Spinal epidural abscess (SEA) is a collection of pus or inflammatory granulation between the dura mater and the vertebral column. Currently the annual incidence rate of SEAs is estimated to be 2.5-3 per 10,000 hospital admissions. Incidence of SEA is on the rise, due to factors such as an aging population, increase in use of invasive spinal instrumentation, growing number of patients with risk factors such as diabetes and intravenous drug use. SEAs are more common in posterior than anterior areas, and the most common location is the thoracolumbar area, where epidural space is larger and contains more fat tissue.

SEAs are more common in males, and can occur in all ages, although highest prevalence is during the fifth and seventh decades of life.

Death occurs in about 10% of cases and people do well about 70% of the time. This is a large improvement from the 1960s due to improved ability to image the head, better neurosurgery and better antibiotics.

Anaerobic and microaerophilic cocci and gram-negative and gram-positive anaerobic bacilli are the predominate bacterial isolates. Many brain abscesses are polymicrobical. The predominant organisms include: "Staphylococcus aureus", aerobic and anaerobic streptococci (especially "Streptococcus intermedius"), "Bacteroides", "Prevotella", and "Fusobacterium" species, Enterobacteriaceae, "Pseudomonas" species, and other anaerobes. Less common organisms include: "Haemophillus influenzae", "Streptococcus pneumoniae" and "Neisseria meningitides".

Bacterial abscesses rarely (if ever) arise "de novo" within the brain, although establishing a cause can be difficult in many cases. There is almost always a primary lesion elsewhere in the body that must be sought assiduously, because failure to treat the primary lesion will result in relapse. In cases of trauma, for example in compound skull fractures where fragments of bone are pushed into the substance of the brain, the cause of the abscess is obvious. Similarly, bullets and other foreign bodies may become sources of infection if left in place. The location of the primary lesion may be suggested by the location of the abscess: infections of the middle ear result in lesions in the middle and posterior cranial fossae; congenital heart disease with right-to-left shunts often result in abscesses in the distribution of the middle cerebral artery; and infection of the frontal and ethmoid sinuses usually results in collection in the subdural sinuses.

An epidural abscess refers to a collection of pus and infectious material located in the epidural space of the central nervous system. Due to its location adjacent to brain or spinal cord, epidural abscesses have the potential to cause weakness, pain, and paralysis.

Risk factors for abscess formation include intravenous drug use. Another possible risk factor is a prior history of disc herniation or other spinal abnormality, though this has not been proven.

Abscesses are caused by bacterial infection, parasites, or foreign substances.

Bacterial infection is the most common cause. Often many different types of bacteria are involved in a single infection. In the United States and many other areas of the world the most common bacteria present is "methicillin-resistant Staphylococcus aureus". Among spinal subdural abscesses, methicillin-sensitive Staphylococcus aureus is the most common organism involved.

Rarely parasites can cause abscesses and this is more common in the developing world. Specific parasites known to do this include dracunculiasis and myiasis.

Factors increasing the risk of a subdural hematoma include very young or very old age. As the brain shrinks with age, the subdural space enlarges and the veins that traverse the space must travel over a wider distance, making them more vulnerable to tears. This and the fact that the elderly have more brittle veins make chronic subdural bleeds more common in older patients. Infants, too, have larger subdural spaces and are more predisposed to subdural bleeds than are young adults. For this reason, subdural hematoma is a common finding in shaken baby syndrome. In juveniles, an arachnoid cyst is a risk factor for a subdural hematoma.

Other risk factors for subdural bleeds include taking blood thinners (anticoagulants), long-term alcohol abuse, dementia, and the presence of a cerebrospinal fluid leak.

Both sex are equally affected

Any age group can develop a parapheryngeal abscess but it is most commonly seen in children and adolescents. Adults who are immunocompromised are also at high risk.

RPA is usually caused by a bacterial infection originating from the nasopharynx, tonsils, sinuses, adenoids or middle ear. Any upper respiratory infection (URI) can be a cause. RPA can also result from a direct infection due to penetrating injury or a foreign body. RPA can also be linked to young children who do not have adequate dental care or brush their teeth properly.

Skin abscesses are common and have become more common in recent years. Risk factors include intravenous drug use with rates reported as high as 65% in this population. In 2005 in the United States 3.2 million people went to the emergency department for an abscess. In Australia around 13,000 people were hospitalized in 2008 for the disease.

Infection can occur from:

- Pharynx: acute and chronic infection of tonsil and adenoids

- Teeth: dental infection occurs from lower last molar tooth

- Ear: bezold abscess and petrositis

- Other space: infection of parotid retropharyngeal space

- External trauma: penetrating injuries of neck, injection of local anaesthetic

Subdural hematomas are most often caused by head injury, when rapidly changing velocities within the skull may stretch and tear small bridging veins. Subdural hematomas due to head injury are described as traumatic. Much more common than epidural hemorrhages, subdural hemorrhages generally result from shearing injuries due to various rotational or linear forces. Subdural hemorrhage is a classic finding in shaken baby syndrome, in which similar shearing forces classically cause intra- and pre-retinal hemorrhages. Subdural hematoma is also commonly seen in the elderly and in alcoholics, who have evidence of cerebral atrophy. Cerebral atrophy increases the length the bridging veins have to traverse between the two meningeal layers, hence increasing the likelihood of shearing forces causing a tear. It is also more common in patients on anticoagulants or antiplatelet drugs, such as warfarin and aspirin. Patients on these medications can have a subdural hematoma after a relatively minor traumatic event.

A further cause can be a reduction in cerebral spinal fluid pressure which can create a low pressure in the subarachnoid space, pulling the arachnoid away from the dura mater and leading to a rupture of the blood vessels.

