Tonsillitis is an inflammation of the tonsils located at the back of the throat. These tissues serve as part of the body’s immune system, acting as a first line of defense against inhaled or ingested pathogens. While tonsillitis commonly affects both tonsils simultaneously, the condition can occur on only one side, known as unilateral tonsillitis. When swelling or pain is restricted to a single side of the throat, it warrants attention because it can indicate a range of possibilities, from a common localized infection to a more serious complication that requires prompt medical intervention.
Unilateral Tonsillitis and Common Causes
An infection may affect only one tonsil due to localized factors, such as an uneven initial exposure to the pathogen or an asymmetrical immune response. This uneven presentation can lead to unilateral pain, swelling, and redness on the affected side.
The most frequent culprits behind tonsillitis, whether one-sided or bilateral, are viruses, accounting for approximately 70% to 95% of cases. Viral causes often include common cold viruses, influenza, or the Epstein-Barr virus (EBV), which causes infectious mononucleosis. Mononucleosis, in particular, can cause significant, often asymmetrical, tonsil swelling with white patches, accompanied by severe fatigue and swollen lymph nodes in the neck.
Bacterial infections, such as those caused by Streptococcus pyogenes (Group A Strep), are also a common cause of unilateral tonsillitis. While Strep throat typically affects both sides, the pain and inflammation may feel disproportionately focused on one side. Symptoms of these common infections generally include a fever, throat pain that is worse when swallowing, and the presence of white or yellow spots of exudate on the inflamed tonsil. These conditions are usually manageable with supportive care for viral causes or antibiotics for confirmed bacterial infections.
Emergency Conditions Mimicking One-Sided Swelling
When one-sided tonsil swelling progresses rapidly, a more serious condition known as a Peritonsillar Abscess (PTA) must be considered. A PTA is a collection of pus that forms in the tissue space next to one of the tonsils, typically occurring as a complication of untreated or partially treated tonsillitis. This abscess almost always forms on only one side, making unilateral swelling a significant marker.
Specific symptoms clearly differentiate a PTA from standard unilateral tonsillitis. The pain is not only severe but is often localized to one side and progressively worsens. A hallmark symptom is trismus, a spasm of the jaw muscles that causes difficulty or an inability to fully open the mouth. The inflammation can also displace the uvula, pushing it away from the affected side.
Another distinguishing sign is a change in the voice, often described as a “hot potato” voice, which sounds muffled due to the swelling interfering with vocal resonance. Patients may also experience drooling because the pain makes swallowing saliva nearly impossible. A peritonsillar abscess requires urgent medical attention, as it may need needle aspiration or incision and drainage to remove the pus, followed by appropriate antibiotic treatment.
Non-Tonsil Causes and When to Seek Care
Soreness or swelling felt on only one side of the throat may not always originate from the tonsils themselves. The throat is surrounded by other structures that can cause localized pain, such as the lymph nodes. Swollen lymph nodes, which act as filters during an infection, can become tender and inflamed on one side of the neck, leading to referred pain in the throat.
A dental abscess or infection in a tooth, particularly a back molar, can radiate pain up to the ear and down to the throat, mimicking tonsil discomfort on that side. Chronic irritation from conditions like Gastroesophageal Reflux Disease (GERD) can also cause one-sided throat soreness, especially if a person consistently sleeps on one side, allowing stomach acid to irritate the throat lining. Less common, but part of a thorough medical evaluation, are localized trauma from sharp food or, in rare instances, a mass or tumor.
Actionable guidance is necessary when dealing with one-sided throat issues to ensure a potentially serious cause is not overlooked. Immediate medical care should be sought if certain “red flags” are present:
- Severe difficulty breathing or swallowing.
- Inability to drink fluids.
- A high fever that does not respond to medication.
- The inability to open the mouth fully (trismus) or the presence of a muffled voice.