Fetal type myxoid rhabdomyoma of the vestibular nerve
Rhabdomyomas are benign tumors containing striated muscle and are categorized by location as cardiac and extracardiac tumors. Cardiac rhabdomyomas typically occur in young patients and are often associated with tuberous sclerosis. Among extracardiac rhabdomyomas, adult, fetal and genital types are distinguished with head and neck region as the most common site. Whereas adult-type extracardiac rhabdomyomas mostly occur in adults older than 40 years, fetal-type tumors tend to occur predominantly in infants and may be associated with autosomal dominant nevoid basal cell carcinoma syndrome (Gorlin syndrome) characterized by PTCH mutations. Reports of rhabdomyomas manifesting at cranial nerves are exceptional, and only 4 cases have been published: Zwick et al. reported a rhabdomyoma of the left trigeminal nerve in a 29-month-old male infant replacing the 5th nerve in the posterior fossa and in the region of Meckel's cave. (7) Another report reviewed a case of a 41-year-old man with a tumor emerging from the right the porus acusticus internus, suggesting a schwannoma. (6) This tumor infiltrated the facial nerve root and extended from the brainstem to the geniculate ganglion. Histologically, many mature striated fibers were seen, and the tumor was localized within the nerve, which complicated the differential diagnosis of a schwannoma. However, Antoni A and B regions were not seen. A third case report described a rhabdomyoma of the left 8th cranial nerve in a 6-year old boy manifesting in the cerebellopontine angle with growth into the internal acoustic meatus and adhesion to the pons. (5) In a fourth case a 15-year old boy presented with a rhabdomyoma of the left oculomotor nerve being attached to the midbrain. (2) These cases of intracranial nerve rhabdomyomas were referred to as adult type rhabdomyomas. In contrast, our case affecting a 68-year-old man represents a fetal type rhabdomyoma. The fetal type myxoid rhabdomyoma has been described mainly in infants and is especially localized subcutaneously in the preauricular and postauricular region thus indicating an exceptional localization. (1) Moreover, a fetal type rhabdomyoma occurring in adults seems unusual. Fetal type rhabdomyomas have variable histological patterns, with myxoid/classic and intermediate/cellular forms being distinguished. The myxoid/classic type mainly affects boys in the first year of life. Kapadia et al. reported 8 cases of myxoid/classic rhabdomyomas of which 5 cases had been younger than one year of age. (1) Yet, there was one adult female case of 20 years of age. Among all fetal rhabdomyomas of any type age ranged from 3 days up to 58 years. (1) Thus, fetal type myxoid rhabdomyomas can occur at later stage in life.
In principle, rhabdomyomas occurring in association with nerves, should be distinguished from neuromuscular hamartomas (choristomas), also called intraneural rhabdomyoma-like tumors. These lesions are non-neoplastic. They are composed of well differentiated mature skeletal muscle cells and peripheral nerve tissue and are discussed to arise from displaced muscle fibers during embryonal development or to originate from neuroectodermal cells. Neuromuscular hamartomas (choristomas) are synonymously called benign triton tumors in the literature although some authors have proposed that this term has been incorrectly applied. They suggest that benign triton tumors represent neurofibromas with rhabdomyomatous component. (3, 4)
Although histological criteria in general help to distinguish rhabdomyomas from neuromuscular hamartomas, it may be difficult in certain cases. In particular, clear differentiation between adult type rhabdomyomas associated with nerves and neuromuscular hamartomas may be challenging as reflected for 3 out of 4 reported cases of cranial nerve rhabdomyomas. (2, 6, 7) It may be important for the managements of patients to distinguish between hamartomas and true tumors, both occurring at cranial nerves. A literature review revealed 16 patients with neuromuscular hamartomas/choristomas of cranial nerves involving optic, oculomotor, trigeminal, vestibular, maxillary and facial nerves. Concerning our case, myxoid stroma, absence of nerve structures and nerve fibers as well as recurrence is in concordance with the diagnosis of a fetal type, myxoid rhabdomyoma and excludes a neuromuscular hamartoma or benign triton tumor. Mitoses were absent in our case as has been described previously. (2, 5)
Since only few cases of intracranial rhabdomyoma are reported the prognosis is not clear. Follow-up was reported in only one case with no recurrence after four years. (5) Nevertheless our case demonstrates that recurrence can occur due to incomplete resection.
Contributed by Anja Harder, MD; Eloise Müller-Schulte, Astrid Jeibmann, MD, Johannes van de Nes, MD, Ph.D, Uwe Wildförster, MD; Werner Paulus, MD