Loss of Smell (Anosmia): Causes, Treatment, and the Role of PRP in Singapore

Persistent smell loss — particularly following COVID-19 — occurs because the olfactory receptor neurons are damaged rather than simply blocked, meaning decongestants and nasal sprays are not sufficient. At Absolute ENT in Singapore, Dr Vyas Prasad offers a structured treatment approach including olfactory retraining and Platelet-Rich Plasma (PRP) injection directly into the olfactory niche for patients who have not recovered after six months.

• Anosmia (loss of smell) has many causes including post-viral damage, nasal polyps, head injury, and neurological conditions

• Post-COVID smell loss is now one of the most common presentations

• Smell training is the most evidence-based non-surgical treatment

• Persistent anosmia beyond 6 months warrants specialist assessment to rule out treatable structural causes

In my clinical practice, I have seen a significant rise in patients presenting with persistent loss of smell — particularly in the years following the COVID-19 pandemic. For many, the smell loss began with what seemed like an ordinary viral illness and simply never resolved. Some patients come to me months after the initial infection, having been told to wait and see. Others have tried smell retraining on their own with limited progress. A number describe something arguably worse than not smelling at all: parosmia, where familiar smells have become distorted, unpleasant, or even nauseating.

What strikes me most in these consultations is how profoundly smell loss affects people's lives — in ways that are often invisible to those around them. Food loses its pleasure. Social meals become a source of anxiety. Safety becomes a genuine concern. And for some patients, the psychological toll of a sense that is simply absent, with no visible outward sign, leads to significant distress.

This post explains what we know about smell loss, what assessment involves, and what treatment options are available — including PRP injection, which I have been using in selected patients with encouraging results.

Olfactory nerve fibres under electron microscopy, highlighting the delicate neural structures involved in smell perception

How smell works — and why it is vulnerable

The sense of smell depends on a pathway that begins at the roof of the nasal cavity, where millions of specialised olfactory receptor neurons sit within a small patch of tissue called the olfactory epithelium. These neurons send hair-like projections (cilia) into the nasal airspace, where they detect odour molecules and convert them into nerve signals. Those signals travel through tiny perforations in the skull base (the cribriform plate) to the olfactory bulb, and from there to the brain's smell-processing regions.

This pathway is remarkable in one respect that distinguishes it from most other sensory systems: the olfactory neurons are capable of regeneration. Unlike most neurons in the body, olfactory receptor neurons can regrow from stem cells within the olfactory epithelium. This is why smell loss — even when severe — can sometimes recover, and why treatments aimed at supporting regeneration are a rational therapeutic target.

The vulnerability of this system lies in its exposure. The olfactory epithelium sits directly in the path of inhaled air and the viruses, pollutants, and inflammatory mediators it carries.

Causes of smell loss

Smell loss is not a single condition — it is a symptom with several distinct causes, each with different implications for treatment and recovery.

Post-viral anosmia

Post-viral smell loss is now the most common cause I see in clinic, driven largely by the COVID-19 pandemic but also occurring after influenza, rhinovirus, and other upper respiratory infections. The virus damages the olfactory epithelium — either by directly infecting supporting cells in the nasal lining, by triggering an inflammatory response that disrupts olfactory neuron function, or by causing neurological damage to the olfactory pathway itself.

The degree of damage varies enormously between individuals, which is why some people recover smell within weeks while others remain anosmic for years. Early, complete loss of smell at the time of infection does not necessarily predict a poor outcome — many patients with sudden, total anosmia during acute COVID-19 recovered fully. Persistent loss beyond three months is where outcomes become more variable and specialist assessment is warranted.

Chronic rhinosinusitis and nasal polyps

Persistent nasal inflammation — from chronic sinusitis or nasal polyps — is one of the most common treatable causes of smell loss. In this case, the problem is primarily mechanical and inflammatory: inflamed mucosa and polyp tissue block airflow to the olfactory cleft, and the ongoing inflammation damages the olfactory epithelium over time.

This is an important cause to identify because it responds to treatment. Intranasal and systemic steroids can reduce inflammation and polyp burden, often with meaningful improvement in smell. Functional endoscopic sinus surgery (FESS) to open the sinuses and remove polyps can further restore olfactory function in appropriate patients.

Head trauma

A direct blow to the head — particularly a frontal or occipital impact — can shear the delicate olfactory nerve fibres as they pass through the cribriform plate, causing sudden, complete smell loss. Post-traumatic anosmia is often permanent, though partial recovery is possible in some cases.

It is an important cause to identify as it may have medicolegal implications and benefits from early specialist documentation.

Medications

A number of medications can impair smell as a side effect, including certain antibiotics, antihypertensives, and chemotherapy agents. A careful medication review is part of every assessment for unexplained smell loss.

