Fig. 20.1 Periosteum covers the orbital bones and adheres firmly at the anterior rim and at the apex around the optic canal. It is continuous with intracranial dura through the superior orbital fissure and optic canal. The lateral wall of the orbit is formed by the zygomatic bone and greater wing of the sphenoid. The frontal bone and part of the lesser wing of the sphenoid make up the roof. The fossa of the lacrimal gland lies superotemporally as a recess in the frontal bone, just above the junction with the zygoma. The floor is formed anteriorly by the maxillary process of the zygoma, centrally by the orbital plate of the maxilla and posteriorly by a small portion of the palatine bone. The infraorbital fissure lies between the greater wing of the sphenoid and the orbital plate of the maxilla and communicates with the pterygopalatine fossa posteriorly. The infraorbital groove carries the infraorbital nerve (V_b) and vessels. The medial wall is formed from anterior to posterior by the frontal process of the maxilla, the lacrimal bone (with the fossa of the lacrimal sac lying between), the ethmoid and the body of the sphenoid.

Fig. 20.2 The superior orbital fissure lies between the greater wing of the sphenoid inferiorly and the lesser wing superiorly (see Fig. 20.1). The lacrimal and frontal branches of the fifth nerve, the trochlear nerve and the superior ophthalmic vein are found superiorly and external to the annulus of Zinn; the superior and inferior divisions of the third nerve, the sixth nerve and nasociliary nerve lie within the annulus. The optic canal lies in the body of the sphenoid, medial and superior to the superior orbital fissure. It is lined with dura and transmits only the optic nerve and ophthalmic artery which lies inferior to the nerve. Space-occupying lesions at the orbital apex may compress the optic nerve and ocular motor nerves. Infiltrating lesions can spread through the superior orbital fissure into the cavernous sinus and middle cranial fossa to cause pain and loss of sensation in the distribution of the ophthalmic division of the trigeminal nerve. The sphenoidal sinus lies medial to the optic canal and the two are separated by extremely thin bone, which may even be absent, so that the optic nerve is easily damaged by surgery or disease within the sphenoidal sinus.

Fig. 20.3 The middle cranial fossa and temporal lobes lie posterior to the orbit and are separated from it by the sphenoidal wings. The temporalis muscles and fossa lie laterally and the maxillary sinus lies inferomedially.

Fig. 20.4 Anatomical sections enable a clear correlation to be made between the orbital anatomy and the computed tomography (CT) appearance. Note the close relationship of the frontal lobes to the orbital roof, the anterior and posterior ethmoidal sinuses to the medial orbital wall and the sphenoidal sinus to the optic canal.

Fig. 20.5 CT scanning still has advantages over magnetic resonance imaging (MRI) in the orbit as the bony anatomy and calcification (an important sign of many pathological processes) is readily seen on CT. Axial scans taken along the line joining the external auditory meatus and inferior orbital rim cut the globe through the lens, optic nerve and optic canal in a single plane. The superior orbital fissure can be mistaken for the optic canal, which lies medially and superiorly, adjacent and medial to the anterior clinoid process. The optic nerve has a sinuous course within the orbit and cannot always be seen on a single cut if the section is thin. Similarly, partial sections of the ocular muscles may resemble a space-occupying mass at the apex if the scan is not interpreted carefully. Coronal scans are frequently much more informative and easier to interpret.

Fig. 20.6 A dissection of the lateral aspect of the orbit shows the orbital lobe of the lacrimal gland and its nerve. The lateral rectus muscle and sixth nerve are retracted to demonstrate the ciliary ganglion and short ciliary nerves. The ciliary ganglion (b) contains not only the synapses of the parasympathetic fibres to the iris and ciliary body from the third nerve, but also the efferent (and nonsynapsing) sympathetic fibres to the ocular blood vessels. Afferent sensory fibres from the cornea, iris and ciliary body pass through the ganglion to the nasociliary branch of the ophthalmic nerve.

Fig. 20.9 Orbital disease may arise from abnormalities of the normal constituents of the orbit, contiguous spread of disease from adjacent structures, or invasion from haematogenous spread.

Fig. 20.12 Acute orbital cellulitis presents with fever and malaise, pain, proptosis, restriction of eye movements and conjunctival injection and oedema. This girl has an acutely swollen and painful left upper lid with the globe depressed from cellulitis in the superior part of the orbit secondary to a frontal sinusitis. Patients require hospital admission, nasal and blood cultures and intravenous antibiotics. These should not be delayed for investigations such as imaging. An orbital or subperiosteal abscess generally requires surgical drainage.

Fig. 20.13 CT scan from another patient showing an infective extension from the ethmoidal sinus into the orbit with a small subperiosteal abscess elevating the periosteum.

Fig. 20.14 Cavernous sinus thrombosis is now a rare complication of orbital cellulitis but must be considered when clinical signs progress rapidly with associated meningism, ocular motor and pupillary palsies, and signs of acute inflammation in the cerebrospinal fluid. MRI shows the thrombosis in the cavernous sinus better than CT.

Fig. 20.15 Embryonal sarcomas are tumours of mesenchymal cells with the potential to differentiate into striated muscle (striated or nonstriated rhabdomyomatous differentiation). They are commonly known as rhabdomyosarcomas although they do not arise from striated muscle. They are the commonest primary orbital malignancy of childhood. They can grow very rapidly; this child’s proptosis increased almost daily.

Fig. 20.16 CT scan showing a mass in the region of the medial rectus. After diagnostic biopsy treatment with radiotherapy and chemotherapy carries an excellent prognosis provided the tumour is confined to the orbit. These lesions can invade the orbital walls and metastasize haematogenously to the lung, lymph nodes and bone marrow.