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Disorders of Smell andTaste

Chapter 35 | Part 2: Disorders of Smell and Taste · Part 2 – Cardinal Manifestations & Presentation · Chapter 35

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Key Clinical Points

1. Olfactory dysfunction is a frequent early indicator of neurodegenerative diseases (e.g., Parkinson's, Alzheimer's).
2. Head trauma causes smell loss via shearing/scarring of olfactory fila at the cribriform plate; severity correlates with Glasgow Coma Scale.
3. Over 250 medications can impair taste; common culprits include antineoplastic agents, antibiotics, and blood pressure meds.
4. Presbyosmia (age-related smell loss) affects >50% of people aged 65–80 and 75% of those ≥ 80 years.
5. Most 'taste' complaints are actually olfactory issues due to retronasal stimulation during deglutition.
6. Taste is mediated by CN VII (anterior tongue/soft palate), CN IX (posterior tongue), and CN X (epiglottis/larynx).
7. CN V provides somatosensory information (touch, burning, cooling) but is not a primary gustatory nerve.
8. Zinc supplementation may improve taste in hepatic deficiency but its efficacy for viral-induced anosmia is controversial.
9. COVID-19 induced smell loss is typically independent of nasal inflammation.
10. Topiramate causes reversible loss of ability to detect/recognize tastes and odors.

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DEFINITION & CLASSIFICATION

• Olfaction: Monitoring of environmental chemicals for survival, flavor/palatability, and warning of danger. • Gustation: Detection of basic tastes (sweet, sour, bitter, salty, umami).

Definition (Harrison's 22e): Anosmia (total loss), hyposomnia (reduced ability), or dysosmia (distorted smell).

Definition (Harrison's 22e): Dysgeusia refers to altered taste perception.

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EPIDEMIOLOGY

• Demographics: Women typically outperform men on olfactory tests and retain function longer. • Prevalence: ◦ General population: 13.5% (NHANES 2013–2014). ◦ Age-related (Presbyosmia): → Age 65–80 → >50% prevalence. → Age ≥ 80 → 75% prevalence. • Clinical Impact: Presbyosmia leads to nutritional disturbances and increased risk of accidental gas poisoning.

UPSIT Norms (Table 35-1)

• Trend: High scores (99th percentile) for both sexes in younger cohorts; significant decline observed in older populations. ◦ Note: Table 35-1 provides specific data points showing consistent performance across ages until the 60s, where variance increases.

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ETIOLOGY & PATHOPHYSIOLOGY

• Common Causes: 1. Severe upper respiratory infections (common cold, influenza, pneumonia, HIV, COVID-19). 2. Head trauma. 3. Chronic rhinosinusitis.

• Mechanism of Trauma Loss: ◦ Shearing and scarring of olfactory fila at the cribriform plate. ◦ Severity ∝ poor Glasgow Coma Scale score + length of posttraumatic amnesia. ◦ Recovery: <10% recover full function; ~25% recover if loss is not total.

• Mechanism of Infection Loss: ◦ Direct/permanent damage to olfactory epithelium → reduced receptor cell count, damaged cilia, or replacement with respiratory epithelium. ◦ COVID-19: Loss often independent of nasal inflammation; 30% fail to recover after one year.

Neurodegenerative Diseases

• Early Indicators: Olfactory impairment often predates clinical diagnosis by years. ◦ Parkinson's Disease (PD): Potential first site of damage in olfactory bulbs and dorsomotor nucleus of the vagus. ◦ Alzheimer's Disease (AD): Poor smell linked to higher levels of AD-related pathology even in pre-symptomatic stages. ◦ Dementia with Lewy Bodies (DLB): More marked early impairment than in mild AD. ◦ Narcolepsy: Linked to loss of hypothalamic neurons expressing orexin; intranasal orexin A can improve function. ◦ Exceptions: Minimal/nonexistent in progressive supranuclear palsy and MPTP-induced parkinsonism.