Neurological and systemic causes

Smell loss can be an early feature of several neurological conditions, including Parkinson's disease and Alzheimer's disease — sometimes preceding other symptoms by years. Certain autoimmune conditions and endocrine disorders (including hypothyroidism) can also affect olfactory function. This is part of why thorough assessment and appropriate imaging are important when smell loss is unexplained.

Ageing

A gradual decline in smell sensitivity is a normal part of ageing (presbyosmia), affecting a significant proportion of adults over 60. While this cannot be fully reversed, it is important to distinguish age-related decline from a potentially treatable cause.

Types of smell disturbance

Not all smell problems are the same, and distinguishing between them guides assessment and treatment.

Anosmia — complete absence of the sense of smell. The patient cannot detect any odours.

Hyposmia — a reduced but not absent sense of smell. Odours are detectable but diminished in intensity.

Parosmia — a distortion of smell perception, where an odour that was previously neutral or pleasant is perceived as unpleasant, offensive, or bizarre. Coffee smelling like burning rubber, cooked meat triggering intense nausea, and previously enjoyed foods becoming intolerable are among the most common descriptions I hear. Parosmia is thought to arise during the recovery phase, when olfactory neurons begin to regenerate but make incorrect connections — mismapping odours to the wrong perceptual qualities.

Parosmia is one of the most distressing smell disturbances to live with, precisely because it actively corrupts previously enjoyed experiences rather than simply removing them. The good news is that parosmia is generally a sign that the olfactory system is attempting to recover, and it often — though not always — resolves over time.

Phantosmia — the perception of smells that are not present (olfactory hallucinations). This can occur in the context of neurological conditions and warrants specific assessment.

Cacosmia — the persistent perception of a bad smell, regardless of environmental odours.

Assessment — what to expect at a specialist appointment

When a patient presents with persistent smell loss, my assessment follows a structured approach:

History The timing, onset, and pattern of smell loss are important. Was it sudden or gradual? Did it follow a viral illness, head injury, or change in medication? Is there associated nasal blockage, discharge, or facial pressure that might suggest sinusitis? Are there symptoms of neurological conditions?

Nasal endoscopy A flexible endoscope passed through the nose allows direct visualisation of the nasal cavity, the olfactory cleft (the narrow passage leading to where the olfactory epithelium sits), and the sinuses. This identifies any structural obstruction — polyps, deviated septum, mucosal swelling — that may be contributing to smell loss.

Psychophysical smell testing Standardised smell tests — using validated odour identification, threshold, and discrimination assessments — objectively quantify the degree of smell impairment and provide a baseline to track improvement over time.

Imaging CT scanning of the sinuses is performed when structural sinus disease is suspected. MRI of the brain and olfactory bulbs is indicated when a central or neurological cause needs to be excluded, or when post-traumatic anosmia is being assessed. MRI can also visualise the olfactory bulb volume, which has prognostic relevance — a smaller olfactory bulb is associated with less likelihood of recovery.

Treatment options

Smell retraining therapy

Smell retraining — also called olfactory training — is the best-evidenced conservative treatment for post-viral and post-infectious smell loss. The principle is based on the olfactory system's capacity for neuroplasticity: repeated, deliberate exposure to specific odours encourages the regenerating olfactory neurons to make correct connections and strengthens the brain's smell-processing circuits.

The standard protocol involves sniffing four distinct odours — typically rose, eucalyptus, lemon, and clove — twice daily for at least four to six months. Each sniffing session involves brief, concentrated attention to the smell while actively trying to recall the memory of what it should smell like. Passive exposure is less effective than engaged, attentive sniffing.

The evidence supports olfactory training as a safe, low-cost, and genuinely beneficial intervention — particularly for patients who start within the first year of smell loss. Results are variable but a meaningful proportion of patients show measurable improvement. The key requirements are consistency and patience; this is a months-long commitment, not a quick fix.

For patients with parosmia, the odours used in training may need to be modified to avoid those that trigger the worst distortions.

Treatment of underlying causes

Where smell loss is caused by chronic rhinosinusitis, nasal polyps, or other treatable structural conditions, addressing the underlying cause is the primary treatment. This may involve:

• Intranasal corticosteroid sprays

• A short course of oral corticosteroids to rapidly reduce inflammation

• Functional endoscopic sinus surgery (FESS) in appropriate cases

• Management of allergic rhinitis contributing to mucosal inflammation

Oral and intranasal steroids for post-viral anosmia

Short courses of systemic corticosteroids are sometimes used in the acute phase of post-viral smell loss to reduce inflammation, though the evidence for benefit beyond the acute period is limited. Topical steroid drops delivered in a head-down position to reach the olfactory cleft have been explored, with modest evidence of benefit in some patients.

Platelet-Rich Plasma (PRP) injection

PRP is an area where I have developed specific experience in the management of persistent post-

viral anosmia, and I want to explain it carefully — both what it is and what we currently know.

