Pneumosinus dilatans (PSD) is a rare disease of unknown cause in which one or more paranasal sinuses become hyperpneumatized and enlarged beyond normal anatomical boundaries. The sinus mucosa is normal, and the bony walls become thinned but without bone erosion. It was first named “Pneumosinus frontalis dilatans” by Benjamins in 1918 1).
PSD must be distinguished from similar conditions. It is conceptually different from hypersinus (sinus enlargement within normal anatomical boundaries) and pneumatocele (enlargement with bone erosion) 2).
With approximately 145 reported cases in the literature, it is a rare disease, and its exact incidence is unknown. It is more common in males (about twice as often as females), with an average age of onset of 33 years, showing a bimodal age distribution between 16–25 years and 36–45 years.
The most commonly involved sinus is the frontal sinus (62.8%), followed by the sphenoid sinus (24.1%), maxillary sinus (19.3%), and ethmoid sinus (18.6%). When the sphenoid or ethmoid sinuses are involved, the enlarged sinus can compress the optic nerve, potentially causing compressive optic neuropathy.
Only about 145 cases have been reported in the literature, and the incidence rate has not been clearly established. It is more common in males (approximately twice as often as in females), with an average age of onset of 33 years, showing a bimodal distribution between 16–25 years and 36–45 years.
PSD may be discovered as an incidental radiological finding without symptoms. Subjective symptoms vary depending on the location of the affected sinus.
In PSD involving the sphenoid and ethmoid sinuses, findings of compressive optic neuropathy are observed.
Abri Aghdam et al. (2021) reported a case of bilateral visual impairment in a 20-year-old woman 2). She had been aware of gradual vision loss since 4 years ago in the right eye and 1 year ago in the left eye. At the initial visit, visual acuity was light perception in the right eye and 20/400 in the left eye. The right eye showed a positive relative afferent pupillary defect (RAPD), and color vision was 2 out of 14 plates (Ishihara) in the left eye. Fundus examination revealed optic atrophy in both eyes, optical coherence tomography (OCT) showed severe bilateral thinning of the retinal nerve fiber layer (RNFL), and computed tomography (CT) confirmed bilateral optic canal stenosis and protrusion of the optic nerves into the sphenoid sinus.
Slowly progressive painless vision loss is typical. It ranges from transient monocular blindness to permanent vision loss, and about two-thirds of PSD patients have some visual impairment 2). Long-term disease can lead to optic atrophy, and vision may not recover even after decompression surgery.
PSD is usually idiopathic (primary), but can be secondary to underlying conditions.
The following four hypotheses have been proposed regarding the pathophysiology of PSD.
Ball-valve theory
Mechanism: A polyp or mucosal abnormality at the ostium forms a one-way valve, allowing air to enter but not exit, creating a pressure gradient.
Limitations: Anatomically, this can only explain frontal sinus PSD and is difficult to apply to sphenoid and ethmoid sinus PSD. Reports have noted mucosal changes at the ostium.
Fibro-osseous lesion theory
Mechanism: PSD associated with fibrous dysplasia, ossifying fibroma, and congenital bone growth disorders has been reported.
Evidence: Cases have been reported where surgical resection of deformed bone halted the progression of PSD.
Hormonal Theory
Mechanism: The paranasal sinuses enlarge in response to hormonal changes in genetically susceptible individuals.
Evidence: There are cases with onset and rapid progression during puberty, and a bimodal age distribution supports this hypothesis. Associations with parathyroid hormone, thyroid, and gonadal hormone abnormalities have been reported2).
Intracranial Lesion-Related Theory
Mechanism: Meningiomas contribute to the development of PSD by inducing bone proliferation through changes in intracranial pressure or local growth factor release.
Note: Optic neuropathy associated with PSD may arise from a coexisting optic nerve sheath meningioma rather than from PSD itself.
