The US FDA’s proposed rule on laboratory-developed tests: Impacts on clinical laboratory testing

Hypogeusia can be defined as the reduced ability to taste things.^[1] Due to a lack of stratification, the prevalence of hypogeusia, as well as hyposmia, may not be accurately known.^[2] Additionally, reviews do not always make distinctions between ageusia and hypogeusia, often classifying them as the same in certain circumstances and studies.^[3] The severity of the loss of taste from hypogeusia is not clearly outlined in current research due to these reasons.

Covid-19 causes symptoms that affect the central nervous system (CNS), peripheral nervous system (PNS), and skeletal muscle. Hypogeusia falls under a neurological disease and a PNS symptom, while also being the highest occurring PNS symptom, closely followed by anosmia. Due to hypogeusia being a significant symptom of Covid-19, it is often accompanied by hyposmia, even when many other Covid-19 symptoms are absent. Both can be considered early indications of a Covid-19 infection.^[1] Further, hypogeusia is often developed following early symptoms of hyposmia, usually developed from olfactory epithelium damage from upper respiratory infections.^[4]

Hypogeusia tied to oral cancer and tumors can affect sweet, sour, salty, and bitter tastes, but bitter taste hypogeusia occurs significantly more often compared to the rest of the tastes. Inhibition of gustatory papillae found in the base, often due to oropharyngeal tumors, is thought of to be the cause of this. Oral cancer treatments, such as chemotherapy, radiation therapy, and surgical treatments, are further causes of taste and smell loss with up to 70% of oral cancer patients noting dysgeusia. Specifically, chemotherapies and radiation treatments may impair or damage various taste related cells, and certain surgeries may even remove minor to major parts of the tongue depending on the severity of the tumor.^[5]

Nutritional zinc deficiency may cause various problems, hypogeusia being one of them.^[6] Chronic Rhinosinusitis (CRS) may cause olfactory dysfunction as well as gustatory problems, with either or both leading to the noticeable presence of hypogeusia in CRS patients.^[7] The connection between hypogeusia and Parkinson's Disease is less well described. PD patients have an increased dysregulation in their taste receptors, as well as their olfactory receptors. The receptors affected in PD patients were those associated with the perception of bitterness in most cases.^[3]

Hypogeusia tied to Covid-19 can serve as an indicator of Covid-19, which can allow appropriate treatments to be administered earlier to patients.^[1]

When treating oral cancer and related tumors, there is no clear treatment for hypogeusia. Precautions need to be studied and taken to prevent hypogeusia and related symptoms from forming. However, if the treatments have led to the formation of hypogeusia, than patient specific nutrition plans may be used to treat the loss of taste.^[5]

While zinc supplementation may treat certain taste dysfunctions, there is a lack of evidence for treatment regarding hypogeusia and dysgeusia not caused by low zinc concentrations in the body.^[6] While the mechanisms surrounding hypogeusia from PD are hypothesized, specific treatments are not researched enough.^[3] Similarly, while treatment of olfactory related issues is known in CRS research, the treatment of gustatory problems, including hypogeusia, are unknown.^[7]