The olfactory nerve is the most commonly skipped cranial nerve and the one that, when an abnormality is found, most often catches an examiner by surprise. It is not part of routine testing in most settings — and that is a reasonable choice when there is no specific suspicion. But there are a few situations in which the olfactory exam reaches up and changes a differential entirely, and those are the situations a neurologist has to recognize. A patient with frontal lobe symptoms in whom a meningioma of the olfactory groove or planum sphenoidale is hiding. A patient with early Parkinson disease or dementia with Lewy bodies, in whom hyposmia precedes motor symptoms by years. A patient with a head injury whose persistent dysosmia tells you the cribriform plate was sheared. A patient with a temporal lobe seizure whose ictal aura is a foul or strange smell. When any of these is on the differential, the olfactory exam moves from skippable to mandatory.

The clinical message: routine testing of CN I in every patient is unnecessary; knowing when to test it is the entire point.

Functional Anatomy

Olfaction is the only sensory modality that bypasses the thalamus on its way to cortex. Odorant molecules dissolve in the mucus of the olfactory epithelium in the roof of the nasal cavity. Bipolar olfactory receptor neurons send their distal processes into the mucus and their proximal processes — which together constitute the olfactory nerves — through the cribriform plate of the ethmoid into the cranial vault. There they synapse in the olfactory bulb, which sits on the inferior surface of the frontal lobe. Mitral and tufted cells of the bulb send their axons in the olfactory tract directly to several primary olfactory cortical areas, principally the piriform cortex, the entorhinal cortex, parts of the amygdala, and the olfactory tubercle. Only from there does information reach the thalamus and then the orbitofrontal cortex for conscious perception.

Two anatomical facts shape clinical practice:

  • The cribriform plate is a fragile, fenestrated bone, and the olfactory nerves are tethered to it. Even minor head injury — a forehead strike against a steering wheel — can shear the filaments and produce immediate, often permanent, anosmia.
  • The olfactory bulb sits on the inferior frontal lobe, in the path of any mass arising from the floor of the anterior cranial fossa. An olfactory groove meningioma can grow silently for years, presenting with isolated anosmia long before frontal lobe symptoms appear. The classical Foster Kennedy syndrome — ipsilateral anosmia, ipsilateral optic atrophy, and contralateral papilledema — describes such a mass compressing both the olfactory bulb and the ipsilateral optic nerve, with contralateral papilledema from raised intracranial pressure.

The Examination

What to Use

Test each nostril separately. Use a non-noxious odorant: coffee grounds, peppermint, cloves, or one of the specifically designed smell identification tests (UPSIT, Sniffin’ Sticks). Do not use ammonia, alcohol prep pads, or vinegar. These stimulate trigeminal nociceptors in the nasal mucosa rather than the olfactory receptors, and a patient with no functioning CN I will still recoil from them. Using a trigeminal irritant is one of the most common errors in this exam.

How to Test

Have the patient close their eyes. Occlude one nostril with finger pressure on the side of the nose. Bring the odorant to the open nostril and ask the patient first to detect that something is there, then to identify it. Test detection and identification independently — patients with mild hyposmia may be able to detect odors but unable to name them. Repeat with the other nostril.

Documentation should specify both detection and identification, by nostril:

  • Normosmia: detects and correctly identifies a familiar odor.
  • Hyposmia: detects with effort, or fails identification.
  • Anosmia: fails to detect at all.
  • Parosmia: detects but with a distorted, often unpleasant character.
  • Phantosmia: perceives an odor that is not present (an olfactory hallucination).

Patterns and Their Meaning

Bilateral Anosmia

The most common cause of bilateral anosmia is sinonasal disease — chronic rhinosinusitis, nasal polyposis, allergic rhinitis. These are not neurological. Any patient with bilateral anosmia therefore deserves a careful look at the nose first, including a question about congestion, allergies, prior surgery, and nasal sprays. After viral upper respiratory infection, post-infectious anosmia (made famous by COVID-19, but a recognized post-viral syndrome long before) can persist for months and sometimes permanently.

Once nasal disease and recent viral infection are excluded, the neurological differential becomes more focused. Head trauma is the most common cause of post-traumatic bilateral anosmia — patients often do not recall the injury as significant. Toxic exposures (chronic cocaine use, certain organic solvents, some chemotherapeutics) and neurodegenerative disease are the other principal categories.

