Despite their similarity in name, and the fact that they are both infections of the eye, the severity of each of their condition differs vastly. Specifically speaking, periorbital cellulitis is the infection from surrounding the outside of the eye, whereas orbital cellulitis is the infection from inside the eye socket. If left untreated, both can progress into a life-threatening stage. 

Introduction

Periorbital cellulitis, also known as preseptal cellulitis, is the infection of the skin and soft tissue surrounding the eye. It is important to note that it is anterior to the orbital septum and it is usually a unilateral swelling and edema. Periorbital cellulitis is caused primarily by the spread of rhinosinusitis infection or general infection after local trauma.¹ It is often mistaken as orbital cellulitis, which has a similar clinical presentation in the eye to the untrained eyes. Orbital cellulitis, also known as postseptal cellulitis, is often a continuum of periorbital cellulitis. It is usually caused by the progression of ethmoid sinusitis, or a bacterial infection of the Staphylococcus aureus and Streptococci species.² In comparison, it is important to note that, unlike periorbital cellulitis, the condition is posterior to the orbital septum, but it also presents as swelling and oedema like periorbital cellulitis. 

It is essential to be able to distinguish between these two conditions as their severity differs greatly. In simpler terms, periorbital cellulitis is less severe and dangerous compared to orbital cellulitis as it affects the skin and soft tissue in front or outside the actual eye structure. Whereas, orbital cellulitis is the infection and swelling behind the orbital septum and is inside the eye structure.³ Its name preseptal and postseptal cellulitis is derived from “orbital septum”. The orbital septum is a fibrous, thin membrane that separates the superficial lids from the orbit.4 It acts as the distinguishing factor between these two conditions – “preseptal” meaning “before” the septum would be periorbital cellulitis, whereas “postseptal” means “after” the septum would be classified as orbital cellulitis. Some other distinguishable factors of orbital cellulitis from periorbital cellulitis are the clinically presented conditions; ophthalmoplegia (pain with eye movement), and or proptosis (the protrusion of the eyeball which is also a common characteristic of people with Grave’s disease).⁵

Periorbital cellulitis

Clinical presentation

The clinical presentation of periorbital cellulitis is unilateral eyelid swelling and oedema (swelling caused by trapped fluid). Assuming without early intervention treatment, according to the Chandler classification of orbital complications, it can also progress to cause orbital cellulitis, subperiosteal abscess, orbital abscess, and cavernous sinus thrombosis, in that order of severity chronologically.¹ For those with even more severe cases, it can even result in intracranial infections, although these are unlikely for those with only periorbital cellulitis. 

Risk factor

Periorbital cellulitis can occur in all ages without any other discrimination. However,  it is presented most commonly in the paediatric unit as children are more likely to be subjected to periocular or facial trauma, practise poor hygiene habits that expose them to unforeseen pathogens, and their comparably weaker immune system also prevents effective immune response to pathogens. In terms of inciting events to periorbital cellulitis, the list includes sinusitis, direct inoculation of the skin on the eye (such as insect bites), and impetigo (skin infection with sores and blisters). 

Diagnosis

To diagnose periorbital cellulitis, a physical examination is first overseen. On the contrary to orbital cellulitis, periorbital cellulitis only involves extraocular muscles and should have no involvement with intraocular muscles, making the vision, globe motility, and intraocular pressure normal.¹ It also usually has erythema (redness of the skin caused by infection), oedema, and eyelid swelling.

To confirm this condition, imaging studies such as Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) scans are implemented. CT scan is also a good resource to identify the presence of proptosis and limitations of extraocular movement⁶. In addition, it can not only differentiate between periorbital cellulitis and orbital cellulitis, but it can also determine the extent of the infection – allowing easier consideration of the appropriate treatment plan.¹ In terms of the imaging of periorbital cellulitis, grey figures can be seen in front of the eye instead of behind the eye and within the eye socket. 

Treatment

For mild periorbital cellulitis, oral antibiotics such as Cefalexin or Cefuroxime are typically administered to treat the spread of pathogens like the Streptococcus species and anaerobes which had caused or accelerated the condition in the first place. As for a moderate periorbital cellulitis condition, Flucloxacillin or Ceftriaxone is administered intravenously (IV) in addition to oral therapy. However, if the moderate condition is not quickly improve after administration of both IV and oral therapy within 24-48 hours, the condition is then treated as severe periorbital cellulitis (the same as orbital cellulitis). For severe periorbital cellulitis, 3rd generation cephalosporin and Flucloxacillin are administered intravenously, along with Amoxicillin with clavulanic acid orally.⁷ For more information, see the following link. For those with complications with any treatments for the condition and if the condition continues to progress, it could require hospital stay in case of severe progression to critical conditions like cavernous sinus thrombosis. 

