Year : 1988 | Volume
: 36 | Issue : 4 | Page : 171--175
Computerised tomography of the orbits
Inder Talwar, GS Burde
C.T. Scan Department, Bombay Hospital, 12, Marine Lines, Mumbai - 400 020, India
G S Burde
C.T. Scan Department, Bombay Hospital, 12, Marine Lines, Mumbai - 400 020
Computerised Tomography has revolutionised the approach to many problems, especially orbital diseases. It is a simple, safe, non-invasive diagnostic examination done on an out patient. Computerised Tomography not only shows the presence of a lesion but also shows the extent of the disease and its relationship to other intraorbital and adjacent structures. With coronal scans, relationship of orbital disease to the optic nerve and extraocular muscles can be well demonstrated. However it must be remembered that Computerised Tomography is not a microscope to give histopathological diagnosis but we can often draw a conclusion of the nature of the disease by Computerised«SQ» Tomography appearances and clinical details. Now Computerised Tomography has become the primary investigation of choice in evaluating orbital disease.
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Talwar I, Burde G S. Computerised tomography of the orbits.Indian J Ophthalmol 1988;36:171-175
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Talwar I, Burde G S. Computerised tomography of the orbits. Indian J Ophthalmol [serial online] 1988 [cited 2020 Mar 29 ];36:171-175
Available from: http://www.ijo.in/text.asp?1988/36/4/171/26117
Computerised Tomography is a simple, safe, non-invasive diagnostic examination performed on an out patient basis. Before the advent of Computerised Tomography the orbit was a blind spot in radiology. Now, with the introduction of new faster high resolution scanners, Computerised Tomography offers new insight to orbital radio-diagnosis. It is used to confirm an orbital lesion and also to determine the nature of the disease with anatomical extent in detail. Computerised Tomography has become the primary, defenitive and diagnostic imaging modality in investigation of the orbit.
MATERIAL AND METHODS
In our study of the orbital disease, we have evaluated 40 cases in the last 2 years. Patients ranged from 3 months to 68 years. 26 were males and 14 were females. This was highly selected group of patients in a referral hospital. Computerised Tomography Scanning was performed with a Somatom DR3 (Siemens) machine, a third generation scanner using an average time of 5 seconds per `cut' (section). Scout view (Topogram) simulating lateral skull x-ray was taken routinely in all cases which helps to localise accurately, the axial scans to the exact anatomical level in the orbit. For axial scans, patients were in supine position and for coronal scans, which from an integral part of the orbit examination, patients were in a prone position. Routinely 5 mm, contiguous sections were obtained from Reid's base line upto the 3rd ventricle. In all patients initial plain CT was followed by post-contrast scans after administration of 60 ml of 76% Urograffin intravenously. The only preparation required was for all patients to be starving for 3 hours prior to the examination to minimize the side effects of nausea and vomiting caused by intravenous contrast. All patients were followed up. In all cases, where specimen or material was available, pathological corelation was done and final diagnosis obtained.
Observation and Results
In our series, the break up of various lesions is as follows:
I .Frequency of Orbital Disease
Slight discrepancy found is attributed to the type of patients referred to our institution.
Orbital anatomy is inherently well suited for study by Computerised Tomography, since the low density fat within and the dense bony perimeter of the orbit present a high contrast surrounding for the soft tissue structures within the orbit. Simultaneous visualisation of the soft tissues and bones together with adjacent paranasal sinuses, nasopharynx and brain, had made Computerised Tomography Scanning the primary radiological investigation of choice.
To interpret an orbital Computerised Tomography Scan, study of normal Computerised Ton ography anatomy of the orbit is a must. Extraocular muscles devide the retrobulbar space into intraconal and extraconal anatomic compartments. The first one is formed by the posterior wall of the globe, four rectimuscles, and orbital apex. At the orbital apex, the optic canal is separated by a bony strut from the superior orbital fissure. The intraconal optic nerve and the ophthalmic artery and the extraconal lacrimal gland were visualised consistently.
Orbital diseases may be divided into:
1. Primary: Originating within the orbit.
2. Secondary: Extension of pathology from adjacent structures i.e. paranasal sinuses, nasopharynx, brain.
In our series, we limit ourselves, to the primary intra-orbital disease.
Specific (1) Graves disease: We had the opportunity to study a case of this otherwise common condition, diagnosed with ease clinically. Bi-lateral enlargement of an extra ocular muscle is the hallmark of this disease. (ii) We have come across a case of cysticercosis, where oval calcific densities were scattered intra-cranially and within the left eye ball.
Non-specific inflammations of uncertain aetiology, i.e. Pseudo tumors, may be present as a mass of heterogenous density with ill-defined edge and variable form of enhancement. `Dirty Orbit' appearances may be seen due to altered orbital fat, and thickening of the posterior coats of the globe may occur. Air or air/fluid level may be visualised in the presence of acute cellulitis. Single rectus enlargement due to inflammation may be differtiated from Graves disease, where in the latter, the muscle insertion is spared.
Computerised defined orbital details, show the extent of soft tissue injury to the eyeball, extra-ocular muscles and optic nerve along with bony injuries. Foreign bodies embedded can be localised accurately. Orbital trauma is invariably associated with injuries where usually patients being uncooperative, role of conventional radiography is limited, whereas Computerised Tomography helps to detect associated intra-cranial bleeding if any. In our experience, Computerised Tomography has become an integral part in the evaluation and management of a patient with orbital trauma and is the preliminary investigation of choice.
