Introduction
Paget’s disease of the bone (PDB) is a relatively common condition that affects nearly 3 percent of individuals over 40, with its incidence rising with age. The primary pathophysiological abnormality is irregular, abnormal, and unbalanced bone remodeling, which can lead to significant long-term disabilities, such as neurological deficits, multiple fractures, and, rarely, osteosarcoma (bone cancer). Although PDB is the second most common bone disorder after osteoporosis, its exact cause remains unclear. Potential etiopathogenesis includes viral infections, endocrine dysfunction, or autoimmune disorders.
Genetics also plays a significant role in the disease's development. In most cases (70 to 95 percent), PDB is asymptomatic and is either discovered incidentally or goes undiagnosed throughout the individual's life. In about two-thirds of patients with Paget’s disease, the skull and temporal bones are affected, which can lead to hearing impairment or vestibular dysfunction. Hearing is impaired in 30 to 50 percent of symptomatic patients, typically mixed and bilateral.
Vertigo occurs in 25 percent of patients, and tinnitus, usually pulsatile, occurs in 20 percent. Various lesions have been proposed to underlie the audiovestibular symptoms. Still, different studies show a wide variation in pathology, and the sensorineural and conductive components of hearing loss are poorly understood.
What Is Hearing Loss In Paget's Disease?
Paget's disease involves a dysfunction of osteoclasts, but its cause remains unknown. Evidence suggests that both a paramyxovirus and genetic factors play a role. Pagetic osteoclasts are more giant and resorb bone with significantly more significant activity, sometimes by an order of magnitude. This leads to the formation of progressively enlarging lytic lesions. Osteoblasts (bone-forming cells) respond to the increased osteolysis (the process of bone destruction) by rapidly forming new, poorly organized bone.
When medication controls the bone resorption rate, the rate of bone formation gradually decreases. Hearing loss (HL) is a significant characteristic of Paget's disease of bone when it affects the skull. Auditory studies have shown that most patients experience high-frequency sensorineural HL and a low-frequency air-bone gap. The exact mechanism causing HL needs to be better understood. Due to the absence of an animal model for Paget's disease of bone, research on the mechanisms of HL is restricted to postmortem examinations and in vivo clinical studies. These studies have proposed several mechanisms for sensorineural hearing loss, including compression of the auditory nerve and vascular shunts. Changes in the stiffness of the middle ear's soft tissue elements might cause an air-bone gap. Paget's disease may result in bone density, mass, and form changes that disrupt the finely tuned motion mechanics of the middle and inner ears. However, direct evidence to support this suggestion still needs to be provided.
What Are the Mechanisms of Hearing Loss in Paget's Disease?
Studies have frequently reported the narrowing of the auditory canal. Compression of the acoustic nerve within this narrow space, like the internal auditory canal, can lead to a lack of blood flow, atrophy, and nerve degeneration. However, studies did not observe this in any of the cases. In one specimen, a small acoustic neuroma was found in the top coil of the cochlea. Schuknecht previously commented on this specimen, suggesting that the pressure from the newly formed pagetic bone caused this growth.
Degeneration or atrophy of the stria vascularis or the spiral ligament has been reported in some studies, and researchers have frequently observed similar findings in other cases. These abnormalities can lead to ion imbalances and changes in the endocochlear potential. Inflammatory cytokines released during pagetic bone remodeling might also contribute to the dysfunction of these structures. Abnormal cytokine levels in the spiral ligament can disrupt ion and fluid balance, resulting in sensorineural hearing loss.
After the blockage (occlusion), a decrease in cochlear microphonics (the electrical signals generated by the hair cells in the cochlea in response to sound) was observed, along with reduced blood flow and widening of the small blood vessels in the stria vascularis (part of the cochlea) and some blood vessels in the vestibular labyrinth (part of the inner ear involved in balance). However, no significant congestion was seen in other blood vessels, suggesting that alternate blood flow (collateral circulation) was available. Occasionally, some blood vessels burst, leading to bleeding within the inner ear.
How to Diagnose Hearing Loss In Paget's Disease?
The following are the methods to diagnose hearing loss in Paget’s disease:
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Computed Tomography (CT Scan) - The images showed the most significant dimensions of the internal auditory canal (IAC) at different points along its length in each case. The results of images of CT scans measure the bone density of the internal ear.
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Radiology - The bone density measurement of the cochlear is done by radiographic images.
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Histology - Histological studies reveal that the pagetic process affects the cochlear capsule in nearly all observed cases.
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Magnetic Resonance Imaging (MRI) - The abnormal changes in the internal ear are found in MRI images.
What Is the Treatment Of Hearing Loss In Paget's Disease?
The goal of pharmacological treatment for Paget’s disease is to relieve pain and slow down the rate of bone remodeling. Bisphosphonates (medications that help prevent the breakdown of bone by inhibiting bone resorption) are commonly used to manage Paget’s disease, and monitoring bone markers such as serum ALP (alkaline phosphatase) is done regularly. Restoring normal bone turnover might reduce blood flow to the bone and slow disease progression.
Calcitonin treatment may help prevent hearing loss for 5 to 8 years compared to no treatment. However, hearing loss tends to be permanent in most Paget’s disease cases. Additionally, neither a bone-conduction hearing aid (a device that transmits sound vibrations directly to the inner ear through the bones of the skull) nor an air-conduction hearing aid has effectively improved conductive hearing loss. Cochlear implants are the final treatment of choice in the patients. Cochlear implantation may be considered for cases of Paget’s disease with severe hearing loss.
Conclusion
Clinicians treating Paget’s disease of bone have observed that, while pain relief may be achieved with treatment, improvement in hearing sensitivity is not typically noticed. Normal cochlear capsule bone undergoes minimal remodeling. It is possible that hearing does not improve with treatment because the affected bone has limited ability to restore normal structure. Therefore, early diagnosis and aggressive long-term treatment might be necessary to manage hearing loss as a complication of Paget’s disease.
