Primary Orthostatic Tremor

Primary orthostatic tremor (POT), or ``shaky legs syndrome'', is a rare movement disorder characterized by fast, task-specific tremor, affecting the legs and trunk while standing.

Epidemiology

The prevalence is unknown. To date, around 390 cases have been reported in the literature. Women are predominantly affected (sex ratio: 2:1) with disease onset occurring in middle aged or elderly people.

Clinical description

POT only appears on standing, is associated with an intense and disabling sense of unsteadiness and a fear of falling without actually falling, and is reduced or abolished by sitting, walking or the use of a support. Symptoms may persist or worsen when walking backwards. Unsteadiness may lead to stasibasiphobia and patients develop strategies through stamping, leaning on a support, and walking. This fast tremor may be visible as a fine-amplitude rippling of the leg muscles, palpable as a thrill, and heard by muscle auscultation as a thudding sound, similar to that of a distant helicopter.

Etiology

At present, the pathophysiology of POT is unknown. The strong coherence of tremor frequency seen on electromyography (EMG) between muscles of the leg, trunk and arm (unilaterally and bilaterally) is thought to be driven by a supraspinal oscillator as tremors at 16 Hz have been reported in cranial nerve innervated muscles. Possible anatomic regions of the oscillator include the cerebellum and brainstem. The same central oscillator that is active in POT may also be active in healthy subjects who are made unsteady and POT may represent an exaggerated activity in otherwise physiological pathways involved in postural responses. An alternative hypothesis is that POT is triggered by the weight of the body when standing still, and that the slight imbalance which ensues aggravates the tremor.

Diagnostic methods

Diagnosis is based on a history of unsteadiness and muscle thrill while standing with an enlargement of the sustentation polygon in patients that do not usually show any other neurological abnormalities. EMG reveals high-frequency bursts (13-18 Hz) of muscle activity in weight-bearing muscles (synchronous between homologous muscles) with a short latency tremor onset upon standing and abrupt cessation after sitting, occurring sometimes in association with a postural upper limb tremor of a similar frequency. Computed tomography (CT) and magnetic resonance imaging (MRI) are normal.

Differential diagnosis

Differential diagnoses based on clinical and EMG findings include slow orthostatic tremors (4-12 Hz) that have been described in patients with Parkinson's disease and parkinsonism, cerebellar lesions and atrophy.

Genetic counseling

POT is sporadic but exceptional familial cases have been reported.

Management and treatment

There is no cure or standard treatment for POT but benzodiazepines, antiepileptics (valproate, gabapentin), beta-blockers (propranolol), and muscle relaxants are all used as symptomatic treatments. Clonazepam appears to be the most effective. The therapeutic effect is dose-dependent and appears from 1.2 mg/24 h. The dose may be increased from 4- 6mg and sometimes up to 8mg /24 h t.i.d. (but with sedative side effects). Other benzodiazepines may be effective, such as diazepam. Although there is evidence that the dopaminergic system is involved in POT pathogenesis, the response to L-dopa or dopaminergic agonists is variable.

Prognosis

Prognosis is good and treatment greatly improves quality of life.