What Is a Phlegmon? Causes, Signs, and Treatment

A phlegmon is a spreading area of infected, inflamed tissue that has not yet walled itself off into a pocket of pus. Unlike an abscess, which forms a contained fluid collection you could theoretically puncture and drain, a phlegmon is a solid, diffuse mass of swollen tissue teeming with bacteria and inflammatory cells. The distinction matters because you cannot drain what is essentially solid, and the treatment strategy shifts accordingly. Phlegmons can develop almost anywhere in the body, from the tissues around an inflamed appendix to the spaces deep in the neck, and they range from manageable with antibiotics alone to life-threatening when infection spreads to critical structures.

How a Phlegmon Differs From an Abscess

The words “phlegmon” and “abscess” get used loosely, sometimes even by clinicians, but they describe two meaningfully different stages of the same infectious process. A phlegmon is an ill-defined, mass-like inflammatory process within soft tissue. An abscess, by contrast, is a fluid collection surrounded by a wall, sometimes containing gas, that shows up on imaging as a distinct pocket with a rim around it.¹ Think of it this way: a phlegmon is infected tissue that is uniformly swollen and angry, while an abscess is what happens when the body manages to corral the infection into a walled-off pool of pus.

This difference directly affects treatment. A phlegmon contains no pus to drain, so needle aspiration or catheter drainage is pointless. An abscess, on the other hand, often needs to be drained to resolve. When doctors look at a liver infection on imaging, for example, a solid-appearing lesion is called a hepatic phlegmon, and a fluid-filled one with a core of pus is called a hepatic abscess. You treat the phlegmon with antibiotics; the abscess typically needs drainage as well.² In practice, the line between phlegmon and abscess is not always crisp. A phlegmon can evolve into an abscess over time as pus accumulates and the body walls it off, or a phlegmon can resolve on its own without ever forming a drainable collection.

What Causes a Phlegmon

Most phlegmons are caused by bacterial infections, and the specific organisms depend heavily on where in the body the infection develops. In the head and neck, the microbiology has been studied most closely in the context of tonsillar and peritonsillar infections. The usual cast of characters includes a mix of aerobic and anaerobic bacteria. Group A Streptococcus is the dominant pathogen, but cultures from most patients also grow other streptococcal species, Haemophilus bacteria, and a range of anaerobes including Fusobacterium and Prevotella species, with polymicrobial infections found in roughly 80 to 90 percent of cases.³

Abdominal phlegmons tend to develop from an organ that has become inflamed or perforated, seeding infection into the surrounding fat and tissue. An inflamed appendix is the classic example: the appendiceal wall breaks down, bacteria leak into the surrounding tissue, and a phlegmonous mass forms. The same process can occur around a diseased gallbladder, an inflamed segment of bowel in Crohn’s disease, or a pancreas damaged by acute pancreatitis. In many of these abdominal cases, the bacteria involved are the gut organisms you would expect: Escherichia coli, Bacteroides, and other enteric species. Dental infections are another major source, particularly for neck phlegmons, where bacteria from the mouth spread along tissue planes into deeper spaces.

Where Phlegmons Tend to Develop

Phlegmons are not restricted to one body region, but a few locations account for the vast majority of cases that bring patients to hospitals.

Head and Neck

The tissue spaces of the head and neck are separated by layers of fascia, and infections can track along these planes with alarming speed. A tonsillar phlegmon, for instance, can spread from the area around the tonsil into the parapharyngeal space and down into the floor of the mouth, causing a dangerous condition called Ludwig’s angina. In one reported case, a tonsillar phlegmon in an otherwise healthy 50-year-old woman spread along the peritonsillar and parapharyngeal planes to cause diffuse infection in the submental, sublingual, and submandibular spaces, even despite intravenous antibiotics.⁴ Dental infections are another common starting point. When bacteria from a decayed tooth or gum infection spread into the surrounding neck tissues, an odontogenic phlegmon can develop and, if not treated promptly, extend downward toward the chest.⁵

Appendix

When appendicitis goes untreated long enough, the inflamed appendix can perforate or its wall can begin to break down, and the surrounding fat and bowel loops stick together to form an appendiceal phlegmon. On a CT scan, this appears as fat stranding around the appendix, thickening of nearby tissues, clustering of adjacent bowel loops, and sometimes small amounts of free fluid, but without a distinct abscess cavity.⁶ Appendiceal phlegmons are common enough that the question of how to manage them has generated a substantial body of surgical literature.

