. Myoclonic jerks mainly involve the muscles of the neck or upper body and are exacerbated by action, particularly writing or outstretching of the arms. The jerks abate during sleep and are ameliorated by alcohol. The beneficial effects of the latter are at times dramatic and nearly diagnostic; however, following withdrawal of alcohol, the condition frequently becomes worse on the rebound.y
_TABLE 34-7 -- DIAGNOSTIC CRITERIA FOR ESSENTIAL MYOCLONUS
1. Onset of myoclonus in first or second decade
2. Males and females equally affected
3. Benign course
4. Dominant mode of inheritance with variable severity
5. Absence of seizures, dementia, gross ataxia, or other deficit
6. Normal EEG
Some individuals with essential myoclonus have associated dystonic movements, including spasmodic torticollis and blepharospasm. The presence of dystonia is an exception to the diagnostic criteria, as noted above. Further complicating these diagnostic criteria is the occurrence of myoclonus in patients with hereditary dystonia, so-called myoclonic dystonia or dystonia with alcohol-responsive lightening-like jerks. Whether these entities represent different disorders remains to be elucidated by future genetic studies.^
In contrast to the hereditary form, the characteristics of sporadic essential myoclonus are more heterogeneous. Patients with this disorder range in age from 2 to 64 years at onset, have normal neurological findings on examination, and normal EEG results. The myoclonic jerks may be oscillatory, rhythmical segmental, nonrhythmical segmental, or nonrhythmical multifocal; the trunk and proximal limb muscles, followed by the muscles of the neck and face, are most affected. Generally, patients do not demonstrate marked progression of symptoms or disability.
Differential Diagnosis. Because essential myoclonus has no known cause, its diagnosis is one of exclusion, and other causes must be evaluated. The coexistence of myoclonus and epilepsy suggests the presence of a form of epileptic myoclonus, which may present with either focal or generalized myoclonus. In these patients epilepsy is the dominant feature, the syndrome is generally age-related, and in most cases EEG abnormalities are associated (see Ch.a.pie.L5.2.). Myoclonic syndromes associated with underlying diseases that cause a predominant encephalopathy, ataxia, dementia, or pyramidal or extrapyramidal signs as major features of the illness are termed symptomatic myoclonias. Syndromes comprising symptomatic myoclonus with seizures are termed progressive myoclonic epilepsies (PME) (see Chapter...52 ), although the seizures are generally minor or late developments. Symptomatic causes of myoclonus include a variety of disorders ( .TableS^-S).
Evaluation. The evaluation of essential myoclonus relies on a detailed history and a thorough neurological examination. Neuroimaging studies (head MRI and CT) in patients with essential myoclonus are normal, but these studies are helpful in patients with alternative diagnoses such as stroke, degenerative disorders, and trauma. Electrophysiological tools, including EMG, electroencephalography (EEG), and somatosensory evoked potentials (SEPs), can be particularly valuable in distinguishing the levels of neural dysfunction in patients with myoclonic syndromes. In essential myoclonus, these studies are normal; however, EEG results are generally abnormal in patients with epileptic myoclonus and the PMEs, showing epileptiform discharges. Patients with metabolic or toxic causes of myoclonus may also show
_TABLE 34-8 -- SYMPTOMATIC MYOCLONUS_
Hypoxia, uremia, chronic hemodialysis, hepatic failure hypercarbia, hypoglycemia, hyponatremia, nonketotic hyperglycemia
Bismuth, cocaine, organic mercury, methylbromide, strychnine, tetraethyl lead, tetanus
III. Degenerative Dementias--Alzheimer's disease
Movement disorders--corticobasal degeneration, multiple systems atrophy, progressive supranuclear palsy, Hallervorden-Spatz disease, Huntington's disease
IV. Neurovascular Ischemia Hemorrhagic
Transmissible spongiform encephalopathies--Creutzfeldt-Jakob disease, kuru, viral--measles, Epstein-Barr virus, etc.
VI. Neoplastic Neuroblastoma
Paraneoplastic disorders--ovarian, lung, breast
VII. Autoimmune--multiple sclerosis
Penetrating trauma--brain, spinal cord Heat stroke Electric shock Decompression illness
Medications (amantadine, etomidate, penicillin, buspirone, diclofenac, lithium, metoclopramide, tricyclic antidepressants, levodopa, dopamine agonists) Procedures (water-soluble eontrast media, post-thalamotomy)
diffuse slowing on EEG. SEPs are also normal in patients with essential myoclonus, as well as subcortical and spinal forms of myoclonus, but are abnormal in those syndromes originating in the cortex, including most PMEs. Blood, cerebrospinal fluid, and tissue biopsies, although normal in patients with essential myoclonus, should be used in the appropriate clinical settings.
Treatment. Despite its nonprogressive, benign nature, essential myoclonus can significantly impair the performance of routine daily activities, and symptomatic treatment of the myoclonic jerks can result in substantial improvement in the patient's quality of life. Although alcohol has been shown to alleviate the movements in a number of patients with essential myoclonus, the limitations of this pharmacological therapy are obvious. Manipulation of brain serotonin levels, although beneficial in some forms of myoclonus, has generally been unsuccessful in the treatment of essential myoclonus. A variety of medications may be used in this and other myoclonic disorders (,.T§Me.3.4-:9. ). The benzodiazepines, particularly clonazepam, are the most consistently effective medications used for the treatment of essential myoclonus. In addition, favorable treatment results have been observed with certain anticholinergic medications including benztropine and trihexyphenidyl.
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