What causes internal tremors in an adult with a possible history of neurological or movement disorders?

What Causes Internal Tremors

Internal tremors—the subjective sensation of trembling without visible shaking—are most commonly caused by anxiety, medication side effects (particularly SSRIs, lithium, or dopaminergic drugs), or early manifestations of movement disorders like Parkinson's disease. 1, 2

Primary Causes to Consider

Medication-Induced Tremor

• Drug-induced tremor is the most frequent cause of new-onset tremor in adults, with common offending agents including SSRIs/SNRIs, lithium, valproate, amiodarone, β-adrenoceptor agonists, dopamine receptor antagonists, and tricyclic antidepressants like amitriptyline. 1, 3
• Lithium specifically can cause both visible and internal tremor sensations, with toxicity occurring at doses close to therapeutic levels, making this a critical consideration. 4
• Antipsychotic medications and dopamine antagonists can induce parkinsonism with associated tremor sensations through functional disruption of dopaminergic pathways. 4, 1
• Risk factors for drug-induced tremor include polypharmacy, male gender, older age, high doses, and immediate-release preparations. 1

Neurological Movement Disorders

• Parkinson's disease should be considered, particularly if the patient has bradykinesia (slowness of movement) plus either rigidity or resting tremor, as internal tremor sensations can precede visible tremor. 5, 2
• Early-onset Parkinson's disease risk is increased in certain genetic conditions like 22q11.2 deletion syndrome, where patients may experience tremor along with other movement disorders. 3
• Essential tremor can manifest as both visible action tremor and subjective internal tremor sensations, typically affecting the hands but potentially involving the head, voice, or trunk. 3, 6

Metabolic and Endocrine Causes

• Hypocalcemia can induce or worsen tremors and movement disorders, making serum calcium the single most important initial laboratory test. 3, 7
• Hypomagnesemia contributes to tremor and should be checked alongside calcium levels. 3, 7
• Thyroid dysfunction (both hyperthyroidism and hypothyroidism) can cause tremor sensations and should be evaluated with TSH. 7
• Hypoglycemia in diabetic patients can produce internal tremor sensations as part of autonomic activation. 3

Autonomic Dysfunction

• Primary or secondary autonomic nervous system failure can cause internal tremor sensations along with orthostatic hypotension, early impotence, and disturbed micturition. 3
• Autonomic dysfunction occurs in diabetes mellitus, amyloidosis, and various polyneuropathies, all of which can present with tremor-like sensations. 3

Psychiatric and Functional Causes

• Anxiety disorders commonly produce internal tremor sensations without visible shaking, often accompanied by palpitations, sweating, and hyperventilation. 3
• Functional (psychogenic) tremor may present with sudden onset, distractibility, entrainment with contralateral movements, and arrest with specific maneuvers. 1

Critical Diagnostic Workup

Initial Laboratory Assessment

• Check serum calcium first, as hypocalcemia is a treatable cause that can induce or worsen any tremor syndrome. 3, 7
• Obtain parathyroid hormone (PTH) if calcium is low to evaluate for hypoparathyroidism. 7
• Check magnesium levels, as hypomagnesemia frequently coexists with hypocalcemia. 3, 7
• Measure TSH to exclude thyroid dysfunction. 7
• Consider vitamin B12, folate, and vitamin D levels, as deficiencies can impact neurological symptoms. 7

Medication Review

• Immediately review all current medications for tremor-inducing agents, including over-the-counter drugs and supplements. 1
• If on lithium, obtain serum lithium level immediately to rule out toxicity. 4
• Evaluate for recent medication changes, dose increases, or drug interactions that may precipitate tremor. 1

Clinical Examination

• Assess for visible tremor at rest, with posture holding, and during action to distinguish tremor types. 6
• Examine for bradykinesia by testing finger tapping, hand opening/closing, and rapid alternating movements. 5
• Check for rigidity by passively moving limbs while the patient relaxes, noting any cogwheel phenomenon. 5
• Evaluate for postural instability, though this typically appears later in Parkinson's disease. 5

When to Obtain Neuroimaging

• MRI brain is NOT routinely indicated if the clinical picture suggests medication-induced tremor, anxiety, or typical essential tremor without red flags. 7, 5
• Obtain MRI brain if new focal neurological signs develop, atypical features suggest alternative diagnoses, or rapid progression raises concern for structural lesions. 7
• DaTscan (ioflupane SPECT) can differentiate Parkinson's disease from essential tremor or drug-induced tremor when diagnostic uncertainty exists, but is not indicated for evaluating progression in established Parkinson's disease. 7, 5

Common Pitfalls to Avoid

• Do not assume internal tremor equals anxiety without first ruling out metabolic causes (especially hypocalcemia) and medication effects. 7, 1
• Do not order routine brain imaging in the absence of red flags, as this adds cost without changing management in typical cases. 7
• Do not miss drug-induced parkinsonism by failing to recognize that anticholinergics, phenothiazines, and tricyclic antidepressants can cause functional dopaminergic blockade. 3, 1
• Do not overlook that internal tremor sensations may precede visible tremor in early Parkinson's disease by months to years. 5, 2
• Avoid endless searches for physical causes when psychiatric illness (anxiety, somatization) is the likely diagnosis after appropriate workup. 3

Management Approach

• If medication-induced, reduce or discontinue the offending agent when clinically feasible, as drug-induced tremor usually resolves after discontinuation. 1
• For persistent drug-induced parkinsonism, consider adding anticholinergic agents or mild dopaminergic agents like amantadine. 4
• Correct metabolic abnormalities (calcium, magnesium, thyroid) before attributing symptoms to disease progression or psychiatric causes. 7
• Propranolol is useful for most types of tremor, though it can fail in some cases of essential tremor. 8
• Refer to movement disorders neurology if diagnostic uncertainty exists about whether symptoms represent early Parkinson's disease, atypical parkinsonism, or another condition. 7, 5