Vasovagal Syncope or Vasovagal Response: This condition is characterized by a reflex of the involuntary nervous system that can cause nausea, among other symptoms like fainting or near-fainting, often triggered by stress or certain sights. The absence of other symptoms makes this a plausible explanation for waves of nausea.

Anxiety or Panic Disorder: Anxiety and panic attacks can manifest with nausea as a primary symptom, especially if the individual is experiencing significant stress or anxiety without other overt symptoms.
Gastroesophageal Reflux Disease (GERD): Although typically associated with heartburn, some individuals with GERD may experience nausea without the classic symptoms of reflux.
Motion Sickness: This could be a consideration if the nausea is related to travel or movement, even if other typical symptoms like vomiting are not present.

Myocardial Infarction (Heart Attack): Nausea can be a presenting symptom of a heart attack, especially in women, and it is crucial not to miss this diagnosis due to its high mortality rate if untreated.
Pulmonary Embolism: Similar to myocardial infarction, pulmonary embolism can present atypically with nausea, and missing this diagnosis can be fatal.
Intracranial Hemorrhage: Although less common, nausea can be a symptom of increased intracranial pressure or hemorrhage, making it a critical diagnosis not to overlook.

Cyclic Vomiting Syndrome: A rare condition characterized by recurrent episodes of severe nausea and vomiting, which could present with nausea waves without vomiting.
Chiari Malformation: A structural defect in the cerebellum that can cause a variety of symptoms, including nausea, especially with certain head movements or positions.
Superior Mesenteric Artery Syndrome: A rare condition where the duodenum is compressed between the abdominal aorta and the superior mesenteric artery, leading to nausea and vomiting, among other symptoms.