Discussion

It is well known that irreversible middle-ear or mastoid pathology can develop behind an intact tympanic membrane.^{Reference da Costa, Paparella, Schachern, Yoon and Kimberley6,Reference Jaisinghani, Paparella, Schachern and Le7} Acquired middle-ear cholesteatomas behind intact tympanic membranes have received increasing attention because the principal complaint is hearing loss.^{Reference Matsuzawa, Iino, Yamamoto, Hasegawa, Hara and Shinnabe3,Reference Lee, Hong, Kim, Park and Baek4,Reference Di Lella, Bacciu, Pasanisi, Ruberto, D'Angelo and Vincenti8,Reference Mills9}

Some studies on human temporal bones have found that granulation tissue is the most common form of middle-ear pathology behind an intact tympanic membrane.^{Reference da Costa, Paparella, Schachern, Yoon and Kimberley6,Reference Jaisinghani, Paparella, Schachern and Le7} da Costa et al.^{Reference da Costa, Paparella, Schachern, Yoon and Kimberley6} observed that various extents of granulation tissue were the most prominent pathological features of 96 per cent of temporal bones with tympanic membrane perforations and 97 per cent of bones without such perforations. Paparella et al.^{Reference Paparella, Schachern and Cureoglu10} found that over 80 per cent of patients lacked tympanic membrane perforations when a total of 155 temporal bones were associated with chronic otitis media. However, most cases may be asymptomatic or exhibit only minor symptoms. A granulation pathology may grow slowly over many years, to be detected only when symptoms appear. Meyerhoff et al.^{Reference Meyerhoff, Kim and Paparella11} reported that only 24 of 36 ears with granulation tissue and an intact tympanic membrane exhibited otological symptoms.

In the present study, all tympanic membranes exhibited pars flaccida retraction. Granulation and retraction pathologies may be closely associated with Eustachian tube dysfunction and negative pressure within the middle ear. Chronic Eustachian tube dysfunction causes negative pressure within, and ventilation abnormalities of, the middle ear, gradually inducing retraction associated with two pathological changes,^{Reference Yoon, Schachern, Paparella and Aeppli1} one of which is persistent inflammation of the middle-ear mucosa (which may trigger epithelial invagination) and the other which is acquired cholesteatoma.

Some authors have reported attic cholesteatomas associated with tiny pars flaccida retractions.^{Reference Matsuzawa, Iino, Yamamoto, Hasegawa, Hara and Shinnabe3–Reference Kim, Jung, Sung, Kim and Kim5} Negative pressure within the middle ear may trigger oedema and granulation tissue proliferation. If the latter tissue is quiescent during the development of the Eustachian tube, no cholesteatoma develops. The granulation pathology may grow silently over many years, becoming very large at the time of detection.^{Reference Di Lella, Bacciu, Pasanisi, Ruberto, D'Angelo and Vincenti8} Surgical exploration and CT confirmed that the granulation pathologies of our patients were confined principally to the Prussak space and aditus ad antrum. Kim et al.^{Reference Kim, Jung, Sung, Kim and Kim5} examined temporal bone CT scans of patients with tinnitus and intact tympanic membranes and found dense soft tissue lesions within the Prussak spaces of 12.3 per cent such patients (18 of 146). Another study found that the aditus ad antrum was obstructed by granulation tissue in 75 per cent of patients.^{Reference Alleva, Paparella, Morris and da Costa12} The granulation pathology was confined to the mastoid in only two patients of our present study. A previous work suggested that out of the protympanum, mesotympanum, isthmus, attic, aditus ad antrum and isolated groups of mastoid air cells, any or all of these may become narrowed or obstructed.^{Reference Alleva, Paparella, Morris and da Costa12,Reference Schachern, Paparella, Sano, Lamey and Guo13}

Low-pitched tinnitus was the chief complaint of our patients (100.0 per cent), followed by ear fullness (23.7 per cent); no patient complained of significant hearing loss. The pre-operative mean air–bone gap was 14.7 ± 2.3 dB. These findings differ from those in patients with cholesteatomas behind intact tympanic membranes; the chief complaint is hearing loss.^{Reference Matsuzawa, Iino, Yamamoto, Hasegawa, Hara and Shinnabe3–Reference Kim, Jung, Sung, Kim and Kim5} This may reflect the extent of the lesion. Previous studies have suggested that a cholesteatoma behind an intact tympanic membrane may damage the ossicular chain. However, we found no such damage. The tinnitus disappeared completely or improved significantly in 96.7 per cent of our patients after surgery.

We speculate that the presence of middle-ear granulation tissue may correlate with tinnitus. However, the mechanism remains unclear. Ventilation and drainage path obstructions may be important. Obstructions may trigger anatomical malformation of, or variations in, the middle ear or mastoid.^{Reference Paparella, Schachern and Cureoglu10} Although some such blockages do not affect hearing, persistent obstruction triggers effusion, release of inflammatory mediators, and local pathological changes (scarring and mucosal fibrosis), thereby inducing tinnitus. In addition, local obstructions may change the middle-ear airflow direction or reduce the diameter of the airflow passage, triggering tinnitus. The mechanism is similar to that of snoring caused by upper airway obstruction. Also, certain granulation pathologies may obstruct or stimulate the round window niche. Vambutas and Paparella^{Reference Vambutas and Paparella14} reported that granulation tissue was most frequently (76 per cent) located in the epitympanum and round window niche, involving the mastoid antrum in only 52 per cent and the mastoid bone in 61 per cent of cases. Hidden inflammation or a bacterial infection damaged the inner ear via the round window. Some authors have suggested that tinnitus reflects inappropriate interactions of the middle and inner ear that are often unrecognised because the symptoms of labyrinthine disease may be non-specific.^{Reference Vambutas and Paparella14}

Although CT often underestimates the extent of disease, CT showed local dense soft tissue without bony erosion in the present study, and a granulation pathology was surgically confirmed in all patients. We performed canal wall up mastoidectomy on 88.1 per cent of our patients and endoscopic endaural atticoantrotomy on 11.9 per cent of patients. The latter procedure is appropriate for patients with attic pathologies, but mastoidectomy remains the principal procedure. No complication (iatrogenic sensorineural hearing loss, facial nerve palsy or vertigo) was observed during the follow-up period.