Proptosis is defined as abnormal protrusion of the eyeball. It is also used interchangeably with exophthalmos. Purists are comfortable defining exophthalmos as proptosis associated with lid lag.
Anatomy of orbit and its relationships with paranasal sinuses:
Orbit is related to paranasal sinuses in two ways: 1. Anatomically by its location and by 2. venous drainage (They both share the same venous drainage).
The paranasal sinuses surround the orbit from 11 o clock position superiorly to 6 o clock position inferiorly.
The orbit is pyramidal shaped formed by several bones. The superior wall of the orbit is shared by the floor of the frontal sinus, the floor of the orbit is shared by the roof of the maxillary sinus, the medial wall of the orbit is shared by the lateral wall of ethmoidal sinus. These shared bones are really thin enabling infections to travel from either direction. The medial wall of the orbit is so thin that it is termed as lamina papyracea.
Peculiarities of venous drainage in this area: The veins draining this area are peculiar in the following aspects:
1. The whole venous system in this area is devoid of valves, consequently a two way pathway of infection between the orbit, nasal cavity and paranasal sinuses become a reality.
2. The superior ophthalmic vein connects the facial veins to the cavernous sinus thus causing infections from face to spread to the cavernous sinus.
3. One branch of the inferior ophthalmic vein connects the orbit with that of the veins of pterygoid plexus and the other branch connects the orbit with that of the cavernous sinus.
The approximate volume of the orbital cavity is about 30 ml. Since the contents of the orbit are within a rigid confines of the orbital walls any disease process within the orbit or adjacent region has a tendency to displace the orbital contents forwards, this displacement of the globe is known as the proptosis. Other symptoms of globe displacement include diplopia, and visual loss.
Proptosis must be distinguished from pseudoproptosis. Enophthalmous involving the opposite eye may cause apparent proptosis of the uninvolved eye. Similarly unilateral high myopia and lid retraction may cause apparent proptosis. A careful history will go a long way in ascertaining the exact cause of proptosis. Clinically significant proptosis is defined as a minimum difference of 2mm or more as measured by exophthalmometer.
Image showing proptosis
Measuring proptosis using exopthalmometer:
Hertel mirror exophthalmometers are used to measure the degree of protrusion of the eyeball. The distance between the lateral orbital rim and corneal apex is used as an index for measuring proptosis. Under normal conditions this distance is roughly 18 mm, there may be individual and racial variations.
Procedure: The examiner is seated infront of the patient at the eye level of the patient. The exophthalmometer is then positioned with the blue arched support at the temporal lateral orbital walls. The instrument is maneuvered using both hands and firmly propped first against the right-hand orbital wall on the temporal side (which should be felt against the lowest part of the support point). The moveable part is then set in such a way that the left-hand orbital wall lies against the lowest part of the arched support. The distance between the lateral orbital walls can then be read from the upper side of the scale; this distance can be noted for future reference. The examiner asks the patient to look straight ahead with eyelids wide open. The examiner measures for proptosis in each eye Seperately by looking into the mirror (which has a millimeter scale marked on it) with one eye and moving the head horizontally until the red fixations line is at 22mm. The examiner can now determine the position of the corneal apex of the patient from the millimeter reading.
The mnemonic VEIN is helpful in remembering the causes of proptosis.
V - Vascular causes
E - Endocrine causes
I - Inflammation and infective causes
N - Neoplastic causes
CT scan and MRI scan of the orbit may prove beneficial in diagnosing the cause for proptosis. The presence of fat in the orbit serves as an inherent contrast medium for the study. 3 mm cuts are ideal for the study of orbit. Ultrasound studies using either A or B mode may help in rapid diagnosis of the cause for proptosis.
Coronal CT of sinuses showing proptosis due to frontal mucocele
Vascular causes of proptosis:
Vascular causes of proptosis can be classified into arterial and venous causes. Venous causes are due to the formation of dilated veins known as varices. Patients with these varices give a classic history of positional proptosis (proptosis varying with positions) or proptosis being induced by valsalve manuver. In patients with long standing varices there is also an associated orbital fat atrophy leading on to a transient stage of enophthalmos. In these patients a valsalva manuver may reveal proptosis. CT scan performed with jugular venous compression or during a valsava manuver may prove diagnostic. Surgical intervention in these patients may prove disastrous, hence observation and treatment of complications is advisable.