We have seen 4 such cases Computerised Tomography appearances vary with each type of congenital disorder. Computerised Tomography is of value in assessing the extent of congenital defects and to rule out any organic causes. The 3 dimensional views of bone deformities and soft tissue planes is provided by Computerised Tomography so as to plan plastic surgery and obtain excellent cosmetic results.
Visualisation of an intraorbital lesion enhancing well on intravenous contrast in association with dilated orbital. vessels is diagnostic of vascular malformation. Computerised Tomography is non-invasive and gives useful diagnostic information, even in the presence of partial or complete thrombosis, not demonstrable by angiography. Hence the role of invasive procedures like angiography and venography is reduced. We have seen two such cases. One case of carotico-cavernous fistula seen by us was confirmed by angiography. The other patient refused the invasive procedure.
Intra-orbital tumours formed the bulk of our case series.
9 cases of suspected retinoblastoma were referred to us for further evaluation. It is the commonest tumour in childhood, originating in the retina, but has the potential to disseminate widely. Intraocular disc calcification is a diagnostic finding, but bilateral disc calcification is pathognomonic. Computerised Tomography showed the extent of the tumour and its relation to adjacent intracranial structures, which helped in planning the field of radiotherapy. Subsequent, follow-up of patients was maintained by Computerised Tomography, which helped us to detect recurrence in one of our cases. Because of early detection and improved therapy, the mortality rate had decreased from 95% to 9% over the past century (11).
Computerised Tomography is highly sensitive in detecting calcium as compared to conventional radiography. Computerised Tomography employing 13 mm sections was found 5-15 times more sensitive than a skull x-ray in detecting intra-cranial calcification (10).
Cavernous haemangioma is the most common orbital tumour usually seen in middle age with a high female preponderence (70%). This is a solitary, well-defined, invariably intraconal lesion of high attenuation value which shows dense homogenous enhancement. Calcification though known to occur was not seen by us. Neurilemmomas simulate a haemangioma, but are larger in size. There is no need to differentiate the two of them as excision by lateral orbitotomy is advised for both. Orbital varies should not be a problem to differentiate since it is slightly irregular and associated with dilated vessels.
Lacrimal gland tumours are diagnosed on Computerised Tomography by their typical location and appearance. We have diagnosed 3 such cases later confirmed by biopsy. There was an extraconal soft tissue density lesion of variable form, characteristically located supero-laterally in the anterior part of the orbit and showing non-specific contrast enhancement. Associated bowing of the zygomatic bone may be present due to chronic pressure.
We have seen 4 cases of lymphoma referred to us for unilateral proptosis. 3 cases were diagnosed by excision biopsy. One case is presumed to be a lymphoma because of its Computerised Tomography appearance and resistance to antibiotics and steroids. On Computerised Tomography it may be difficult to distinguish between a lymphoma and a granuloma. In such a situation, the patient may be given a therapeutic trial of steroids and antibiotics. If the patient does not respond to the treatment, the tumour is excised for biopsy. Lymphoid infiltrations have 4 common patterns of presentation:
1. Diffuse orbital inflammation.
2. Involvement of one or more extraocular muscles.
3. A solid mass lesion, often involving the lacrimal
4. Involvement of orbital fat with secondary scleritis,
and optic neuritis.
It is particularly difficult to differentiate benign from malignant lymphoid infiltrations.
Dermoids have a characteristic Computerised Tomography appearance. An extraconal, cystic, well-defined mass, usually in the anterior superior orbit with negative attenuation value is diagnostic of dermoid (negative values: 10 to 100 HU represents fat). 2 of our cases were proved by excision biopsy and one patient was not operated upon because of the patients age (3 months).
A large, variegated tumour with intracranial extension and calcification was seen. New bone formation, bony destruction and marked enhancement suggested the diagnosis of haemangiopericytoma on Computerised Tomography and was found to be the same after excision on histopathology. Detection of intraorbital lesions in 5 patients referred to us for proptosis who had undergone surgery for malignancy elsewhere, favoured the diagnosis of metastases. Computerised Tomography appearance is not specific. Metastases simulate a granuloma or lymphoma. A known primary gives the clue.
We had two cases of optic nerve tumours. Computerised Tomography showed a mass along the optic nerve of higher precontrast attenuation value which enhanced markedly after IV contrast, with calcification suggesting a meningioma and was confirmed at surgery. We have not however come across an optic nerve glioma which is the most common optic nerve tumour (80%). Gliomas are common before ten and in girls as opposed to meningiomas seen in middle aged men.
Corneal scanning is important to differentiate an optic nerve tumour from an extra-neural retrobulbar mass. Study of the optic nerve is extremely helpful in analysing orbital Computerised Tomography Scan. In patients referred for papilloedema, a thickened and tortuous optic nerve is visualised. Computerised Tomography provided the best technique to evaluate the extent of lesion, plan the field of radiotherapy and study the response to treatment and detect recurrence if any.
Our study shows that to some extent it is possible to give a pathological diagnosis, based on location and characteristics of a lesion on pi [-]sub am and contrast study. Lesions like dermoids, lacrimal gland tumours, meningiomas and vascular malformation are quite diagnostic on Computerised Tomography. However, it must be remembered that Computerised Tomography is not a microscope though it is often possible to draw a conclusion by taking into account clinical features and Computerised Tomography characteristics. One may mistake tumours with pseudo tumours especially lymphomas and granulomas, when biopsy is the only means to get the final answer.
The overall accuracy rate is greater than 90% in orbital radiodiagnosis.
(Lloyd 1977 - Forbes 1980)
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