Pancreas

Acute pancreatitis can produce a pancreatic phlegmon, which shows up on CT as a solid mass in and around the pancreas without a fluid-filled core or a fibrous capsule. Patients who develop this complication tend to have more severe pancreatitis overall, with longer durations of fever, pain, and elevated white blood cell counts compared to those without a phlegmon.⁷ Distinguishing a pancreatic phlegmon from a pancreatic abscess is tricky on clinical grounds alone because patients look similar. The good news is that most pancreatic phlegmons resolve on their own without surgery, while abscesses generally do not.⁷ That said, pancreatic phlegmon is a marker of severe disease. All deaths in one study of pancreatic phlegmon occurred in patients who also met the clinical criteria for severe pancreatitis.⁸

Stomach and Liver

Phlegmonous gastritis is a rare but dramatic condition in which the stomach wall itself becomes infected, turning thickened and edematous. This is a distinct diagnosis from the more common forms of gastritis most people are familiar with. In the liver, a phlegmon can mimic a solid tumor on imaging, creating a diagnostic puzzle that may require biopsy to sort out.²

Signs and Symptoms

The symptoms of a phlegmon depend on its location, but there are some common threads. Fever, pain, and swelling at the affected site are almost universal. In the abdomen, a phlegmon often presents as a palpable mass or area of tenderness, sometimes with the patient looking sicker than you would expect for a simple infection. Blood tests typically show elevated white blood cell counts and markers of inflammation like C-reactive protein.

In head and neck phlegmons, the swelling can be dramatic and visible from the outside. Difficulty swallowing, a muffled voice, trismus (difficulty opening the mouth), and pain radiating to the ear are common. The danger in this location is that swelling can compromise the airway. In appendiceal phlegmons, patients usually present with a history of abdominal pain that has been going on for days, sometimes with a palpable right-lower-quadrant mass. With pancreatic phlegmons, prolonged fever and abdominal pain in the context of known pancreatitis raise the suspicion.

There is no single blood test that confirms a phlegmon specifically. Elevated C-reactive protein and ferritin, along with CT findings such as thickened fascia or extraluminal gas, have been identified as predictors of disease severity and recurrence in appendiceal phlegmon.⁹ But the diagnosis itself is made on imaging, not on labs.

How Phlegmons Are Diagnosed

CT scanning with contrast is the workhorse for identifying phlegmons in the abdomen and pelvis. The classic appearance is a poorly defined area of inflammatory tissue, often with fat stranding, that does not contain a drainable fluid collection. On ultrasound, a phlegmon appears as a solid, poorly marginated mass within inflamed tissue, whereas an abscess shows up as a fluid collection with an irregular wall.¹

For pancreatic phlegmons, the CT criterion involves a solid mass in the pancreatic and peripancreatic region measuring above a certain density threshold, without liquid collections or a fibrous capsule.⁸ In the liver, a phlegmon can look remarkably like a solid tumor, which sometimes leads to unnecessary biopsies or surgical explorations before the infectious nature of the lesion becomes clear.² In the neck, contrast-enhanced CT or MRI helps map how far the infection has spread along fascial planes, which is critical for surgical planning.

The challenge is that imaging cannot always distinguish a phlegmon from an early or organizing abscess with certainty. When clinical suspicion for an abscess is high but imaging suggests a solid inflammatory mass, doctors sometimes attempt needle aspiration to see if any pus can be withdrawn. If the needle comes back dry, you’re dealing with a phlegmon.

