What are the causes of altered taste in adults, particularly those with a history of medical conditions such as diabetes, hypertension, or neurological diseases like multiple sclerosis or Parkinson's disease?

Causes of Altered Taste in Adults

Most Common Etiologies

Altered taste in adults is most commonly caused by medications (particularly through drug-drug interactions), viral upper respiratory infections including COVID-19, and chronic rhinosinusitis, with the critical understanding that most perceived "taste loss" actually reflects impaired retronasal olfaction rather than true gustatory dysfunction. 1, 2

Medication-Induced Causes (Primary Consideration)

• Medications represent the most significant yet underappreciated cause of taste disorders in adults, especially in elderly patients with polypharmacy 3
• Drug-drug interactions that inhibit P-glycoprotein efflux transporters or cytochrome P450 metabolism create supratherapeutic plasma concentrations, inducing taste disturbances through intravascular taste mechanisms 3
• Major drug classes associated with taste alterations include:
  • Antihypertensives (particularly ACE inhibitors and calcium channel blockers) 4
  • Antidepressants (tricyclics and SSRIs) 4
  • Antimicrobials 4
  • Anticholinergics (phenothiazines, tricyclic antidepressants) 5
  • Chemotherapeutic agents (vincristine causes direct neurotoxicity) 5
  • Other medications: baclofen, clonidine, phenytoin, verapamil 5
• AChE inhibitors used in dementia treatment paradoxically increase saliva production, while many antidepressants cause xerostomia (dry mouth), both affecting taste perception 5

Viral and Post-Infectious Causes

• COVID-19 affects 59-86% of infected patients with taste/smell loss, presenting as the initial or sole symptom in 11.9-22% of cases 1
• Post-viral olfactory loss accounts for 11% of community cases and 20-25% of specialist clinic presentations 1
• Upper respiratory infections (including influenza) damage the olfactory neuroepithelium, secondarily affecting taste perception 1
• Most viral-related "taste loss" actually represents loss of retronasal olfaction (flavor perception), not true taste dysfunction, since true taste only differentiates sweet, sour, salty, and bitter 1

Sinonasal and Structural Disorders

• Chronic rhinosinusitis with nasal polyposis causes conduction loss from sinonasal passage obstruction, with CT-documented severity correlating with worse olfaction 1
• Allergic rhinitis and chronic rhinosinusitis (with or without polyps) are among the most common causes of chemosensory dysfunction 2

Neurological and Neurodegenerative Conditions

In patients with diabetes, hypertension, or neurological diseases:

• Diabetes mellitus causes autonomic neuropathy affecting parasympathetic/sympathetic nerves that innervate the gut and can indirectly affect chemosensory pathways 5
• Parkinson's disease is strongly associated with olfactory dysfunction and enteric neuropathy, serving as an early diagnostic marker 5, 1
• Multiple sclerosis can cause enteric neuropathy and affect chemosensory pathways through brainstem lesions or leukoencephalopathy 5
• Alzheimer's disease demonstrates strong association with olfactory dysfunction as an early marker 1
• Head trauma (particularly frontobasal trauma) causes neural damage to olfactory pathways 1
• Brainstem lesions, spinal cord injury, and basal ganglia calcification all affect chemosensory function 5

Cranial Nerve Pathology

• Bell's palsy (acute unilateral facial nerve paralysis) causes taste disturbance from the anterior tongue along with xerostomia 1
• Temporal lobe dysfunction can cause taste hallucinations or delusions 1

Nutritional and Metabolic Causes

• Zinc deficiency is strongly associated with taste disorders, and oral zinc supplementation has been shown to improve taste function 6
• Vitamin B12 deficiency causes neurological symptoms including sensory disturbances affecting taste 7
• Iron deficiency, particularly in patients with restrictive diets, contributes to taste alterations 7
• General malnutrition disturbs the regeneration cycle of chemoreceptors 8

Autoimmune and Inflammatory Causes

• Autoantibodies directed at enteric neurons (voltage-gated potassium channels, acetylcholine receptors, VGCC) occur in both paraneoplastic and non-paraneoplastic disorders 5
• Inflammatory lesions including sarcoidosis and granulomatosis with polyangiitis impair chemosensory function 1

Neoplastic Causes

• Tumors affecting the cribriform plate (squamous cell carcinomas, meningiomas, esthesioneuroblastomas) impair olfaction and secondarily affect taste 1
• Paraneoplastic syndromes from small cell lung cancer, carcinoid tumors, neuroblastoma, and thymoma with anti-Hu antibodies cause enteral neuropathy 5

Toxic and Environmental Exposures

• Lead poisoning represents a rare but reversible cause 5
• Exposure to volatile toxic chemicals in occupational settings causes chemosensory disturbances 9
• Radiation therapy alters chemoreceptor regeneration cycles 8

Age-Related Changes

• Salivary flow rate declines with age, and aging itself represents one of the most common causes of smell disorders 5, 1
• Losses in taste perception and gustatory distortions occur with greater frequency in older individuals 3

Psychiatric Associations

• Depression is associated with olfactory dysfunction, though organic causes must be excluded first 1, 9

Critical Clinical Pitfalls to Avoid

• Never overlook COVID-19 as a potential cause, especially when taste/smell loss presents without traditional nasal congestion or rhinorrhea 1
• Do not rely on patient self-assessment of severity, as objective testing reveals much higher dysfunction rates (98.3% by UPSIT testing versus 35% self-reported) 1
• Avoid ordering unnecessary neuroimaging when there is clear temporal relationship to viral infection 1
• Always review medication lists for drug-drug interactions and consider medication-induced causes before pursuing extensive workup 4, 3
• Do not assume psychiatric origin without excluding organic causes through appropriate neurological examination and imaging 9
• Recognize that stopping offending medications may not immediately resolve symptoms, as full recovery can take several months 8

Diagnostic Approach for Patients with Diabetes, Hypertension, or Neurological Disease

• Perform comprehensive medication review focusing on antihypertensives, antidepressants, and anticholinergics 4, 3
• Assess for autonomic neuropathy in diabetic patients 5
• Evaluate for progression of underlying neurological disease (Parkinson's, MS) 5, 1
• Check nutritional status including zinc, B12, and iron levels 7, 6
• Examine nasal passages for chronic rhinosinusitis or polyps 2
• Assess salivary flow rate, as xerostomia from medications or autonomic dysfunction significantly affects taste 5
• Consider neurological examination focusing on cranial nerves I, VII, IX, and X 2