What is fenestral otosclerosis?

What is Fenestral Otosclerosis?

Fenestral otosclerosis is a form of otosclerosis characterized by abnormal bone growth (otospongiosis) at or adjacent to the oval window of the middle ear, causing conductive hearing loss by fixating the stapes footplate and preventing normal sound transmission to the inner ear. 1

Pathophysiology and Location

Fenestral otosclerosis represents one of two main types of otosclerosis, distinguished by its specific anatomical location 1:

• The abnormal bony excrescences develop at or adjacent to the oval window, most commonly in the fissula ante fenestram (FAF) region anterior to the oval window 2, 3
• This pathologic bone formation causes fixation of the stapes footplate, preventing the normal mechanical transmission of sound waves through the ossicular chain to the cochlea 4
• The disease process involves an otodystrophy of the otic capsule, typically manifesting in the 2nd to 4th decades of life 1

Clinical Presentation

Fenestral otosclerosis causes conductive hearing loss, which is the hallmark presenting symptom 1, 5:

• Patients experience progressive hearing loss due to impaired sound conduction through the fixed stapes
• The hearing loss is characteristically conductive in nature, though mixed hearing loss can occur if retrofenestral (cochlear) involvement develops 1
• Acoustic reflexes are characteristically absent on audiometric testing 6

Diagnostic Imaging

High-resolution CT (HRCT) of the temporal bone using 1-mm or thinner sections is the imaging modality of choice for diagnosing fenestral otosclerosis 4, 1:

• CT demonstrates abnormal bony excrescences at or adjacent to the oval window on axial and coronal sections obtained at 0.5-mm intervals 2
• The fissula ante fenestram region shows the most reliable diagnostic findings, with bone density measurements below 1,871 Hounsfield units (HU) demonstrating 96.64% sensitivity and 100% specificity for diagnosis 3
• However, the sensitivity of HRCT as interpreted by general radiologists remains only 47.1%, with many cases requiring expert re-review for accurate diagnosis 6

Imaging Subtypes

Three subtypes of fenestral otosclerosis have been defined based on CT appearance 7:

• "Focal" fenestral otosclerosis: Limited, small lesions that are frequently misdiagnosed
• "Transitional" fenestral otosclerosis: Intermediate involvement
• "Typical" fenestral otosclerosis: More extensive, easily recognizable disease

Distinction from Retrofenestral Otosclerosis

Fenestral otosclerosis must be distinguished from retrofenestral (cochlear) otosclerosis, which has different clinical implications 1:

• Fenestral disease involves only the oval window region and causes pure conductive hearing loss
• Retrofenestral disease involves the cochlear capsule and causes sensorineural or mixed hearing loss
• Extensive otosclerosis with both fenestral and retrofenestral involvement shows significantly poorer preoperative bone conduction thresholds (30.3 dB) compared to isolated fenestral disease (24.6 dB) 8

Treatment Implications

The primary treatment for fenestral otosclerosis is surgical stapedectomy or stapedotomy with prosthesis insertion 1:

• Surgical outcomes are significantly better in isolated fenestral otosclerosis compared to extensive disease 8
• Overclosure of the air-bone gap greater than 10 dB occurs in 20% of isolated fenestral cases but only 2.85% of extensive otosclerosis cases 8
• Preoperative CT evaluation helps identify surgical obstacles including abnormally wide cochlear aqueduct, high jugular vein, or dehiscent facial nerve 2

Clinical Pitfalls

Several important caveats exist in the diagnosis of fenestral otosclerosis:

• Normal otoscopic examination is compatible with fenestral otosclerosis, as the disease process occurs within the temporal bone and may not be visible on routine ear examination 5
• Negative HRCT does not exclude the diagnosis—surgical confirmation may reveal stapes fixation even when imaging appears normal, particularly in focal disease 6
• In children without family history or in cases with mixed hearing loss, CT examination becomes more important for diagnostic confirmation 5