Treatment Approaches

Because a phlegmon is a solid mass of infected tissue rather than a drainable collection, antibiotics are the backbone of treatment. The strategy varies depending on the location and severity of the infection.

Antibiotics First

In many cases, intravenous antibiotics alone can resolve a phlegmon. This has been demonstrated particularly well in phlegmonous gastritis, where prompt antibiotic treatment leads to rapid resolution in most patients.¹⁰ In reported cases, patients with phlegmonous gastritis who were recognized early and started on parenteral antibiotics improved quickly without needing surgery.¹¹ Antibiotic choice is typically broad-spectrum initially, covering both aerobic and anaerobic organisms, and then narrowed based on culture results when available.

The Appendiceal Phlegmon Debate

Appendiceal phlegmon has been the testing ground for one of the bigger questions in surgical practice: when you find an inflammatory mass around the appendix, do you operate right away or treat with antibiotics first and consider surgery later? The evidence leans strongly toward the conservative approach. A systematic review and meta-analysis found that immediate surgery was associated with roughly three times the odds of complications compared to nonsurgical treatment initially.¹² Operating into an inflamed phlegmonous mass is technically difficult: tissues are swollen, friable, and don’t separate cleanly, which raises the risk of bowel injury and other surgical complications.

With antibiotics alone, the early success rate for nonoperative management of complicated appendicitis, including phlegmon, ranges from roughly 83 to 97 percent.¹³ The catch is that recurrence rates range from about 5 to 30 percent, meaning some patients will need appendectomy down the road after the acute inflammation settles.¹³ In the meta-analysis data, the recurrence rate was about 7 percent.¹²

One reason surgeons still recommend follow-up colonoscopy or interval appendectomy after a phlegmon resolves is that a small percentage of cases turn out to be something other than simple appendicitis. Malignant disease is detected in about 1.2 percent of patients during follow-up after successful nonsurgical treatment, and a significant benign condition in another 0.7 percent.¹² That is a small number, but it is not zero, and it is the kind of finding that weighs on the decision about whether to proceed with an interval appendectomy once the inflammation has cooled.

When Surgery Cannot Wait

Not every phlegmon can be managed with antibiotics and patience. If there are signs of sepsis, if the patient is deteriorating despite antibiotics, or if imaging shows the infection spreading into dangerous territory, surgery becomes necessary. Neck phlegmons that extend toward the chest, for example, require surgical drainage to prevent mediastinitis. In phlegmonous gastritis, patients who do not respond to antibiotics or who develop perforation need emergency surgery, which can involve partial or even total gastrectomy.

When Phlegmons Become Dangerous

A phlegmon that stays localized and responds to antibiotics is a manageable problem. The fear is what happens when it does not. In the head and neck, infection can spread along fascial planes into the mediastinum, the space between the lungs in the chest. Mediastinitis is an extremely dangerous complication of neck phlegmons, carrying a high risk of death if not caught early.⁵ Parapharyngeal phlegmon specifically can spread to fascial spaces throughout the face and neck and lead to sepsis and septic shock.¹⁴

Septic embolism is another feared complication. When bacteria from a phlegmon enter the bloodstream, they can seed distant organs. One striking case involved a 43-year-old man who developed a nasal phlegmon after squeezing a pimple on his nose. The infection led to septic embolic pneumonia with multiple lung abscesses and fluid collections around both lungs.¹⁵ The so-called “danger triangle” of the face, the area from the bridge of the nose to the corners of the mouth, drains through veins that connect to the brain’s venous sinuses, making infections in this zone particularly prone to intracranial complications.

People with weakened immune systems, whether from diabetes, chronic steroid use, chemotherapy, or HIV, are at higher risk for both developing phlegmons and having them progress to these serious complications. Alcohol use disorders and malnutrition also impair the body’s ability to wall off infections, making phlegmon formation more likely and resolution slower.