The optic nerve and posterior sinuses are adjacent through extremely thin bone, providing an anatomical background that facilitates the spread of lesions to the optic nerve. Sinus lesions in the posterior ethmoid sinus are particularly prone to compressing the optic nerve.
In secondary PSD, the association with anterior skull base meningiomas or optic nerve sheath meningiomas is most important, so contrast-enhanced CT and MRI are recommended for detailed examination 1). Some cases of optic neuropathy associated with PSD may be due to a coexisting optic nerve sheath meningioma rather than PSD itself.
Diagnosis of PSD requires a combination of ophthalmic examinations and imaging tests.
The differentiation of three types of paranasal sinus expansile lesions is shown below.
| Disease | Bone wall changes | Mucosal findings |
|---|---|---|
| PSD | Thinning only | Normal |
| Hypersinus | No change | Normal |
| Pneumatocele | Erosion present | Abnormal |
The following baseline tests are performed at the initial visit.
The differential diagnosis of PSD with optic neuropathy is broad.
The treatment strategy for PSD is determined based on symptoms, presence of optic neuropathy, and underlying diseases.
The principle of treatment for rhinogenic optic neuropathy is surgical curettage of the sinus lesion, which also serves as a biopsy, and optic canal decompression if necessary.
In a case report by Abri Aghdam et al. (2021) of a 20-year-old woman, optic nerve decompression via transnasal sphenoidotomy was performed after correction of primary hypothyroidism, but no visual improvement was observed at 9-month follow-up 2). This was attributed to permanent axonal loss confirmed by OCT, emphasizing that visual recovery is not always achieved even in idiopathic PSD.
Early surgical decompression may allow recovery. However, if compression of the optic nerve persists for a long time, permanent axonal loss may occur, and visual improvement may not be achieved even after surgery2). Therefore, early decompression is recommended when vision loss occurs.
As pneumatization of the sphenoid sinus progresses, optic neuropathy occurs through the following mechanisms.
The common pathophysiology of compressive optic neuropathy often involves lesions at the orbital apex. Initially, optic disc swelling is observed, but if treatment is delayed, atrophy occurs, and at this stage the visual prognosis becomes poor.
The bimodal age distribution—young adults aged 16–25 and middle-aged adults aged 36–45—may reflect different pathophysiological processes. It has been suggested that in younger patients, sinus expansion in response to hormonal changes may be involved, while in middle-aged patients, different mechanisms such as ball-valve obstruction may play a role.
Ball-valve mechanisms and gas-producing bacterial infections have also been proposed, but none have been definitively proven2).
The pathogenesis of PSD remains unclear, and further research is needed for this disease.
Demir et al. (2025) reported the world’s first case of a sphenoethmoidal PSD associated with a right frontal off-midline mature teratoma (35×40×43 mm) in an 18-year-old male1). It was discovered following a left-sided convulsive seizure, and total resection was performed surgically, with pathology confirming a mature teratoma. Previously, only meningiomas were noted as lesions associated with PSD, but this report indicates that off-midline mature teratomas should also be included in the differential diagnosis.
Abri Aghdam et al. (2021) reported a case of bilateral visual impairment due to idiopathic PSD in which visual recovery was not achieved even 9 months after surgical decompression, emphasizing that visual impairment is not always reversible in idiopathic PSD 2). They discussed the importance of early decompression and also reported an association between PSD and hypothyroidism.
Demir MK, Yapicier O, Kiliç D, Kilic T. Intracranial off-midline mature teratoma and pneumosinus dilatans: a unique clinical report. Asian J Neurosurg. 2025;20:165-169.
Abri Aghdam K, Aghajani A, Soltan Sanjari M. Bilateral visual loss caused by pneumosinus dilatans: idiopathic cases are not always reversible. J Curr Ophthalmol. 2021;33:197-200.
Pereira S, Vieira B, Maio T, Moreira J, Sampaio F. Susac’s Syndrome: An Updated Review. Neuroophthalmology. 2020;44(6):355-360. PMID: 33408428.