In dural venous sinus fistula the shunt is low flow in type and proptosis is insiduous in onset, high index of suspicion is necessary in diagnosing these patients.
Carotid cavernous fistula (high flow shunts) may arise as a result of trauma or spontaneously. These patients have subjective bruits, proptosis, chemosis and vision loss. The conjunctival vessels become arteriolised assuming a cock screw pattern. A fistula of spontaneous occurence has a better chance of spontaneous resolution, but in intractable cases the shunt must be closed with a baloon or carotid artery ligation.
Endocrine causes of proptosis: are the most common cause of exophthalmos. The diagnosis is fairly simple because it is invariable associated with lid signs like lid lag. The major endocrine cause for proptosis is thyrotoxicosis. This condition is also known as Graves disease.
Characteristic features of endocrine causes of proptosis:
1. Presence of lid lag / lid retraction
2. Presence of temporal flare in the upper eyelid
3. Presence of orbital congestion
CT scan of the orbit show enlarged extra ocular muscles, there may also be a bulging of orbital septum due to protrusion of fat. This is pathognomonic of Grave's disease. TSH estimation show elevated levels in the serum.
Inflammatory causes of proptosis: In inflammatory proptosis the lesion could be either an idiopathic inflammatory orbital pseudotumor, or due to specific orbital inflammation. Proptosis in these patients appear suddenly and acutely. These patients are invariably toxic and febrile. Myositis of extra ocular muscles may cause pain when eyes are being moved. There may also be associated acute dacryo adenitis. There may also be peri optic neuritis causing blindness. Orbital inflammation, perioptic neuritis and dacryo adenitis are highly responsive to oral prednisolone.
Inflammations involving the paranasal sinuses may involve the orbit causing proptosis. The interveining walls between the medial orbital wall and the ethmoidal sinuses is paper thin (lamina papyracea) which can be easily breached by infections from the ethmoidal sinuses causing spread to the orbit. In proptosis caused by ethmoidal sinus pathology the eye is pushed laterally, where as proptosis due to maxillary sinus pathology causes deviation of the eye upwards and outwards. In frontal sinus pathology the eye is deviated downwards and outwards. Commonest sinus inflammatory cause for proptosis is the formation of mucoceles in the paranasal sinuses. This commonly occur in the fronto ethmoidal regions.
Neoplastic causes of proptosis: Neoplasms involving orbit may cause proptosis. Here the eye is pushed directly forwads. This type of proptosis is known as axial proptosis. Tumors involving the optic nerve can cause axial proptosis. These patients have pain free disease. The only exception to lack of pain is patients with adenocystic carcinoma of lacrimal gland. These patients have excessive pain because the tumor infiltrates the nerves.
Neoplastic lesions involving the paranasal sinuses can also cause proptosis. The common benign tumor involving the sinuses causing proptosis are:
1. Inverted papilloma
2. Fungal infections involving the paranasal sinuses
3. Mucoceles involving the paranasal sinuses
4. Fibrous dysplasia of the maxilla
5. Osteomas involving the frontal and ethmoidal sinuses
6. Juvenile nasopharyneal angiofibroma
Picture showing proptosis of left eye due to fibrous dysplasia of maxilla
Malingnant tumors involving the nasal cavity and paranasal sinuses can involve the orbit causing proptosis. This is because of the common intervening walls between the orbit and the paranasal sinuses.
Preventing complications like drying of the cornea should take precedence. This is done by prescribing artificial tears. Specific management is directed towards treating the cause for proptosis. Orbital decompression may be resorted to in extreme cases of proptosis due to Grave's disease. Common type of orbital decompression is medial orbitotomy where the medial wall of the orbit is removed facilitating collapse of the orbital contents into the nasal cavity. It can also be combined with lateral orbitotomy if the result of medial orbitotomy is not satisfactory. Inflammatory causes of proptosis must be treated with antibiotics.