What could be causing a young to middle-aged adult's symptoms of déjà vu, brain fog, nausea, and vomiting lasting 12 hours?

Episodic Neurological Syndrome with Déjà Vu, Brain Fog, and Prolonged Nausea/Vomiting

This symptom constellation—déjà vu followed by brain fog, nausea, and vomiting lasting 12 hours—is most consistent with temporal lobe epilepsy (complex partial seizures) or cyclic vomiting syndrome, and requires urgent neurological evaluation to exclude seizure activity and rule out life-threatening causes.

Primary Diagnostic Considerations

Temporal Lobe Epilepsy (Most Likely)

Déjà vu is an established symptom of temporal lobe seizures and results from faulty activity in the parahippocampal gyrus recognition memory system 1. The specific pattern described—déjà vu as a prodrome followed by cognitive impairment ("brain fog") and prolonged gastrointestinal symptoms—strongly suggests seizure activity originating from the medial temporal structures.

• Epileptic activity in the entorhinal cortex of the parahippocampal area specifically elicits déjà vu experiences 2
• The "brain fog" represents the postictal confusion state that follows complex partial seizures 3
• Nausea and vomiting lasting 12 hours can occur as part of the ictal or postictal phase, mediated through autonomic dysfunction 4

Critical red flag: The stereotypical nature of these episodes (recurring pattern with consistent duration) is pathognomonic for seizure activity rather than other causes 2.

Cyclic Vomiting Syndrome (Secondary Consideration)

Cyclic vomiting syndrome presents with stereotypical episodes of acute-onset vomiting, with a prevalence of approximately 2% in US adults 4. However, this diagnosis is less likely given the prominent déjà vu component.

• Episodes are characterized by acute nausea and vomiting lasting less than 7 days, with symptom-free intervals between episodes 4
• The 12-hour duration fits within the typical timeframe for cyclic vomiting syndrome 4
• Brain fog and déjà vu are not typical features, making this diagnosis secondary to temporal lobe epilepsy 4

Algorithmic Diagnostic Approach

Step 1: Exclude Life-Threatening Neurological Causes

Immediate neuroimaging (MRI brain with epilepsy protocol) is mandatory if any of the following are present 5, 6:

• Dysarthria or dysphagia accompanying episodes
• Visual disturbances beyond typical aura
• Motor or sensory deficits
• Severe imbalance disproportionate to symptoms
• New-onset severe headache, particularly occipital
• Any focal neurological signs

Step 2: Characterize the Déjà Vu Component

• Déjà vu in temporal lobe epilepsy is typically intense and prolonged, lasting minutes rather than the fleeting seconds experienced by healthy individuals 7, 1
• The patient may describe feeling as if "living the same life again" during episodes 2
• Isolated déjà vu without other symptoms can occur with serotonergic medications (5-hydroxytryptophan), but the addition of prolonged nausea/vomiting makes medication effect less likely 7

Step 3: Evaluate the Nausea/Vomiting Pattern

The 12-hour duration is critical for narrowing the differential 5, 4:

• Ménière's disease causes episodes lasting 20 minutes to 12 hours maximum, but would include hearing loss, tinnitus, or aural fullness—not déjà vu 5
• Vestibular neuritis causes vertigo lasting 12-36 hours but presents with rotational vertigo, not déjà vu 5
• Cyclic vomiting syndrome episodes last up to 7 days, but 12 hours falls within the typical range 4

Step 4: Assess for Seizure-Specific Features

Mental fogginess is a strong predictor of neurological etiology and may indicate slower recovery from seizure activity 3:

• Postictal confusion typically follows complex partial seizures
• The cognitive symptoms ("brain fog") combined with déjà vu strongly suggest temporal lobe origin 3
• Retrograde and anterograde amnesia should be assessed by asking about memory of events before and after symptom onset 3

Essential Diagnostic Workup

Immediate Evaluation Required

• EEG with sleep deprivation and nasopharyngeal leads to capture temporal lobe epileptiform activity 2
• MRI brain with epilepsy protocol focusing on hippocampal and parahippocampal structures 2
• Basic metabolic panel to exclude metabolic causes of nausea (hypercalcemia, uremia, diabetic ketoacidosis) 8, 9
• Pregnancy test in all women of childbearing age, as pregnancy is the most common endocrinologic cause of nausea 8

Advanced Testing if Initial Workup Negative

• SPECT imaging during an episode can demonstrate hyperperfusion in the entorhinal cortex during déjà vu, confirming seizure activity 2
• Gastric emptying scintigraphy (performed for at least 2 hours) if gastroparesis is suspected, though less likely given the episodic nature 4

Critical Pitfalls to Avoid

Do not dismiss this as anxiety, migraine, or benign vertigo without excluding temporal lobe epilepsy 6, 10:

• The combination of déjà vu and prolonged gastrointestinal symptoms is not typical of migraine, even though migraineurs can experience both phenomena separately 10
• Overlooking subtle neurological signs in the presence of obvious gastrointestinal symptoms is a common error 6
• Before treating as migraine with triptans, exclude other potentially serious neurological conditions, as triptans are contraindicated if cerebrovascular events are occurring 10

Do not attribute déjà vu to normal experience when it occurs with other neurological symptoms 1:

• While déjà vu is common in healthy individuals, it is fleeting (seconds) and isolated 1
• Prolonged or intense déjà vu, especially when stereotypical and recurring, indicates temporal lobe pathology 2, 1

Immediate Management Pending Diagnosis

Symptomatic Treatment for Nausea/Vomiting

• Ondansetron 4-8 mg orally/sublingual is the preferred initial antiemetic for acute nausea 4
• IV normal saline or lactated Ringer's for patients unable to tolerate oral intake 4
• Avoid serotonergic medications until seizure disorder is excluded, as they can exacerbate déjà vu 7

Seizure Precautions

• Advise against driving, swimming, or operating machinery until neurological evaluation is complete
• Document episode frequency, duration, and associated symptoms to establish pattern
• Consider empiric antiepileptic therapy if episodes are frequent and EEG/MRI are delayed, in consultation with neurology

When to Hospitalize

Immediate hospitalization is warranted if 5, 6:

• Any neurological red flags are present (speech difficulties, motor deficits, severe imbalance)
• Inability to maintain hydration due to persistent vomiting
• First-time presentation with this symptom complex requiring urgent diagnostic workup
• Concern for status epilepticus or cluster seizures