The Terminology Problem

If you find the concept of a phlegmon confusing, you are in good company. The term has been used inconsistently throughout medical history and across different specialties. Pancreatic surgeons, for example, have debated and revised their terminology for peripancreatic collections multiple times. In older literature, “phlegmon” was applied broadly to any inflammatory mass around the pancreas, whereas more recent classification systems have moved toward terms like “acute necrotic collection” and “walled-off necrosis” to describe specific stages of the process more precisely.

In head and neck surgery, “phlegmon” tends to mean a diffuse cellulitis of the deep tissue spaces, while in abdominal surgery it often specifically implies a solid inflammatory mass that formed around a perforated or inflamed organ. The underlying concept is the same: infected tissue that has not liquefied into drainable pus. But the practical details of how that concept gets applied vary enough between specialties that reading about phlegmons across different body regions can feel like reading about different diseases. If your doctor uses the word, the question worth asking is whether the mass contains fluid that can be drained, or whether it is solid tissue that needs to be treated with antibiotics and time. That distinction is the one that actually drives decisions.

Facial Phlegmons and the Risks of Self-Treatment

The case of a nasal phlegmon triggered by squeezing a facial pimple deserves its own mention because it represents a genuinely common behavior leading to a rare but devastating outcome. The skin of the face, particularly around the nose, has a rich blood supply and a venous drainage system that communicates directly with the cavernous sinus inside the skull. Manipulating infected skin lesions in this area can push bacteria deeper into the tissue, creating a phlegmon where a simple pimple existed before. From there, the infection can disseminate through the bloodstream. The patient described in the literature developed septic embolic pneumonia as a downstream consequence of this cascade.¹⁵ Intracranial complications, including Lemierre’s syndrome and septic embolism to the brain, are among the possible outcomes of infections that originate in this region.

Dental infections that go untreated are another everyday scenario that can escalate to phlegmon. A tooth abscess that is ignored or treated with repeated courses of oral antibiotics without definitive dental work can spread into the floor of the mouth or the neck. Odontogenic phlegmons can extend into the deep spaces of the neck and, from there, track into the mediastinum, turning what started as a toothache into a surgical emergency.⁵ The message here is not about fear; it is about respecting infections in certain anatomical locations. A swelling on the arm is unlikely to threaten your life. A rapidly expanding swelling in the neck or central face warrants urgent medical attention.

1
PubMed Central. Similar Long-Term Outcomes in Children Presenting With Abscess vs Phlegmon at Diagnosis of Crohn Disease
2
PubMed Central. Klebsiella liver phlegmon mimicking a solid liver tumour
3
New Microbes and New Infections. Methanobrevibacter smithii tonsillar phlegmon: a case report
4
PubMed Central. Parapharyngeal and floor-of-mouth abscess secondary to tonsillar phlegmon: A rare and unusual cause of Ludwig’s angina
5
EMERGENCY MEDICINE. A case of surgical treatment of odontogenic phlegmon of the neck with abscess of the pterygoid space and submandibular area on the right, complicated by anterior superior and posterior superior mediastinitis
6
PubMed Central. A rare case of appendiceal tip complete attachment to a sigmoid diverticulum: An appendiceal phlegmon case report
7
PubMed. Pancreatic phlegmon. Clinical features and course.
8
PubMed. Pancreatic phlegmon: a potentially fatal form of acute pancreatitis
9
Surgery and Oncology. Outcomes of appendiceal phlegmon: diagnosis and treatment tactics
10
PubMed Central. Successful Antimicrobial Treatment of Phlegmonous Gastritis: A Case Report and Literature Review
11
PubMed Central. Phlegmonous Gastritis: A Case Report of Successful Early Antibiotic Treatment
12
PubMed. Nonsurgical treatment of appendiceal abscess or phlegmon: a systematic review and meta-analysis
13
Annals of Coloproctology. Clinical outcomes and optimal indications for nonoperative management of acute appendicitis in adult patients: a comprehensive literature review
14
Wiedza Medyczna. Craniofacial and neck phlegmon with sepsis as a complication of the parotid gland abscess – a case report
15
Meditsinskiy sovet = Medical Council. Case report of necrotizing pneumonia complicating a nasal phlegmon