Medications and Systemic Factors

• Drug Impact: >250 medications affect taste; common culprits include antineoplastic agents, antirheumatic drugs, antibiotics, and blood pressure meds. ◦ Terbinafine: Linked to taste disturbance lasting up to 3 years. ◦ Eszopiclone (Lunestar): Causes bitter dysgeusia (more severe in women). ◦ Topiramate: Results in reversible loss of ability to detect/recognize tastes and odors. ◦ Zinc: Used for some conditions; efficacy for viral anosmia is controversial.

• Systemic Conditions: Chronic renal failure, end-stage liver disease, vitamin/mineral deficiencies, diabetes mellitus, and hypothyroidism.

• Pregnancy: ◦ 1st Trimester → increased dislike/intensity of bitter tastes (avoidance of poisons). ◦ 2nd & 3rd Trimesters → increased preference for salt and bitter (electrolyte balance).

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CLINICAL FEATURES

• Symptom Presentation: ◦ Anosmia, hyposmia, dysosmia. ◦ Dysgeusia. ◦ Metallic sensations (often from CN VII damage). ◦ Burning mouth syndrome (glossodynia/glossalgia) → likely linked to CN V dysfunction.

• Mechanisms of Taste Loss: 1. Oral cavity: Foul-tasting materials from gingivitis, sialadenitis, or appliances. 2. Transport: Drying, infections, or inflammation of orolingual mucosa. 3. Taste Buds: Local trauma or invasive carcinomas. 4. Neural Pathways: e.g., middle ear infections affecting CN pathways. 5. Central Structures: MS, tumor, epilepsy, stroke. 6. Systemic: Diabetes, thyroid disease, medications.

Cranial Nerve Involvement

• CN VII (Facial): Anterior tongue and soft palate; damage from mastoidectomy/tympanoplasty can cause metallic sensations. ◦ Bell's palsy is a common cause of CN VII-related taste disturbance. ● CN IX (Glossopharyngeal): Posterior tongue; relatively protected but susceptible to tonsillectomy or radiation. ● CN X (Vagus): Laryngeal surface of epiglottis, larynx, and proximal esophagus.

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DIFFERENTIAL DIAGNOSIS

• Table 35-1: Disorders Associated with Compromised Olfactory Function * Endocrine/Metabolic: Addison's, Cushing's, Diabetes, Hypothyroidism, Liver/Renal disease, Pregnancy. * Immune-Related: SLE, MS, Rheumatoid arthritis, Sjögren's, Cystic fibrosis. * Nasosinus: Rhinosinusitis, Laryngopharyngeal reflux, Upper respiratory infections. * Neurologic: Alzheimer's, Parkinson's, ALS, Huntington's, Stroke, Head trauma, Epilepsy. * Infections: COVID-19, HIV, Hepatitis C, Candidiasis, Lyme disease. * Other/Factors: Alcoholism, Chemical exposure, Nutritional deficiencies (Vitamin B), Smoking. * Psychiatric: Depression, Schizophrenia, OCD, PTSD.

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DIAGNOSTIC APPROACH

1. Clinical History: Determine nature, onset, duration, and pattern of fluctuations.
2. Temporal Analysis: → Sudden loss → Head trauma, ischemia, infection, or psychiatric condition. → Gradual loss → Progressive obstructive lesion (or post-traumatic). → Intermittent loss → Inflammatory process.
3. Risk Factor Screening: → Trauma history → assess severity via GCS and duration of amnesia. → Substance use → alcohol, nicotine, cocaine. → Medication review → identify agents like Topiramate or Terbinafine.
4. Comorbidity Assessment: Evaluate renal failure, liver disease, hypothyroidism, diabetes, and dementia.
5. Developmental Markers: → Delayed puberty + anosmia + craniofacial abnormalities → suspect Kallmann's syndrome.

