Paranasal sinuses and orbit
Nasal cavity, paranasal sinuses, orbit and its contents: anatomy and imaging
Coronal CT shows the sinuses, their drainage channels and the thin bone separating them from the orbit and brain; MRI shows the orbital contents. CPSP asks where each sinus drains, how infection and fractures reach the orbit, which nerve explains each eye sign, and the imaging signature of common masses.
Lateral nasal wall and sinus drainage
| Opening | What drains there |
|---|---|
| Sphenoethmoidal recess (above and behind the superior concha) | Sphenoid sinus |
| Superior meatus | Posterior ethmoidal cells |
| Middle meatus (hiatus semilunaris and ethmoidal infundibulum) | Frontal, maxillary, anterior and middle ethmoidal sinuses |
| Inferior meatus | Nasolacrimal duct |
- The ostiomeatal complex (uncinate process, infundibulum, maxillary ostium, ethmoidal bulla, middle meatus) is the shared drainage pathway. Blockage opacifies the maxillary, anterior ethmoid and frontal sinuses together, with the posterior ethmoid and sphenoid clear.
- The maxillary ostium is high on the medial wall, so drainage depends on cilia beating towards the natural ostium.
- Upper premolar and molar roots lie against the maxillary sinus floor: extraction can leave an oroantral fistula, and sinusitis causes toothache via the superior alveolar nerves.
- Present at birth: maxillary and ethmoid. The frontal sinus appears last, so its absence in a small child is normal.
Variants the radiologist must report
| Variant | What it is | Why it matters |
|---|---|---|
| Concha bullosa | Air inside the middle turbinate | Can narrow the middle meatus |
| Haller cell | Ethmoid cell below the orbital floor | Narrows the infundibulum |
| Onodi cell | Posterior ethmoid cell above and lateral to the sphenoid | Optic nerve runs in its wall |
| Deep olfactory fossa | Long lateral lamella of the cribriform plate | Thinnest bone; CSF leak at surgery |
| Carotid bulge in the sphenoid | ICA indenting a dehiscent lateral wall | Torrential bleeding in pituitary surgery |
- The nasal cycle swells each side's turbinates in turn, so asymmetry on one scan is normal.
Nasal cavity: lining, blood supply and the septum
- The vestibule has keratinised squamous epithelium with hairs; most of the cavity has respiratory epithelium; the roof has olfactory neuroepithelium (source of olfactory neuroblastoma, a dumbbell mass through the cribriform plate).
- Little's area on the anterior septum (anterior ethmoidal, sphenopalatine, greater palatine and superior labial arteries) is the site of most nosebleeds in the young. Posterior epistaxis comes from the sphenopalatine artery; bleeding high above the middle concha after sphenopalatine ligation is from the anterior ethmoidal artery.
- Septal haematoma: after nasal trauma, blood collects between the cartilage and its perichondrium on both sides. Septal cartilage has no vessels of its own and is nourished by diffusion from the perichondrium, so the separated cartilage dies (or an abscess forms) and the bridge collapses into a saddle nose within days. It is soft and fluctuant, unlike a deviated septum, and must be drained urgently.
- Other causes of saddle nose: granulomatosis with polyangiitis (with epistaxis and haematuria), congenital syphilis, cocaine use.
- Juvenile angiofibroma: adolescent boy, intensely enhancing mass at the sphenopalatine foramen, widening the pterygopalatine fossa and bowing the back wall of the maxillary sinus forwards; fed by the internal maxillary artery; never biopsied.
Sinonasal lesions on CT and MRI
| Lesion | Imaging clue |
|---|---|
| Mucocele (usually frontal) | Expanded airless sinus, bone remodelled, not destroyed; pushes the eye down and out |
| Fungal ball | Hyperdense contents with fine calcification |
| Antrochoanal polyp | From the maxillary sinus through a widened ostium into the nasopharynx |
| Inverted papilloma | Lateral wall at the middle meatus, convoluted cerebriform pattern |
| Silent sinus syndrome | Small airless maxillary sinus, orbital floor bowed down, enophthalmos |
Orbit: walls and openings
- Medial wall (lamina papyracea) is the thinnest: the route for ethmoid sinusitis to reach the orbit. The floor breaks in a blowout fracture: limited upgaze from inferior rectus entrapment, cheek numbness (infraorbital nerve), enophthalmos.
- Superior orbital fissure: lacrimal, frontal and trochlear nerves and the superior ophthalmic vein pass outside the common tendinous ring; III, nasociliary and VI pass inside it.
- The orbital septum separates preseptal cellulitis (lids only) from orbital cellulitis.
- Valveless ophthalmic veins link the face to the cavernous sinus, so facial infection spreads backwards.