The neurodegenerative association is the most clinically important point. Hyposmia is one of the earliest and most reliable prodromal features of Parkinson disease, predating motor symptoms by years or even decades. The same is true for dementia with Lewy bodies and, to a lesser extent, Alzheimer disease. A patient who has lost smell years before they develop a tremor or memory complaints, and who has no nasal pathology to explain it, is at substantially elevated risk. In contrast, hyposmia is not characteristic of progressive supranuclear palsy, corticobasal degeneration, multiple system atrophy, or essential tremor — its presence helps distinguish PD from these mimics.

Unilateral Anosmia

Unilateral anosmia is far more specific to neurological disease than bilateral. It implies a problem upstream of the cribriform plate on one side. The differential is narrow: a mass on the floor of the anterior cranial fossa (olfactory groove meningioma, planum sphenoidale meningioma, frontal lobe tumor pressing inferiorly), traumatic shearing of the olfactory filaments on one side, or rarely, an unilateral congenital anomaly. Unilateral anosmia without an obvious cause warrants neuroimaging, with particular attention to the inferior frontal region and the anterior cranial floor.

Parosmia and Phantosmia

Distortion or hallucination of smell points away from a peripheral problem and toward central involvement. Parosmias are common after viral infection, in early Parkinson disease, and in head injury — typically familiar smells become unpleasant or perverse. Phantosmias of foul, burning, or chemical odors are characteristic of temporal lobe seizures: the so-called uncinate seizure, named for the involvement of the uncus and amygdala, produces brief stereotyped olfactory hallucinations, often with a sense of dread and sometimes followed by automatisms. A patient describing recurrent brief episodes of an unpleasant smell that comes from nowhere is having focal aware seizures until proven otherwise.

🔍 Did You Know?

Hyposmia precedes the motor symptoms of Parkinson disease by an average of four to six years, and a brief smell identification test in a research cohort has predictive value for incident Parkinsonism that rivals dopamine transporter imaging. The clinical implication is that olfactory testing is not a curiosity in early Parkinson disease — it is one of the most sensitive prodromal markers available.

Pitfalls and Pearls

  • Do not use ammonia or alcohol pads. They test trigeminal nociception, not olfaction. A patient with a totally non-functional CN I will still flinch.
  • Always check both nostrils separately. Bilateral and unilateral findings have entirely different differentials.
  • Always check nasal patency first. A congested nose makes the rest of the test uninterpretable.
  • Routine CN I testing is unnecessary. Test in specific clinical scenarios: head injury, frontal lobe complaints, suspected early Parkinson disease, suspected dementia with Lewy bodies, suspected uncinate seizures, or a vague visual complaint where the floor of the anterior fossa is on the differential.
  • Functional anosmia is rare and characteristically clean. Patients who report total loss of all smell and taste, but who detect and respond appropriately to trigeminal irritants, may be exaggerating or feigning. The clinical context matters: secondary gain (litigation, disability claims) raises the index of suspicion.
  • Loss of smell impairs flavor, not taste. Patients often describe “loss of taste” when they mean loss of flavor. True taste (CN VII chorda tympani and CN IX) is sweet, sour, salty, bitter, and umami — these are preserved in pure anosmia, but flavor (the complex perception combining smell and taste) is profoundly impaired.

Bringing It Together

The olfactory nerve rewards selective testing. Most patients do not need a CN I exam; some patients need it desperately. The neurologist’s job is to recognize the second group — the head injury patient, the patient with early extrapyramidal complaints, the patient with frontal lobe signs of unknown cause, the patient whose seizures begin with a foul smell. A bottle of coffee in the drawer of the exam table will, on the right day, change a differential more decisively than an MRI.

References

  1. Campbell WW. DeJong’s The Neurologic Examination. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2013. Chapter 12.
  2. Doty RL. Olfactory dysfunction in Parkinson disease. Nat Rev Neurol. 2012;8(6):329-339.
  3. Hawkes CH, Doty RL. The Neurology of Olfaction. Cambridge University Press; 2009.
  4. Berg D, Postuma RB, Adler CH, et al. MDS research criteria for prodromal Parkinson’s disease. Mov Disord. 2015;30(12):1600-1611.
  5. Boesveldt S, Postma EM, Boak D, et al. Anosmia — a clinical review. Chem Senses. 2017;42(7):513-523.
  6. Hummel T, Whitcroft KL, Andrews P, et al. Position paper on olfactory dysfunction. Rhinol Suppl. 2017;54(26):1-30.