Forgotten patient:

An individual known to healthcare providers with persistent medical issues whose problems may reasonably be considered as not being actively addressed.

Overlooked diseases:

A pathological process for which there is limited interest on the part of healthcare professionals in prevention, mitigation or cure.
Medically unexplained symptoms (MUS): where no disease can be found after appropriate physical examination and investigations. This does not mean the absence of pathophysiological process; rather current testing has not identified a cause. Not understanding the cause can make them distressing and difficult to cope with. Where symptoms persistent or are more complex, many doctors now prefer to use terminology other than MUS. 

Persistent physical symptoms (PPS):

An umbrella term for distressing undiagnosed physical complaints such as pain, weakness or shortness of breath, that last several months or more, regardless of their cause.  

Persistent somatic symptoms (PSS - considered by some to be synonymous with PPS):  

Physical symptoms of emotional distress are called somatic symptoms. A person is not diagnosed with somatic symptom disorder solely because a medical cause can or can’t be identified for a physical symptom. The emphasis is on the extent to which the thoughts, feelings and behaviours related to the illness are excessive or out of proportion.3  This is not ‘faking’, rather the person is experiencing symptoms and believes they are sick. PSS are associated with substantial disability and represent a major burden for patients, health-care professionals, and society. 

Causes of persistent physical symptoms:
Persistent physical symptoms can follow infections, injuries, medical diseases, stressful life events, or arise de novo. As symptoms persist, their link to clearly identifiable pathophysiology often weakens, making diagnosis and treatment challenging. 
Multiple biological and psychosocial risk factors and mechanisms contribute to the persistence of somatic symptoms, including persistent inflammation; epigenetic profiles; immune, metabolic and microbiome dysregulation; early adverse life experiences; depression; illness-related anxiety; dysfunctional symptom expectations; symptom focusing; symptom learning; and avoidance behaviours, with many factors being common across symptoms and diagnoses.

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Unbearable Unexplained Complaints (UUC):

“A sense of unease and helplessness on behalf of the physician when faced with a patient who repeatedly presents with serious complaints, but no cause can be found. The physician cannot shake off the thought that a diagnosis is being missed. 'It just can't be true that there's nothing wrong with this patient. What am I overlooking? Something unusual, something rare?''

This definition provided by the Witte Raven (van Leeuwen et al. 2019), reflects the sentiment or feeling often generated by a patient who does not complain unnecessarily yet no specialist has been able to provide a satisfactory explanation. “UUC could represent the first step in a challenging search, an analytical process that might eventually lead to a diagnostic solution, or at least point toward a more targeted referral.” 

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