Nerves and pupils: reading eye signs
| Sign | Lesion and reason |
|---|---|
| Ptosis, small pupil, dry flushed half of face | Horner syndrome: cervical sympathetic chain (loss of Müller muscle, dilator pupillae and facial sweating) |
| Ptosis, eye down and out, dilated pupil, sudden headache | Compressive III palsy, classically a posterior communicating artery aneurysm: pupillomotor fibres lie superficially on the nerve and are compressed first |
| Ptosis, eye down and out, pupil spared, diabetic | Ischaemic III palsy: the vasa nervorum supply the core of the nerve, so the superficial pupillary fibres survive |
| Cannot look laterally in cavernous sinus thrombosis | Abducens (VI) is affected first because it runs free in the sinus beside the ICA; III, IV, V1 and V2 lie in the lateral wall |
| Small irregular pupils that react to near but not to light | Argyll Robertson pupil (neurosyphilis): lesion near the pretectal nucleus spares the near-response pathway |
| Touching the right cornea gives no blink in either eye; left cornea gives both | Right trigeminal (V1), the afferent limb; the efferent facial nerves are intact. Loss of the corneal reflex is an early sign of an acoustic neuroma |
- Acute angle-closure glaucoma: painful red eye, haloes, vomiting, fixed mid-dilated pupil. Avoid atropine and other mydriatics: dilating the pupil crowds the iris into the angle and blocks aqueous outflow further.
Orbital masses on imaging
| Lesion | Imaging clue |
|---|---|
| Thyroid eye disease | Enlarged muscle bellies with tendons spared (inferior and medial rectus most often) |
| Cavernous venous malformation | Well-defined intraconal oval mass that fills progressively from one point |
| Orbital varix | Distensible vein that enlarges with Valsalva or prone positioning; intermittent proptosis on bending or straining; may look normal on a routine supine scan |
| Orbital lymphoma | Older adult, painless lid swelling, salmon-pink conjunctival patch; homogeneous mass that moulds around the globe without deforming it or eroding bone; stage for systemic disease |
| Dermoid cyst | Child, firm lump at the outer end of the eyebrow (frontozygomatic suture); fat density (about −80 HU) and smooth bone scalloping |
| Optic nerve sheath meningioma | Tram-track enhancement with calcification |
| Optic nerve glioma | Pilocytic astrocytoma: fusiform, kinked nerve without calcification in a child with NF1 (café-au-lait macules, axillary freckling); bilateral gliomas strongly suggest NF1 |
| Carotid–cavernous fistula | Dilated superior ophthalmic vein, pulsatile proptosis after trauma |
| Retinoblastoma | Calcified intraocular mass in a child with leukocoria |
| Choroidal melanoma | Mushroom shape, bright on T1, dark on T2 (melanin) |
- A midline nasal lump in a child may connect to the brain (encephalocele, nasal glioma); image before excision.
Remember Medial wall thinnest, floor breaks: infection goes medially, blunt force goes down.
Exam trap The frontal sinus drains into the middle meatus, not the superior meatus or the sphenoethmoidal recess.
Exam trap A dilated pupil with a third nerve palsy means compression (aneurysm) until proven otherwise; pupil sparing suggests diabetic ischaemia.
How FCPS asks about this
- Drainage of frontal, maxillary and anterior ethmoid sinuses: middle meatus; nasolacrimal duct to the inferior meatus.
- Thinnest orbital wall: medial (lamina papyracea); floor fractures in blowout injury.
- First nerve affected in cavernous sinus thrombosis: abducens.
- Painful III palsy with dilated pupil: posterior communicating artery aneurysm.
- Collapsed nasal bridge a week after trauma: septal haematoma; cartilage lost its nutrition.
- Fusiform optic nerve enlargement in NF1: optic pilocytic astrocytoma.
- Drug to avoid in acute angle-closure glaucoma: atropine.
Test yourself on this topic
5 of the 119 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 15-year-old was hit on the nose in a football match and had a soft swelling on both sides of the nasal septum. He was not seen for a week and now has a collapsed nasal bridge. What mechanism caused the collapse?
Answer: Septal cartilage lost its nutrition
A septal haematoma collects between the septal cartilage and its perichondrium. Septal cartilage has no blood vessels of its own and receives its nutrients by diffusion from the perichondrium. When blood lifts the perichondrium off on both sides, the cartilage is starved and necroses within days, especially if the haematoma becomes infected. Loss of septal support gives a saddle-nose deformity.
Septal haematoma
- Blood under the perichondrium
- Cartilage has no own vessels
- Necrosis in days
- Result: saddle nose
Remember Drain a septal haematoma the same day.
Exam trap A septal haematoma is easily mistaken for a deviated septum; it is soft and fluctuant.
Why the other options are wrong
- Displaced fracture of the nasal bones Nasal bone fractures cause deviation, not loss of septal support.
- Osteomyelitis of the nasal bones Osteomyelitis of the nasal bones is rare; the cartilage is the casualty.
- Resorption of the vomer bone The vomer is bone with its own blood supply; it is not the structure lost.
- Weakening of the alar cartilages Alar cartilages support the nostrils, not the bridge.
A 45-year-old man has slow, painless downward and outward displacement of his right eye. At surgery the right frontal sinus is expanded with thinned, bowed bony walls and is filled with thick sterile mucus; there is no tumour. What is the diagnosis?
