Pathology

Diseases of the orbit

 
lntraorbital expanding lesions

With tumours or inflammatory lesions within the orbit the patient can have exophthalmus, poor vision, pain or double vision. Despite the large variation in pathology the symptomatology is fairly similar for all lesions in this area.

Retrobulbar tumours can be classified into intraconal and extraconal depending on whether or not they are situated within or outside the muscle

a   b c Figure 43. Pseudotumoral plaque a) MRl, T2-weighted image: large foci of hyperintensity within the white matter of the left hemisphere. b, c) MRl T1-weighted image following Gd injection. Extensive and marked enhancement of the lesions of the left side; appearance of a small focus also on the right side.

cone. It is important to note that overlapping can occur. Intraconal: optic glioma, optic meningioma, cavernous and capillary haemangioma, haemangiopericytoma, lymphangioma and neurofibroma. Extraconal: lacrimal gland tumour (pleomorphic adenoma, adenoid cystic cancer), dermoid, lymphoma, pseudotumour, rhabdomyosarcoma and metastases.

Of intrabulbar tumours, retinoblastoma and malignant melanoma should be mentioned. Retinoblastoma usually occurs during the first two years of life as a calcified les ion in the posterior part of the bulb. Another cause of calcification in the area of the papilla is drusen which is a calcium-containing lesion of the papilla and is often familial. Malignant

Figure 44. Cavernous haemangioma within the left orbit.

melanoma has a pathognomonic signal quality at MRI, relatively high signal with T1-weighted images and relatively low signal with T2-weighted images.

Thickening of the optic complex can be caused by several conditions; optic glioma, optic meningioma, optic neuritis, lymphoma, leukaemia, sacroidosis, thyrotoxicosis, oedema with raised intracranial pressure and high subarachnoid space.

Optic glioma is often seen in childhood and has an increased incidence in patients with neurofibromatosis. The commonest configuration is fusiform but it can be exophytic or tubular. The tumour is usually homogeneous. However, on CT, varying contrast enhancement of different parts of the tumour is commonly seen. Large tumours can exhibit cyst formation. The tumour may have a significant intracranial component and this is best shown by MRI.

Meningioma of the optic sheath usually occurs in adults but can be seen in young patients suffering from neurofibromatosis. In contrast to optic glioma which respects the dura the meningioma invades the dura and this may appear irregular. Meningiomas often contain calcification in contrast to optic gliomas. A tubular shape is commonest but fusiform and exophytic tumours occur. A characteristic but not pathognomonic, finding at intravenous injection of contrast medium is that the sheath meningioma exhibits higher attenuation than the optic nerve.

Cavernous haemangiomas is the commonest orbital tumour in adults (Fig. 44). They have a round or oval shape and smooth borders. If contrast medium is administered at CT, the attenuation increases from negative to approximately isodense with the eye muscles. In capillary haemangioma, which occurs in children, MRI may reveal a discrete vacularity.

Figure 45. Marked muscle thickening in dysthyroid myopathy with compression of the optic nerves in the apex regions.

 

Lacrimal gland tumours occur in the upper lateral quadrant of the orbit. Both benign (pleomorphic adenoma) and malignant forms (adenoid cystic cancer) occur. The latter invades bone.

Dermoids are well defined and often situated anteriorly in the upper half of the orbit. The cystic part is in general hypodense at CT but this is not always the case and depends on the dermoid's content. Similar varying signal intensity is seen at MRI. Erosion is not uncommon in the upper lateral part of the orbit. A coexisting fluid level (fat and fluid) is pathognomonic.

Lymphoma is most common in older people and can occur as a primary tumour of the orbit or may be secondary to a systemic illness. Usually lymphomas are diffuse, infiltrating, retrobulbar lesions which are iso- or hyperdense at CT. Bone erosion is common.

Pseudotumour is a chronic non-specific inflammation of unknown aetiology. It may appear similar to lymphoma. It is a diffuse expanding lesion which, in its extreme form, can stretch from the apex anteriorly to the bulb. The sclera is often thickened as are individual muscles.

Rhabdomyosarcoma is the commonest malignant orbital tumour in children. It usually occurs extraconally but can also occur intraconally. At CT it displays medium density without contrast medium. It is important that the diagnosis is made and treatment started as soon as possible because of the tumour's aggressive behaviour.

Metastasis is commonest from breast, lung, kidney, and colon cancer. Of these metastases, those from breast and lung tumors occur most commonly. The metastases display very varying morphological patterns.

Muscle thickening is commonest in hyperthyroidism (dysthyroid myopathy (Fig. 45) and can occur before, during and after the hyperthyroid stage. If only individual muscles are thickened, the muscles most

a    b Figure 46. Large intra- and suprasellar pituitary adenoma. MRI in the coronal plane (a) and the sagittal plane (b). The optic chiasm (arrow) is displaced upwards.

commonly affected are the medial and inferior rectus. Isolated thickening of the inferior rectus muscle must not be misdiagnosed as tumour.
Inflammation, in the form of orbital or periorbital cellulitis, can occur as a result of underlying sinusitis or following trauma with a foreign body. The inflammatory lesion can display varying appearances with extra- and intraconal involvement. It may develop into an abscess. Mucocele can invade the orbit and become inflamed.

Trauma

CT is excellent for the mapping of traumatic lesions, haematoma, fractures, retrobulbar air (indicating fracture of the wall of the orbit), bulb laceration and foreign bodies. It should be noted that splinter fragments can display varying attenuation depending on the type of wood and on the water content.

 

Kjell Bergström and Giuseppe Scotti