What is PRP? Platelet-rich plasma is prepared from the patient's own blood. A blood sample is drawn, centrifuged to concentrate the platelet fraction, and the resulting PRP — rich in growth factors including PDGF, VEGF, EGF, and TGF-β — is used therapeutically. These growth factors play important roles in tissue repair, cell proliferation, and — critically for anosmia — nerve regeneration.

How is it administered for smell loss? The PRP is delivered by injection into the region of the olfactory cleft — the nasal passage that leads to the olfactory epithelium — using endoscopic guidance to ensure accurate placement. The procedure is performed in the clinic under local anaesthetic and takes around 30 to 45 minutes. A course typically involves two to three injections spaced four to six weeks apart.

What is the evidence? PRP for anosmia is an emerging treatment. A small but growing body of published research — including randomised controlled trials — has shown statistically significant improvements in olfactory function in patients with post-infectious smell loss treated with intranasal PRP, compared to control groups. The results are most consistent in patients with post-viral anosmia where conventional treatments have not produced adequate improvement.

It is important to be honest: PRP is not a guaranteed cure. Not every patient responds, and the degree of improvement varies. In my clinical experience, some patients have achieved meaningful and in some cases complete recovery of smell following PRP treatment. Others show partial improvement. A proportion do not respond. What the treatment offers is a biologically rational intervention — using the patient's own growth factors to support the regenerative capacity of the olfactory epithelium — at a point where other options are limited.

Who is a candidate? I consider PRP in patients with persistent post-viral anosmia or hyposmia who have not adequately responded to smell retraining and other conservative measures, typically where smell loss has been present for three months or more. A thorough assessment — including nasal endoscopy, smell testing, and appropriate imaging — is carried out before treatment to confirm suitability.

Realistic expectations and timeline

Recovery from smell loss — by any route — takes time. The olfactory system regenerates slowly, and meaningful improvement may take months to become apparent even when recovery is underway. A few points I discuss with every patient:

• Smell retraining requires a minimum of four to six months of consistent practice to assess its benefit

• Post-viral anosmia can recover spontaneously for up to two years after the initial illness — assessment and treatment do not preclude natural recovery, they support it

• Parosmia, while distressing, often signals active olfactory regeneration and may resolve as the system recalibrates

• PRP effects, where they occur, typically become apparent over six to twelve weeks following the injection course

• Some patients with severe or long-standing anosmia — particularly post-traumatic — have a limited prognosis for full recovery, and managing expectations honestly is part of good clinical care

Frequently asked questions

How long after COVID-19 should I wait before seeking specialist assessment? If smell has not returned within eight to twelve weeks of the acute illness, specialist assessment is appropriate. Earlier assessment is warranted if you have associated nasal symptoms (blockage, discharge) that might suggest a treatable cause, or if the smell loss is significantly affecting your quality of life. There is no benefit in waiting a year before being seen — earlier assessment means earlier treatment.

Is parosmia a sign that my smell is recovering? In most cases, yes. Parosmia is thought to arise when regenerating olfactory neurons reconnect imprecisely, sending incorrect signals. The presence of parosmia — even when it is unpleasant — generally indicates that the olfactory system is biologically active and attempting to recover, which is a better prognostic sign than complete, silent anosmia. For most patients, parosmia gradually improves over months, though the timeline is variable.

Can smell retraining make things worse? No. Smell retraining is safe and has no known harmful effects. The main risk is the time investment — months of consistent practice — and the frustration if progress is slow. Modifying the odours used if parosmia is severe can help make the training more tolerable.

Is PRP covered by insurance in Singapore? PRP injection for anosmia is generally not covered by standard insurance plans and is paid for as a private treatment. Your clinic will advise on the cost of assessment and treatment before you proceed.

I had smell loss years ago — is it too late for treatment? It is never entirely too late to be assessed, though the prognosis for recovery does diminish with the duration of smell loss. The olfactory epithelium has some capacity for regeneration even after prolonged anosmia, and some patients with long-standing loss have shown improvement with treatment. An honest assessment of your specific situation — including MRI of the olfactory bulbs — can help give a more accurate indication of whether recovery is likely.

Are there any foods or supplements that help with smell recovery? Omega-3 fatty acids and alpha-lipoic acid have been investigated as potential adjuncts to smell recovery, with some preliminary positive data. Vitamin A has also been explored. The evidence base for these is not strong enough to make firm recommendations, but they are safe and may be worth considering alongside established treatments. I discuss these with patients on a case-by-case basis.

Seeking assessment in Singapore

If you have been experiencing persistent smell loss — whether following COVID-19, another viral illness, or for any other reason — specialist ENT assessment is the right next step. A thorough evaluation identifies the underlying cause, quantifies the degree of impairment, and allows a personalised treatment plan to be developed.

At my clinic at Camden Medical Centre, 1 Orchard Boulevard, Singapore, I offer full assessment of smell disorders including nasal endoscopy, psychophysical smell testing, and access to PRP treatment for suitable patients.

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