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MANAGEMENT & TREATMENT

1. Pharmacologic Intervention: • Topiramate: Used for taste/smell issues (note: effect is reversible). • Zinc: Supplementation for hepatic deficiency; use in viral anosmia is controversial.
2. Supportive Care: • Address underlying causes (e.g., treating rhinosinusitis with steroids, though this may only provide short-term improvement).

Clinical Monitoring

• Identify and manage specific triggers such as infections or medication side effects.

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PROGNOSIS & COMPLICATIONS

• Trauma Recovery: → <10% of posttraumatic anosmic patients recover full function. → ~25% of those with less-than-total loss recover some function. • Infection Persistence: → Up to 30% fail to regain normal olfactory function one year after COVID-19 diagnosis. → 5–10% experience total loss post-infection.

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SPECIAL POPULATIONS

• Pregnancy: → 1st Trimester: Increased aversion to bitter tastes. → 2nd/3rd Trimesters: Increased preference for salt and bitter (electrolyte support).

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KEY PEARLS & HIGH-YIELD POINTS

• Olfactory vs. Taste: Most 'taste' complaints are actually olfactory because flavor depends on retronasal stimulation. ● Neural Pathway: Olfaction is unique in bypassing the thalamus to reach the primary olfactory cortex (piriform and entorhinal). ● Cranial Nerves: Taste involves CN VII, IX, and X; CN V provides only somatosensory input. ● Presbyosmia: Significant impact on nutrition and safety in patients ≥ 80 years old.

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Reference Tables

Table 35-1 identifiable causes in the clinic are, in

Harrison's 22e, p.240

	5–9	10–14	15–19	20–24	25–29	30–34	35–39 40–44	45–49 50–54	55–59	60–64	65–69	70–74	75–79	80–84	≥85
40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 N =	99	99	89	99	99	99	99 99	99 99	99	99	99	99	99	99	99
	98	94	77	90	89	85	87 91	90 95	93	97	97	99	99	99	99
	94	81	59	68	73	70	64 75	70 89	82	92	93	95	98	99	99
	93	71	44	50	54	53	NNOORRMM 50 75	OOSSMMIIAA 58 73	70	78	83	90	95	99	99
	90	59	35	31	37	37	36 43	49 60	56	66	71	83	87	99	95
	83	52	26	22	23	25	27 26	31 41	42	56	64	77	74	97	95
	80	44	17	16	15	18	19 21	22 33	33	47	54	74	65	93	89
	76	32	11	14	10	11	12 17	15 30	27	34	44	66	53	85	86
	69	28	08	11	07	10	09 13	11 21	23	26	44	62	44	78	84
	63	19	06	07	05	08	06MMIILLDD1 1MMII	CCRRO1O0SSMMIIAA19	23	22	41	58	35	71	81
	59	15	05	06		07	05 08	10 18	23	21	36	52	35	65	81
	49	12		05		05	06	07 14	19	15	31	48	34	63	75
	45	10					06	06 14	18	15	31	42	32	63	74
	36	05					MMOODDEERR0AA6TTEE	MMII0CC5RROOSS1MM0IIAA	18	13	27	36	31	56	68
	34						05	08	12	13	24	34	29	50	63
	29							08	12	13	20	30	27	47	60
	25							08	11	13	20	25	27	46	54
	23							06	11	13	17	23	24	43	47
	21						SSEEVVEERREE MM	IICCRROOSSMMIIAA 05	11	11	15	23	25	40	44
	17								07	11	10	18	23	40	44
	17								05	11	08	16	21	34	44
	15									11	07	14	18	31	42
	10									11	07	13	15	28	40
	06									09	07	13	11	25	40
	05									05	05	10	11	24	37
												08	10	19	32
												08	06	18	32
												08 06	05	16 10	28 25
							AANNOO	SSMMIIAA				05		06	21
														05	12
															07
															05
						PP	RROOBBAABBLLEE MM	AALLIINNGGEERRIINN	GG
	126	145	197	148	186	160	129 103	81 80	73	68	59	77	62	68	57