Mercury-Induced Tremor

Mercury induced tremor is a fine, intention type tremor caused by chronic inhalation of elemental mercury vapour, usually with the neuropsychiatric picture called erethism and with gingivostomatitis and renal effects.

Exposure. Artisanal gold amalgam mining, chloralkali plants, thermometer and instrument manufacture, fluorescent lamp work, dental amalgam handling, felt hat making, and use of mercury containing skin lightening creams and traditional remedies.

Epidemiology. Cases are now concentrated in small scale gold mining and in unregulated product use. Neurological effects usually appear at urinary mercury levels over about 100 micrograms/L, with subtle psychomotor change at lower levels.

Spectrum. Tremor, erethism (irritability, shyness, insomnia and memory loss), gingivostomatitis, salivation, renal tubular injury and proteinuria, and acrodynia in children.

Course. Tremor and erethism usually improve over months to years after exposure ends, but some deficits persist.

Pathophysiology.

Mercury vapour is lipophilic and crosses the blood brain barrier, where it is oxidised to mercuric ions that remain trapped. It binds sulfhydryl groups, inhibits enzymes and disrupts calcium homeostasis.

Cerebellar and cortical neurons are affected, causing tremor and cognitive change. The kidneys accumulate mercury and show proximal tubular damage.

• Fine tremor of the fingers, eyelids, lips and tongue, becoming coarse with intention and affecting handwriting.
• Irritability, excitability, insomnia, shyness, depression and impaired memory.
• Metallic taste, salivation, gingivitis, loose teeth and stomatitis.
• Headache, fatigue, peripheral neuropathy and proteinuria.

Red flags.

• Severe psychiatric disturbance or suicidal thoughts.
• Acute inhalation with dyspnoea, cough and chemical pneumonitis.
• Nephrotic range proteinuria.

• Exposure history including products used at home and source of any spill.
• Urinary mercury (24-hour or creatinine corrected spot): under 10 micrograms/L is usual in the general population. Blood mercury reflects recent exposure only.
• Neurological examination, handwriting sample and neuropsychological testing.
• Urinalysis for protein and renal function; urinary beta-2 microglobulin or N-acetylglucosaminidase for early tubular injury.
• Thyroid function, to exclude thyrotoxicosis.

Differential diagnosis. Essential tremor, Parkinson disease, hyperthyroidism, Wilson disease, drug induced tremor (lithium, valproate, beta agonists), alcohol withdrawal, anxiety disorder, other metal toxicity (manganese, lead).

• Remove from exposure, contain spills and decontaminate clothing, the home and the workplace.
• Chelation for symptomatic patients with a high urinary mercury level: succimer 10 mg/kg orally every 8 hours for 5 days, then every 12 hours for 14 days (19 days in total). Maximum 500 mg each dose. Monitor liver enzymes, full blood count, rash and urinary mercury. Use caution in renal impairment. Chelation increases excretion, but benefit for tremor is uncertain.
• Symptomatic tremor: propranolol 40 mg orally twice daily, increased to 80 to 160 mg daily, maximum 320 mg daily. Review at 4 weeks. Monitor heart rate and blood pressure. Avoid in asthma, bradycardia, heart block and decompensated heart failure; reduce dose in hepatic impairment; no renal adjustment. Evidence for benefit in mercury tremor is limited.
• Treat depression and anxiety by usual practice; cognitive rehabilitation.
• Dental and oral care for gingivostomatitis.

Monitoring. Urinary mercury every 1 to 3 months until under 20 micrograms/L; neurological review every 3 to 6 months; yearly renal function.

Referral and escalation. Toxicology, neurology and occupational physician; nephrology for persistent proteinuria; public health notification for household or community sources; notification and compensation where required.

Special groups. Pregnancy and breastfeeding: mercury crosses the placenta and enters breast milk, so exposure must be removed and chelation is specialist directed. Children with unexplained pain, rash, irritability and hypertension need urinary mercury testing.