Although head and neck paragangliomas are frequently associated with SDHx mutations, SDHA mutations are a rare cause, occurring in only 3%. These mutations may also be associated with gastrointestinal stromal tumours and pituitary adenomas.
A 61 year old gentleman presented with tinnitus, conductive hearing loss and altered balance. Imaging confirmed a 29 x 23 x 31mm enhancing, jugulotympanic mass. Plasma metadrenaline and normetadrenaline levels were within normal limits; however 3 methoxytyramine was elevated at 321 pmol/L (< 110 pmol/L).
Embolisation followed by debulking was performed without pre-operative alpha/ beta blockade. Immunohistochemistry demonstrated negative staining for both SDHA and SDHB and genetic testing confirmed a mutation in exon 2 of the SDHA gene (c.91C>T). Residual tissue measured 8x6x3mm and has remained stable in size over serial imaging. Post operatively the 3 methoxytyramine level has returned to 140 pmol/l (<188 pmol/l).
In addition to the intensely FDG avid skull base lesion, an anterior mediastinal mass with low grade avidity measuring 59 x 53x 68mm was also detected. Core biopsy confirmed this as a thymoma. There is no clinical evidence of myasthenia gravis or compressive symptoms. Over a 2 year timeframe it has increased in size to 89 x 81 x 85 mm. The patient is planned for surgical excision.
Due to the rarity of SDHA mutations, genotype-phenotype associations are still being elucidated. Two previous cases of head and neck paraganglioma with concurrent thymoma have been described in the literature. To our knowledge this is the first case with an associated SDHA mutation. Other groups have proposed a shared neuroectodermal origin of thymic and carotid body tissue as an explanation of coexistent tumours in these locations. This is supported by the co-occurrence of thymomas and neuroendocrine tumours, as well as thymic neuroendocrine tumours in patients with MEN1 mutations.