Sinusitis

Sinusitis, also known as rhinosinusitis, is inflammation of the tissue lining the sinuses — a network of four paired, air-filled cavities within the bones of the face and skull that connect to the inside of the nose. When this lining becomes inflamed, whether from infection or another trigger, it swells and produces excess mucus, blocking the small openings through which the sinuses normally drain into the nose. Trapped mucus and blocked drainage are what produce the characteristic congestion, facial pressure and discoloured discharge of sinusitis.

Sinusitis is extremely common, affecting both adults and children, and most cases follow a viral upper respiratory infection and resolve on their own within one to two weeks. However, sinusitis is not a single, uniform illness — it ranges from a short-lived viral irritation to a bacterial infection requiring antibiotics, to a chronic inflammatory condition lasting months or years, each with a different underlying process and treatment approach.

Understanding the Sinuses: The Anatomy Behind the Condition

There are four paired sinuses, together known as the paranasal sinuses, each named for the facial bone in which it sits: the maxillary sinuses (in the cheekbones, the largest and most commonly affected), the frontal sinuses (in the forehead), the ethmoid sinuses (a honeycomb-like cluster of small air cells between the eyes), and the sphenoid sinuses (deep within the skull, behind the eyes and nose). Each sinus drains into the nasal cavity through a small opening called an ostium.

These ostia are narrow — often only a few millimetres wide — which means it takes only a small amount of swelling in the surrounding tissue to block them completely. Once an ostium is blocked, mucus that would normally drain away instead accumulates within the sinus, creating a low-oxygen, stagnant environment in which bacteria can multiply far more readily than usual. This single anatomical detail — how easily a narrow drainage opening can be blocked — explains why sinusitis develops in the first place and why it can persist or recur so readily in people with anything that narrows these passages further, such as nasal polyps or a deviated septum.

Classifying Sinusitis: Why Duration Matters

One of the most clinically important distinctions in sinusitis is simply how long it has lasted, since duration strongly influences both the likely cause and the right treatment approach.

Acute Sinusitis: Symptoms lasting under four weeks. The great majority of acute sinusitis is caused by a virus (essentially an extension of the common cold) and resolves on its own; only a minority of cases are bacterial and require antibiotics, discussed further below.
Subacute Sinusitis: Symptoms lasting between four and twelve weeks — an intermediate category that has not resolved as quickly as typical acute sinusitis but has not yet met the definition of chronic disease.
Chronic Sinusitis: Symptoms lasting twelve weeks or longer. Chronic sinusitis reflects a fundamentally different, ongoing inflammatory process rather than a single infection that simply hasn't cleared, and is often associated with underlying factors such as nasal polyps, allergies, or structural nasal issues.
Recurrent Acute Sinusitis: Four or more distinct episodes of acute sinusitis within a year, with complete resolution of symptoms between episodes. This pattern often prompts investigation for an underlying anatomical or immune-related cause.

Viral or Bacterial? Why the Distinction Matters

Because the great majority of acute sinusitis is viral and resolves without antibiotics, being able to distinguish it from the minority of cases that are bacterial is important — both to avoid unnecessary antibiotic use and to ensure bacterial infections are treated appropriately.

Suggestive of viral sinusitis: Symptoms lasting under 10 days that are gradually improving are almost always viral, essentially representing an extension of a common cold into the sinuses. These cases are managed with supportive care alone, and antibiotics offer no benefit.
Suggestive of bacterial sinusitis: Two patterns in particular point toward a bacterial cause: symptoms that persist without improvement for 10 days or more, or symptoms that initially start to improve after a typical cold, then suddenly worsen again around day 5 to 6 (sometimes called "double worsening") — a pattern that suggests a secondary bacterial infection has taken hold. Severe symptoms from the outset, including high fever and significant facial pain, also raise suspicion for a bacterial cause.

This distinction is one of the most important, and most frequently misunderstood, aspects of sinusitis care — thick or discoloured (yellow or green) nasal discharge on its own does not necessarily mean a bacterial infection is present, as this is also common in viral sinusitis, and is not, by itself, a reliable reason for antibiotics.

Chronic Sinusitis: With and Without Nasal Polyps

Chronic sinusitis is further divided into two distinct subtypes, which behave differently and can respond differently to treatment, particularly when medical therapy alone is not enough.

Chronic Rhinosinusitis without Nasal Polyps (CRSsNP): The more common subtype, in which chronic inflammation causes swelling and thickened mucus production without the growth of nasal polyps. Often closely linked to persistent nasal allergies or structural issues affecting drainage.
Chronic Rhinosinusitis with Nasal Polyps (CRSwNP): Chronic inflammation leads to the formation of nasal polyps — soft, painless, non-cancerous growths arising from the lining of the nose and sinuses. Polyps can progressively block nasal airflow and sinus drainage, often causing more pronounced loss of smell than CRSsNP, and this subtype is frequently associated with asthma and aspirin sensitivity as part of a recognised symptom cluster.

This distinction matters practically because CRSwNP tends to respond less predictably to standard treatment and more often requires escalation to nasal steroid therapy, biologic medication, or surgery, whereas CRSsNP is more likely to be controlled with saline irrigation and topical treatment alone.

Symptoms of Sinusitis

Symptoms of sinusitis can vary depending on which sinuses are affected and whether the condition is acute or chronic, but a core set of features is common to most cases.

