Primary orthostatic tremor

Causes difficulty in standing still

Heilman described the clinical features of primary orthostatic tremor 10 years ago.1 Subsequent studies have elucidated the specific neurophysiological features of the condition, which clearly distinguish it from other tremulous disorders of the legs.2 3 4 Although the condition is rare, the incidence of this recently described condition remains unknown. Identification of cases is still increasing, with wider recognition of the characteristic clinical symptoms. Doctors should be aware of the typical symptom complex of orthostatic tremor, as many patients are wrongly labelled as suffering from psychiatric symptoms.5

The most prominent and characteristic symptom reported by patients with primary orthostatic tremor is unsteadiness when standing still—for example, at supermarket check outs or bus stops. By contrast, patients have little or no difficulty in walking, which typically relieves their symptoms.3 4 Despite the condition's name few patients complain specifically of tremor. Examination, when the patient attempts to stand still, reveals a fine rippling of the muscles of the legs that may be easier to feel than to see. After a short interval, the patient becomes increasingly unsteady and is forced to take a step to regain balance. Falls and injuries, however, are rare.

The diagnosis is confirmed neurophysiologically.2 3 4 Surface electromyographic recordings show rhythmic activation of lower limb muscles at a frequency of 14–18 Hz. Within any individual patient, the frequency is the same in all muscles and fixed. This frequency of muscle activity is characteristic of the condition and is much higher than can be produced voluntarily and higher than that seen in other tremulous conditions (for example, essential tremor or Parkinson's disease, in which the frequency usually lies between 3 and 8 Hz6). Interestingly, rhythmic activation of upper limb muscles at the same frequency can be seen if patients use their arms to maintain posture—for example, by standing on all fours.3 4 7

The cause of the condition is unknown. The neurophysiological abnormalities suggest a brain stem disturbance, and recent positron emission tomography studies have shown increased activity in the cerebellum in comparison with controls when patients hold their arms outstretched (which in some patients also brings out a tremor).8 Results on radiological examination are normal, and no other disturbance of brain stem function has been associated with the condition. The relation of primary orthostatic tremor to essential tremor is disputed.3 4 7 9 The condition affects both sexes and the age of onset is usually in the sixth or seventh decade of life, although it may begin as early as the third decade. Some patients have had symptoms for more than 20 years.

Although no specific treatment currently exists, patients are often relieved to know the diagnosis, particularly if a psychiatric cause had previously been suspected. Clonazepam,1 4 7 9 phenobarbitone,4 7 primidone,4 9 10 and sodium valproate4 have been used with occasional success. (bela) Blockers and alcohol are ineffective. Stools (in the kitchen) and shooting sticks with rubber ends (for bus queues) may be helpful. With a better understanding of the condition more specific treatment may become available. In the meantime, it is important not to misdiagnose primary orthostatic tremor as a psychiatric illness for want of a careful clinical examination and lack of awareness of the condition.