Retropharyngeal abscess (RPA) is an abscess located in the tissues in the back of the throat behind the posterior pharyngeal wall (the retropharyngeal space). Because RPAs typically occur in deep tissue, they are difficult to diagnose by physical examination alone. RPA is a relatively uncommon illness, and therefore may not receive early diagnosis in children presenting with stiff neck, malaise, difficulty swallowing, or other symptoms listed below. Early diagnosis is key, while a delay in diagnosis and treatment may lead to death. Parapharyngeal space communicates with retropharyngeal space and an infection of retropharyngeal space can pass down behind the esophagus into the mediastinum. RPAs can also occur in adults of any age.

RPA can lead to airway obstruction or sepsis – both life-threatening emergencies. Fatalities normally occur from patients not receiving treatment immediately and suffocating prior to knowing that anything serious was wrong.

The anatomy of the epidural space is such that spinal epidural hematoma has a different presentation from intracranial epidural hematoma. In the spine, the epidural space contains loose fatty tissue and a network of large, thin-walled veins, referred to as the epidural venous plexus. The source of bleeding in spinal epidural hematoma is likely to be this venous plexus.

There is debate as to the cause, although hematogenous seeding of the offending organism is favored as well as direct spread. It is important to differentiate between spontaneous discitis which is usually from hematologic spread from a urinary or respiratory infection versus that from a post-operative complication which usually involves skin flora such as staph aureus.

It can be caused due to spinal tuberculosis and spread along spinal ligament to involve the adjacent anterior vertebral bodies, causing angulation of the vertebrae with subsequent kyphosis.

The cause may be aseptic.

Pancreatic abscesses usually develop in patients with pancreatic pseudocysts that become infected. They may also form as a result of fibrous wall formation around fluid collections or penetrating peptic ulcers. Other causes include gall stones or alcohol consumption and, in rare cases, drugs, blunt trauma and following extension abscess from nearby structures.

A 1994 community-based study indicated that two out of every 100,000 people suffered from SCSFLS, while a 2004 emergency room-based study indicated five per 100,000. SCSFLS generally affects the young and middle aged; the average age for onset is 42.3 years, but onset can range from ages 22 to 61. In an 11-year study women were found to be twice as likely to be affected as men.

Studies have shown that SCSFLS runs in families and it is suspected that genetic similarity in families includes weakness in the dura mater, which leads to SCSFLS. Large scale population-based studies have not yet been conducted. While a majority of SCSFLS cases continue to be undiagnosed or misdiagnosed, an actual increase in occurrence is unlikely.

Abscesses are caused by a high-density infection of (usually) common bacteria which collect in one place or another for any variety of reasons. Anal abscesses, without treatment, are likely to spread and affect other parts of the body, particularly the groin and rectal lumen. All abscesses can progress to serious generalized infections requiring lengthy hospitalizations if not treated.

Historically, many rectal abscesses are caused by bacteria common in the digestive system, such as E. coli. While this still continues often to be the case, there has recently been an uptick in the causative organism being staphylococcus, as well as the difficult to treat community-acquired methicillin-resistant S. aureus. Because of the increasing appearance of more exotic bacteria in anal abscesses, microbiological examination will always be performed on the surgical exudate to determine the proper course of any antibiotic treatment.

In some cases, abscesses may be prevented by draining an existing pseudocyst which is likely to become inflamed. However, in most cases the developing of abscesses cannot be prevented.

Commonly by bacteria – streptococci, spore and non-spore forming anaerobes, etc.

Factors affecting the development of phlegmons are virulence of bacteria and immunity strength.

Spinal epidural hematoma is bleeding into the epidural space in the spine. These may arise spontaneously (e.g. during childbirth), or as a rare complication of epiduralanaesthesia or of surgery (such as laminectomy). Symptoms usually include back pain which radiates to the arms or the legs. They may cause pressure on the spinal cord or cauda equina, which may present as pain, muscle weakness, or dysfunction of the bladder and bowel.

The best way to confirm the diagnosis is MRI. Risk factors include anatomical abnormalities and bleeding disorders.

Treatment is generally with emergency surgery. The risk following epidural anaesthesia is difficult to quantify; estimates vary from 1 per 10,000 to 1 per 100,000 epidural anaesthetics.

Treatment generally consists of surgical drainage, and long-term (6 to 8 weeks) use of antibiotics.

An abscess in the psoas muscle of the abdomen may be caused by lumbar tuberculosis. Owing to the proximal attachments of the iliopsoas, such an abscess may drain inferiorly into the upper medial thigh and present as a swelling in the region. The sheath of the muscle arises from the lumbar vertebrae and the intervertebral discs between the vertebrae. The disc is more susceptible to infection, from tuberculosis and "Salmonella discitis". The infection can spread into the psoas muscle sheath.

Treatment may involve drainage and antibiotics.

PTA usually arises as a complication of an untreated or partially treated episode of acute tonsillitis. The infection, in these cases, spreads to the peritonsillar area (peritonsillitis). This region comprises loose connective tissue and is hence susceptible to formation of an abscess. PTA can also occur "". Both aerobic and anaerobic bacteria can be causative. Commonly involved aerobic pathogens include "Streptococcus, Staphylococcus" and "Haemophilus". The most common anaerobic species include "Fusobacterium necrophorum", " Peptostreptococcus", "Prevotella species", and "Bacteroides".

Bezold's Abscess is an abscess deep to sternocleidomastoid muscle where pus from mastoiditis erodes through the cortex of Mastoid part of the temporal bone, medial to the attachment of sternocleidomastoid, extends into the infratemporal fossa, and deep to the investing layer of the deep cervical fascia . It is a rare complication of acute otitis media.

Severe pain in perimastoid region, difficulty of swallowing, sore throat, difficulty in breathing, nuchal rigidity, and fever.