PHYSIOLOGY OF SYMPTOMS

Owing to the rigid bony structure of the orbit, with only an anterior opening for expansion (Chapter 1), any increase in the orbital contents taking place to the side of or behind the eyeball will displace that organ forward (proptosis). Protrusion of the eyeball is the hallmark of orbital disease. Expansive lesions may be benign or malignant and may arise from bone, muscle, nerve, blood vessels, or connective tissue. A mass may be inflammatory, neoplastic, cystic, or vascular. Protrusion is not in itself injurious unless the lids are unable to cover the cornea. The underlying cause, however, is usually serious and sometimes life-threatening. Pseudoproptosis is apparent prop-tosis in the absence of orbital disease. Such confusion may arise with high myopia, buphthalmos, and lid retraction.

History and examination provide many clues to the cause of proptosis. The position of the eye is determined by the location of the mass. Expansion within the muscle cone displaces the eye straight ahead (axial proptosis), whereas a mass arising outside the muscle cone will also cause sideways or vertical displacement of the globe directly away from the mass (nonaxial proptosis). Bilateral involvement generally indicates systemic disease, such as Graves' disease. The term "exophthalmos" is often used when describing proptosis associated with Graves' disease. Pulsating proptosis reflects the pulse of an orbital vascular malformation or transmission of cerebral pulsations in the absence of the superior orbital roof, as in neurofibromatosis-1. Positional proptosis-which changes with Valsalva's maneuver-is a sign of orbital varices or meningocele. Intermittent proptosis may be the result of a sinus mucocele. The Hertel exophthalmometer (see Chapter 2) is the standard method of quantifying the magnitude of proptosis. Serial measurements are most accurate if performed by the same individual with the same instrument.

With the change in position of the eyeball, especially if it takes place rapidly, there may be enough mechanical interference with the movement of the eye to cause dissociation of ocular movements and diplopia (double vision). Pain may occur as a result of rapid expansion, inflammation, or infiltration of sensory nerves. Vision is not usually affected early unless the lesion arises from the optic nerve. Pupillary signs and color vision testing may identify subtle optic nerve compression or involvement before acuity is reduced significantly. Involvement of the superior orbital fissure by trauma or tumor produces a characteristic combination of diplopia resulting from disturbance of function of the oculomotor, trochlear, and abducens nerves and corneal and facial anesthesia (ophthalmic division of trigeminal nerve), known as the orbital fissure syndrome. Expanding lesions at the orbital apex result in the orbital apex syndrome, characterized by proptosis and optic nerve compression, variably accompanied by the diplopia and corneal and facial anesthesia seen in the orbital fissure syndrome.

DIAGNOSTIC STUDIES

1. IMAGING

CT & MRI

Imaging by computed tomography (CT scan) (  Figures 13-1 and   13-2) was a major advance in orbital diagnosis. Continued improvement in resolution quality-as well as three-dimensional reconstructions-have made CT the single most important diagnostic study in the investigation of orbital disease. Contrast enhancement with CT during study of vascular lesions sometimes provides additional information. Magnetic resonance imaging (MRI) is capable of displaying subtle changes within soft tissue that cannot be imaged with CT, but it is less useful for bony changes. A surface coil applied directly to the orbit enhances image resolution. MRI is contraindicated in the presence of a ferrous intraorbital or intracranial foreign body.

Ultrasonography

The use of ultrasonography in the diagnosis of orbital disease has largely been supplanted by CT and MRI. Although it is a noninvasive and inexpensive form of imaging, its usefulness in both A and B mode is limited to the anterior portion of the orbit. It is of greatest value in the hands of the clinician- ultrasonographer capable of interpreting "real time" images.

Venography

Venography is occasionally useful in defining the extent of orbital venous disease. Although the diagnosis can be made by MRI, contrast injection into the orbital veins via a scalp vein can sometimes reveal the presence of varices that have escaped detection by CT.

Angiography

Selective carotid angiography with bone subtraction is sometimes necessary to make the diagnosis of certain orbital vascular disorders. In spontaneous, low-flow dural carotid artery-cavernous sinus fistula, angiography is required for delineation of the extent of involvement and for treatment by embolization.

Radiography

Plain x-rays are sufficient for diagnosis of many orbital disorders such as fractures. However, the thin walls of the orbit are difficult to visualize even with tomography, and CT or MRI imaging is used to determine the extent of injury. Dacryocystography and radionuclide scanning can sometimes be helpful in localizing the site of lacrimal obstructions, but these procedures are seldom used. The results are difficult to interpret, and treatment is seldom altered by the findings. Positive contrast radiography and pneumo-orbitography are no longer used. Orbital thermography is a research procedure.

Fine-Needle Aspiration

Fine-needle aspiration is an invasive procedure that has proved very useful in orbital diagnosis. Cytology specimens can be aspirated from a lesion the exact location of which is determined by CT imaging. Cytopathology can be inconclusive but is often invaluable.

10.1036/1535-8860.ch13

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