Thick, discoloured (yellow or green) nasal mucus
Postnasal drip, with mucus running down the back of the throat
Nasal congestion, making it hard to breathe through the nose
Tenderness, pressure or swelling around the eyes, cheeks, nose or forehead, which may worsen when bending forward
Reduced sense of smell or taste
Cough, which may worsen at night as postnasal drip pools while lying down
Bad breath or a bad taste in the mouth
Headache, ear pressure, fatigue, and in acute cases, fever

In chronic sinusitis, symptoms tend to be less severe at any single point in time but more persistent, and reduced sense of smell is often more prominent, particularly when nasal polyps are present.

What Causes Sinusitis

Sinusitis develops when something triggers swelling of the sinus lining severe enough to block normal drainage; the underlying triggers vary between acute and chronic cases.

Viral upper respiratory infections: By far the most common cause of acute sinusitis, as viruses that cause the common cold (including rhinovirus and influenza) inflame the same lining that continues into the sinuses.
Bacterial infection: When a viral infection or persistent blockage allows bacteria — commonly including Streptococcus pneumoniae — to multiply within trapped, stagnant mucus, a secondary bacterial infection can develop, usually as a complication of an initial viral illness rather than a standalone cause.
Allergies: Airborne allergens such as dust mites, pollen and pet dander trigger swelling of the nasal and sinus lining in people with allergic rhinitis, which can block sinus drainage and contribute to both acute flares and chronic sinusitis.
Fungal causes: In people with a weakened immune system, or in certain chronic sinus conditions, fungal organisms can colonise or infect the sinuses, ranging from a relatively mild allergic reaction to fungus in the mucus, to rare but serious invasive fungal infection requiring urgent treatment.
Nasal polyps: Non-cancerous growths arising from chronically inflamed sinus lining can progressively block drainage pathways, both resulting from and further worsening chronic sinusitis.
Deviated septum: A shift in the wall between the two nostrils can narrow one side of the nasal passage, impairing normal drainage and predisposing to recurrent or chronic sinusitis on that side.

Risk Factors for Sinusitis

While anyone can develop sinusitis, certain factors make it more likely, particularly for the condition to become chronic or recurrent.

Nasal allergies: Ongoing inflammation from unmanaged allergic rhinitis keeps the nasal and sinus lining swollen, predisposing to blocked drainage.
Asthma: Asthma and chronic sinusitis frequently occur together, sharing overlapping patterns of airway inflammation, and each can make the other harder to control.
Nasal polyps: Existing polyps physically narrow or block sinus drainage pathways, and increase susceptibility to further sinusitis.
Deviated septum: A structurally narrowed nasal passage on one side impairs drainage and predisposes to sinusitis on that side.
A weakened immune system: Conditions or medications that suppress immune function reduce the body's ability to clear both viral and bacterial sinus infections effectively.
Smoking: Tobacco smoke irritates and impairs the normal function of the delicate lining of the nose and sinuses, increasing susceptibility to infection and inflammation.

How Sinusitis is Diagnosed

Most acute sinusitis is diagnosed based on symptoms and a physical examination alone, without the need for imaging. Further investigation is generally reserved for chronic, recurrent, or atypical cases, or when complications are suspected.

Clinical History & Examination: A review of symptom duration and pattern (including whether the "double worsening" pattern is present), together with examination of the nose, throat and face for tenderness, swelling or visible nasal changes, is usually sufficient to diagnose typical acute sinusitis.
Nasal Endoscopy: A thin, flexible or rigid scope is passed into the nose to directly visualise the sinus drainage pathways, allowing a doctor to check for pus, polyps, or structural abnormalities not visible on standard examination — particularly useful in chronic or recurrent cases.
CT Scan: Provides detailed imaging of the sinuses and surrounding bone, used mainly for chronic or recurrent sinusitis, when surgery is being considered, or when a complication is suspected. Not routinely needed for straightforward acute sinusitis.
Allergy Testing: Considered in chronic or recurrent sinusitis to identify whether underlying allergies are contributing, which can meaningfully change the long-term management approach.
Nasal or Sinus Culture: Occasionally used in chronic sinusitis or cases not responding to standard antibiotics, obtained via endoscopy to identify the specific organism involved and guide more targeted antibiotic treatment.

Reducing Your Risk of Sinusitis

A number of everyday measures can help reduce the frequency of sinus infections and support recovery when they do occur.

Protect against respiratory infections: Frequent handwashing and limiting close contact with people who have colds or other respiratory infections reduces exposure to the viruses that most often trigger sinusitis.
Manage allergies proactively: Identifying and avoiding personal allergy triggers, and working with a doctor to keep allergic rhinitis well controlled, reduces the ongoing nasal and sinus swelling that predisposes to sinusitis.
Avoid smoking and irritants: Tobacco smoke and other airborne irritants impair the normal clearing function of the nasal and sinus lining; avoiding smoking and secondhand smoke supports healthier sinus function.
Use a humidifier in dry environments: Adding moisture to dry indoor air, particularly in air-conditioned environments, helps keep nasal secretions thin and mobile, supporting normal sinus drainage.
Consider routine saline irrigation: Regular saline nasal rinses can help clear allergens, irritants and mucus before they contribute to a sinus flare, particularly for those prone to recurrent episodes.