NeurometPlus

Incidence

It is inherited as autosomal recessive. The gene has been located in chromosome 21. This syndrome occurs worldwide especially frequent in Finland and Estonia with an incidence of 1/20,000. More than one hundred cases reported from these countries.

Clinical Characteristics

This is a progressive familial myoclonic epilepsy with degeneration of various groups of neurons in the cerebellum, brain stem and subcortical structures in the brain. No evidence of metabolic disturbance has been found. The onset is in childhood or adolescence from 6 to 16 years and begin with generalized seizures often myoclonic type. It is considered an epileptic syndrome. Absences and drop attacks may be observed. Seizures are more frequent in the morning on awakening when massive myoclonic jerks may be most disturbing. Within a few months it becomes apparent that the condition is very serious and many actions of the patient are associated with arrythmic myoclonic jerks. Polymyoclonic activity is made worse by voluntary movements, maintenance of posture and proprioceptive stimuli such as passive mobilization of limbs or the elicitation of tendon reflexes. They also may be enhanced by excitement. Intensification and spread of the myoclonus can lead to a generalized myoclonic seizure with loss of consciousness. The myoclonus eventually involve the entire body and the movements required for speech are affected, giving a kind of jerking dysarthria and unpredictable dysphagia. A cerebellar ataxia which can be more or less marked may appear but very difficult to distinguish from the intention myoclonus. Pyramidal signs may occur late in the disease. Depressed deep tendon reflexes, wasting of distal musculature and signs of chronic denervation on the EMG may be seen. Scoliosis is noted in some cases. No ocular abnormalities are observed. Intelligence is well preserved but most patients show mild mental deterioration. EEG shows various degrees of slowing of background activity with superimposed paroxysmal bursts. Spikes may appear at the vertex during REM sleep. Giant somesthetic evoked potentials are usual. There is no diagnostic biochemical testings but membrane-bound vacuoles may be detected in sweat glands in some patients. DNA markers linked to the defective gene in the distal part of chromosome 21 seems to be of considerable help in identifying this disorder. The course of the disease is marked by more or less rapid progression and the severity of the polymyoclonus increases. Seizures are usually controlled with treatment. The actual myoclonus progresses relentlessly and over a few years becomes incapacitating. Death occurs in the third or fourth decade. There is no effective treatment for the polymyoclonus, but some antiepileptics help. Occasionally they may improve with baclofen. Phenitoin and Tegretol (carbamazepine) may aggravate the condition and should not be used.

Precipitants

Polymyoclonic activity is made worse by voluntary movements, maintenance of posture and proprioceptive stimuli such as passive mobilization of limbs or the elicitation of tendon reflexes. They also may be enhanced by excitement. Intensification and spread of the myoclonus can lead to a generalized myoclonic seizure with loss of consciousness. The myoclonus eventually involve the entire body

Provocation Tests

no

Diagnostic Procedures

There is no diagnostic biochemical testings but membrane-bound vacuoles may be detected in sweat glands in some patients. DNA markers linked to the defective gene in the distal part of chromosome 21 seems to be of considerable help in identifying this disorder.