Answer: Frontal sinus mucocele
A mucocele is an expanded, mucus-filled sinus whose outflow has been obstructed. Secretions accumulate slowly and the pressure remodels and thins the bony walls without destroying them. The frontal sinus is the commonest site, followed by the ethmoid, and a frontal mucocele pushes the eye down and out by eroding the orbital roof. If it becomes infected it is called a pyocele.
Mucocele
- Obstructed sinus fills with mucus
- Walls thinned and expanded
- Commonest: frontal sinus
- Eye pushed down and out
Remember Expanded, airless sinus: mucocele.
Exam trap Bone remodelling without destruction suggests a slow benign process; true destruction suggests malignancy.
Why the other options are wrong
- Frontal sinus osteoma A very dense bony tumour, not a mucus-filled sinus.
- Inverted papilloma A papillary epithelial tumour, usually from the lateral nasal wall.
- Pott puffy tumour Frontal osteomyelitis with a subperiosteal abscess and doughy forehead swelling.
- Lacrimal gland tumour Lies in the orbit itself and pushes the eye down and in.
A coronal CT of the sinuses shows that the left middle turbinate is enlarged and contains air, pushing the uncinate process laterally. What is this finding?
Answer: Concha bullosa
Concha bullosa is pneumatisation of a turbinate, most often the middle turbinate, by an extension of the ethmoid air cells. It is a common variant and usually harmless. A large one can narrow the middle meatus and ostiomeatal complex and press on the uncinate process, predisposing to sinusitis. It is often seen with deviation of the septum to the opposite side.
Concha bullosa
- Air within a turbinate
- Usually the middle turbinate
- May narrow the middle meatus
- Common, often harmless
Remember A turbinate with air inside is a concha bullosa.
Exam trap Do not report a concha bullosa as a mass; it contains air and is bounded by thin bone.
Why the other options are wrong
- Haller cell An ethmoid cell under the orbit, not within a turbinate.
- Antrochoanal polyp A soft-tissue mass from the maxillary sinus, not an air-filled turbinate.
- Septal spur A bony projection of the septum, not air in a turbinate.
- Mucocele An expanded, airless sinus filled with mucus.
A 40-year-old man has slowly progressive painless proptosis with his left eye pushed downwards and medially. A firm, non-tender mass is felt under the lateral part of the upper orbital rim. From which structure does the mass most likely arise?
Answer: Lacrimal gland
The lacrimal gland lies in the lacrimal fossa of the frontal bone in the superolateral orbit. A mass here pushes the globe down and medially and may be felt under the upper outer orbital rim. A slow-growing painless tumour suggests a benign epithelial tumour, most often pleomorphic adenoma of the lacrimal gland, which scallops the bone without destroying it. Malignant tumours such as adenoid cystic carcinoma are painful and destroy bone.
Lacrimal gland mass
- Lacrimal fossa: superolateral orbit
- Pushes eye down and medially
- Smooth scalloping: pleomorphic adenoma
- Pain + bone destruction: adenoid cystic
Remember Down and in: look up and out.
Exam trap A suspected lacrimal pleomorphic adenoma should be removed whole, not biopsied, to avoid seeding.
Why the other options are wrong
- Superior rectus muscle Lies superiorly but medially within the cone; its masses give axial or downward proptosis.
- Optic nerve sheath Would give axial proptosis from within the cone.
- Frontal sinus A frontal mucocele erodes from above and medial, not from the lacrimal fossa.
- Superior oblique muscle Lies superomedially; a mass would push the eye down and laterally.
A 50-year-old non-diabetic woman in Lahore has a foul smell and right-sided nasal blockage. At endoscopic surgery the right maxillary sinus contains a cheesy, brown-green mass; histology shows densely packed septate hyphae branching at acute angles, with no invasion of the mucosa. What is the lesion?
Answer: Aspergillus fungal ball
A fungal ball (mycetoma) is a dense mass of fungal hyphae packed within one sinus, usually the maxillary, in an immunocompetent person. Narrow septate hyphae branching at acute angles are typical of Aspergillus, the commonest fungus in sinus disease. The fungus does not invade the mucosa, so the bone is intact. Calcium salts concentrated by the fungus give the classic central hyperdensity and calcification on CT.
Fungal ball
- Usually maxillary sinus
- Septate hyphae at acute angles
- Commonest fungus: Aspergillus
- No invasion; host immunocompetent
Remember Packed septate hyphae, no invasion: fungal ball.
Exam trap Broad aseptate hyphae at right angles mean mucormycosis, which is invasive and an emergency.
Why the other options are wrong
- Rhizopus invasive infection Mucor has broad, ribbon-like, pauciseptate hyphae branching at right angles and invades vessels.
- Antrochoanal polyp A mucosal polyp, not a mass of hyphae.
- Maxillary sinus carcinoma A malignant epithelial tumour, not fungal material.
- Bacterial sinusitis with pus Pus contains neutrophils and bacteria, not packed hyphae.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Head and neck, Nasal cavity, paranasal sinuses, orbit and its contents
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