What is the differential diagnosis for a 64-year-old male patient who experienced a possible seizure characterized by muscle rigidity of the upper and lower extremities lasting 16 seconds, with vital signs showing normotension, mild tachypnea, tachycardia, normal oxygen saturation, and normothermia?

Differential Diagnosis for Brief Episode of Muscle Rigidity in a 64-Year-Old Male

The most likely diagnosis is convulsive syncope (syncope with myoclonic jerks), though a true seizure, medication-induced syndrome, or metabolic disturbance must be ruled out through targeted history and examination.

Primary Differential Considerations

1. Convulsive Syncope (Most Likely Given Brief Duration)

• The 16-second duration strongly favors syncope over epilepsy, as syncope typically causes unconsciousness lasting <30 seconds, while epileptic seizures usually last >1 minute 1
• Convulsive syncope occurs when cerebral hypoperfusion produces involuntary myoclonic movements that are asynchronous and limited in scope 1
• Key distinguishing features to assess: Did movements begin after loss of consciousness (favors syncope) or at onset (favors seizure)? 1
• Were movements asymmetric/asynchronous (syncope) or symmetric/synchronous (seizure)? 1
• Cardiac causes of syncope can produce upward gaze deviation, asynchronous myoclonic jerks, and brief automatisms from global cerebral hypoperfusion 2

2. Generalized Tonic-Clonic Seizure

• Critical warning: The brief 16-second duration is atypical for true epileptic seizures, which usually involve prolonged tonic-clonic movements 3
• True seizures present with loss of consciousness coinciding with onset of movements, symmetrical and synchronous movements, and longer duration 3
• Post-ictal confusion lasting more than a few minutes, tongue biting, or muscle pains lasting hours/days point to epilepsy 2
• At age 64, first-time seizure requires urgent evaluation for structural lesions, metabolic causes, or acute symptomatic causes 4

3. Cardiac Arrhythmia with Syncope

• Given the patient's age (64), cardiac causes must be prioritized as syncope can be a precursor to sudden death, particularly in patients with underlying heart disease 2
• Decreased cardiac output can cause convulsive syncope 1
• The normal blood pressure (110/80) and oxygen saturation (95%) at presentation do not exclude a transient arrhythmia that has since resolved 2

4. Medication-Induced Syndromes

• Serotonin syndrome can present with muscle rigidity, though typically accompanied by autonomic hyperactivity and mental status changes 2
• Neuroleptic malignant syndrome (NMS) presents with muscle rigidity but develops over hours to days, not seconds 2
• Obtain medication history including SSRIs, antipsychotics, anti-Parkinsonism drugs, and over-the-counter supplements 2

5. Metabolic/Toxic Causes

• Hypoglycemia can cause seizures and altered consciousness 2
• Electrolyte imbalances, organ failure, and medication toxicity can provoke seizures 5
• The normal vital signs make acute metabolic crisis less likely but do not exclude it 5

Critical Next Steps for Diagnosis

Immediate History to Obtain:

• Position at onset: Standing/sitting strongly favors syncope over seizure 1
• Prodromal symptoms: Nausea, sweating, pallor, blurred vision favor syncope 1
• Witness description: Number of movements (few movements ~10 suggest syncope; many movements 20-100 suggest seizure) 1
• Timing of movements: After loss of consciousness (syncope, average 20 seconds after) vs. at onset or before fall (seizure) 1
• Recovery time: Rapid recovery <1 minute favors syncope; prolonged confusion favors epilepsy 1
• Medication history: Particularly proserotonergic medications, antipsychotics, anti-Parkinsonism drugs 2

Essential Diagnostic Testing:

• ECG immediately to assess for arrhythmias, conduction abnormalities, or inherited ion channel abnormalities 2
• Point-of-care glucose to exclude hypoglycemia 2
• EEG within 24 hours if seizure suspected, though may be normal initially 4
• Serum prolactin 10-20 minutes after event (if still within timeframe) helps differentiate true seizures from psychogenic events 4
• Echocardiogram and cardiac ischemia assessment given age and potential cardiac etiology 2

When to Activate Emergency Response:

• This patient requires EMS activation as this represents a first-time event with loss of consciousness 2, 3
• Additional indications include: seizure lasting >5 minutes, failure to return to baseline within 5-10 minutes, traumatic injuries, or respiratory difficulty 2, 3

Critical Pitfalls to Avoid

• Do not assume all muscle rigidity represents epilepsy: Convulsive syncope is frequently misdiagnosed as epilepsy, leading to unnecessary antiepileptic treatment and potentially delaying diagnosis of life-threatening cardiac causes 1
• Do not dismiss cardiac evaluation based on normal post-event vital signs: Transient arrhythmias may have resolved by time of assessment 2
• Do not start antiepileptic drugs without confirming true seizure: Most patients with secondary seizures do not have epilepsy 5
• The mild tachypnea (RR 24) and tachycardia (CR 96) may represent post-event physiologic response or underlying cardiopulmonary process requiring evaluation 2