Infections of the orbit are relatively rare conditions but they can be rather severe causing destruction of various tissues the orbit is made of. Some patients suffering from orbital infections may lose their sight, develop meningitis or even end up dead depending on potential complications of the primary infection. Diagnostic process must be fast and accurate while treatment is supposed to be aggressive enough to eradicate infectious agents and prevent further spread of the infection to nearby tissues and organs.
What is Orbital Infection?
It is of major importance to differentiate orbital infections from infections affecting the very eye (ocular infections). Namely, the orbit comprises various tissues such as bone tissue, the periorbita (the layer of tissue surrounding the orbit), ocular muscles, retroseptal fat as well as the optic nerve whose proximal part is actually is the part of the retina while the rest of the nerve goes through the orbita, enters the skull and continues in the form of the optic tract reaching specific parts of the brain. Now, if any of the mentioned structures gets inflamed due to certain microorganism we can say that a person is suffering from an orbital infection. On the other hand, if infection affects any of various parts of the eye such as the conjunctiva, cornea, sclera etc., the person is said to be suffering from an ocular infection.
The majority of orbital infections develop as a consequence of infection of the nearby structure, for instance sinusitis. The process of inflammation can be divided into several subcategories depending on the severity of the infection. So, there are inflammation with edema, orbital cellulitis, subperiostal abscess, orbital abscess and cavernous sinus thrombosis.
The previously mentioned categories do not have to imply that the infection will first develop in the form of edema and mild inflammation and then progress towards the most severe form known as cavernous sinus thrombosis but this is the course of the disease in most patients. Symptoms and signs of infection also vary. This is the reason why some forms of infection cause chemosis (edema of the mucous membrane of the eyeball/eyelid lining), proptosis (abnormal protrusion of the eye) or visual disturbances while other forms of infection may be accompanied by tenderness over the inflamed sinuses, headache and fever.
Orbital cellulitis is characterized by inflammation of soft tissues of the orbit without additional abscess formation. The infection either stays localized or soon progress into subperiostal abscess, orbital abscess or the most severe form, cavernous sinus thrombosis. A subperiostal abscess represents a collection of pus localized in the bony part of the orbita, to be more precise between the bone covering the orbit and the periostium. Further, an orbital abscess affects soft tissue of the orbit. It is associated with rather extensive proptosis. And finally, thrombosis of the cavernous sinus, potentially lethal condition, is actually a complication of untreated orbital infection. The sinus drains blood from both orbits and once it gets affected by blood clots, a person starts experiencing intense headaches, increased body temperature, periorbital edema etc. along with paralysis of eye movements.
How Can MRI Help with Orbital Infections?
Both CT scan and MRI can be of great help in differentiating orbital infections from infections affecting both, the orbita and the very eye. Initially, most patients undergo CT scanning because this is a rather easy imaging method and it is available in almost all medical institutions. A well-experienced radiologist can easily determine what particular structures of the orbit are affected by inflammation. The scan also gives insight in the initial infection which may be sinusitis, for instance.
When it comes to magnetic resonance imaging (MRI) of the orbit, this exam is highly precise for detection of early inflammatory changes within the orbit. For example, orbital cellulitis has specific characteristics on T1-weighted and T2-weighted sequences. What is more, MRI of the orbit and surrounding tissues can reveal and asses intracranial extension of the infection and this way conform/rule out cavernous sinus thrombosis. Ischemia and infarction of the optic nerve are two more conditions MRI of the orbit can easily visualize, allowing doctors to act promptly and initiate treatment instantly.
Still, even these two techniques have their flaws. MRI is very precise, but it takes certain length of time to provide with images which are crucial for further action. Furthermore, patients with metal implants as well as those with pacemakers, aneurysm clips and other metallic devices or foreign objects in the body are strictly forbidden to undergo this imaging technique.
On the other hand, CT scans may precipitate damage to the lens if repeated often. But if used with moderation CT scan will not damage any eye structure while at the same time it will provide with sufficient data regarding the infection within optimal period of time.
All in all, orbital infections are serious medical issues which require prompt treatment with potent antibiotics in case irreversible damage to the eye or the brain wants to be avoided. Fortunately, thanks to both CT scan and MRI, doctors can confirm the infection of this type on time.