Orbital cellulitis

Clinical presentation

Orbital cellulitis has a clinical presentation of onset swelling and oedema-like periorbital cellulitis. However, in addition to that, orbital cellulitis also presents pain and restriction with ocular movement, blurred vision, fever, and pupil reactions that may be abnormal (RAPD). Unlike periorbital cellulitis, these symptoms are much more severe and could impact ocular health chronically. 

Risk factor

Based on research, data shows that in around 86% to 98% of orbital cellulitis cases, there are coexisting rhinosinusitis conditions present.⁸ Rhinosinusitis take place in different areas, including maxillary sinuses, ethmoid sinuses, sphenoid, and frontal sinuses. For the infection of the posterior ethmoidal sinus, where its location lies close to the optic canal and optic nerve, it could impact visual acuity alongside orbital cellulitis.⁹

Diagnosis

Like periorbital cellulitis, a physical examination of the clinical presentation of the condition is done to assess and diagnose. Similarly to periorbital cellulitis, imaging studies are done to confirm whether it is orbital cellulitis or not. CT and MRI are both beneficial for confirmation but MRI would be most ideal as it allows the observation of disease progression on soft tissues.¹⁰ These imaging techniques can also specify where exactly in the eye structure the infection is, allowing visualisation of possible intracranial extension of the infection, and also prevent possible misdiagnosis of periorbital cellulitis. 

Treatment

Cases of orbital cellulitis are most likely referred and admitted to the A&E department or ophthalmologist on the same day as an emergency case. In the hospital, the patients would need to stay for tests and antibiotic treatments necessary. Depending on each unique case, different specialists may be involved; ophthalmologists, ear, nose and throat (ENT) specialists, and paediatricians.¹¹ For those with severe and rapid progression of orbital cellulitis, perhaps with orbital abscess, surgical intervention may be required. Some instances where surgery may be required are; the extension of infection to the intracranial section, if visual acuity continues to worsen after 24-48 hours of treatment, or if the abscess is larger than 10mm, surgery would be implemented to physically drain the abscess.² By doing so, samples of the abscess can be obtained for culturing, offering insight into the source of infection. If the intracranial abscess is left untreated, meningitis, encephalitis, sepsis, and dural sinus thrombophlebitis could occur, leaving the patient in life-threatening danger.¹²

Key differences between periorbital and orbital cellulitis

Anatomical location and involvement

The orbital septum is a membrane from the orbital periosteum lining. It is this boundary that determines if the infection is periorbital (hence preseptal) or orbital (hence postseptal). If the infection is anterior to the orbital septum, it is periorbital cellulitis, and if the infection is posterior to the orbital septum, it is orbital cellulitis. 

Severity of symptoms

The symptoms of orbital cellulitis are more strenuous, having pain and restriction to eye movement in addition to visual complications. Furthermore, orbital cellulitis is able to extend the abscess to the cranium, making it life-threatening as the intracranial pressure increases and the abscess intrudes into the brain. Some resulting symptoms are headache, fever, changes in consciousness, confusion, neck stiffness, vomiting, seizures, weakness, trouble moving, and changes in vision.¹³

Complications and prognosis

If the complications are left untreated for long, the prognosis is unlikely to improve without lasting effects on health. Some lasting effects are full or partial vision loss, meningitis, septicemia, and cavernous sinus thrombosis (blood clot in a cavity of the base of the brain).¹⁴ In addition to that, when receiving treatments the following complications may arise; some being GI symptoms and or antibiotic resistance. 

Summary

While periorbital and orbital cellulitis differ in terms of anatomical location and severity of symptoms, both are difficult circumstances that require appropriate treatment. With early treatment, the condition can improve more quickly and prevent even more complications. Prompt treatment for periorbital cellulitis can prevent further progression into orbital cellulitis. For orbital cellulitis, early treatment prevents further progression into permanent or dangerous conditions like vision loss or abscess formation.