Discussion

Aneurysmal bone cysts were first described in 1942 by Henry L Jaffe and Louis Lichtenstein, and were defined as locally aggressive, blood-filled cystic lesions of the skeletal system.^{Reference Jaffe and Lichtenstein2} The pathogenesis of aneurysmal bone cysts remains unclear. Edling believed that aneurysmal bone cysts are a manifestation of fibrous dysplasia of bones or a defect in epiphyseal plate development, but this does not explain their occurrence in mature bone.^{Reference Edling3} Lichtenstein suggested that aneurysmal bone cysts may be caused by local circulatory disturbances due to sudden vascular occlusion of the venous drainage or the occurrence of arteriovenous shunts.^{Reference Lichtenstein4} Aneurysmal bone cysts are also thought to be secondary to an underlying vascular malformation due to a pre-existing primary bone lesion, such as osteoblastoma, chondroblastoma, chondromyxoid fibroma and giant cell tumour.^{Reference Wong, Chantharasamee, Nelson, Eckardt, Motamedi and Hornicek5,Reference Gutierrez, Link, Horvai, Joseph, O'Donnell and Motamedi6} Trauma is additionally considered to be a cause of aneurysmal bone cyst development.^{Reference Gutierrez, Link, Horvai, Joseph, O'Donnell and Motamedi6} Our reported patient denied having any pre-existing lesion of the temporal area or any history of trauma. Thus, our case can be considered as a primary aneurysmal bone cyst.

A review of the literature indicated that there have been 21 patients with aneurysmal bone cysts in the temporal bone, with the most common sites of involvement being the petrous apex, the squamous portions and the mastoid.^{Reference Blanchard, Abergel, Williams and Ayache7–Reference Saez, Sharma, Barnes, Lu, Hsu and Huoh9} Very few cases involve the internal auditory canal. Symptoms of aneurysmal bone cysts depend on the location and size of the lesion. The most common manifestations of temporal aneurysmal bone cysts are painless masses,^{Reference Saez, Sharma, Barnes, Lu, Hsu and Huoh9,Reference Wang, Cao and Liu10} headache,^{Reference Kim, Lee, Chang, Jung, Kim and Sohn11,Reference Kletke, Popovic, Algird, Alobaid and Reddy12} ear pain,^{Reference Sharma, Velho and Kharosekar13} tinnitus,^{Reference Kletke, Popovic, Algird, Alobaid and Reddy12} hearing loss,^{Reference Sharma, Velho and Kharosekar13,Reference Sayama and Macdonald14} facial paralysis,^{Reference Kletke, Popovic, Algird, Alobaid and Reddy12,Reference Sayama and Macdonald14} vertigo^{Reference Sharma, Velho and Kharosekar13,Reference Kumar, Retnam, Krishnamoorthy, Parameswaran, Nair and Bhattacharya15} and otorrhoea.^{Reference Sharma, Velho and Kharosekar13}

Our patient presented with facial palsy and hearing loss, accompanied by loss of vestibular function (based on vestibular-evoked myogenic potential testing results), all of which indicated that nerves running through the internal auditory canal were involved. Based on the absence of vertigo and the pre-operative vestibular-evoked myogenic potential testing results, we presumed that the patient's vestibular nerve was completely damaged because of tumour compression of the inner auditory canal, and therefore vertigo could not have occurred post-operatively. Interestingly, the patient developed vertigo 3 days after surgery, which implies that vestibular function was partially restored after tumour resection and decompression therapy, resulting in a new binaural imbalance. Whether this situation arises from reversible damage to the vestibular nerve and labyrinth or from neuroregenerative ability is unclear. We presume that segments of the facial and vestibular nerves located in the internal auditory canal, rather than the vestibular end organs or the arteries supplying the inner ear, were suppressed by the tumour. Once the suppression was alleviated, nerve impulse conduction was restored, followed by partial restoration of nerve function. Further clinical data are needed to clarify these findings. Fortunately, the patient's vertigo disappeared after vestibular rehabilitation.

Aneurysmal bone cysts can be visualised radiographically. Multiple small fluid interfaces are an important feature of aneurysmal bone cysts on CT, which represents the sedimentation of red blood cells within the blood-filled cavities. In addition, aneurysmal bone cysts demonstrate expansile bone destruction, with patchy and banded shadows indicating ossification.^{Reference Hashemi, Heidarpour, Eshaghian, Ansari, Hashemi and Yaghoobi16,Reference Gotecha, Punia, Chugh, Patil, Kashyap and Raghu17} Characteristic MRI findings include distinct expansile lesions surrounded by a hypointense fibrous capsule shadow. Because of the formation of a fibrous envelope, sclerotic rim and bone shell,^{Reference Edling3} there may be a hypointense internal septum that separates the lesion into multi-cystic compartments, appearing as a ‘soap bubble’ pattern.^{Reference Unadkat, Adams, Richards and Bajaj18} In our case, the lesion was mainly concentrated within the tympanic cavity, with no extending compressive bone destruction or absorption; this was probably because the lesion was at an early stage.

The only way to definitively diagnose aneurysmal bone cyst is by histopathological examination; a positive diagnosis is based primarily on a honeycomb-appearing cyst, consisting of multiple blood-filled sinusoidal spaces, with bone destruction and osteoid formation. These cysts are separated by fibrous septa without endothelial cells, and various structures exist within the lesion, including haemosiderin, osteoclasts, polynuclear macrophages, inflammatory cells and new bone.^{Reference Kim, Lee, Chang, Jung, Kim and Sohn11,Reference Kletke, Popovic, Algird, Alobaid and Reddy12} Therefore, aneurysmal bone cysts cannot be diagnosed by needle biopsy.

En bloc resection is the ‘gold standard’ treatment and can usually result in good local control.^{Reference Gotecha, Punia, Chugh, Patil, Kashyap and Raghu17} The likelihood of recurrence is related to the patient's age, lesion size, mitotic stage and completeness of surgical resection. Given the rapidly expansile nature of aneurysmal bone cysts, early surgical intervention is required; the risk of intracranial and intradural extension is high. Considering the highly vascular nature of these tumours, pre-operative embolisation is often required, to control intra-operative and peri-operative bleeding. Radiotherapy is recommended for patients with residual or recurrent lesions post-operatively and deep skull base lesions; whether radiotherapy can induce sarcoma transformation still requires extensive clinical data to confirm.^{Reference Kumar, Singh, Phadke, Diwakar, Agarawal and Jain19,Reference Sodhi, Salunke, Agrawal and Gupta20}