1. Sporadic forms (the most frequent ones; there is a possibility of concealed paraganglioma)

2. Familial forms (relatively frequent) related to PGL syndromes:

- PGL1 (mechanism detected in SDHD gene). Frequency in multicentric PGs.

- PGL2 (genetic mechanism in SDHAF2 gene)

- PGL3 (mechanism detected in SDHC gene)

- PGL4 (mechanism detected in SDHB gene). Frequency in malignant forms.

3. Forms involving multiple systems (exceptionally associated with head and neck PGs):

3.1. Associated with syndromes:

- Von Hippel-Lindau (VHL) disease (mechanism detected in VHL gene)

- Multiple endocrine neoplasia type II (mechanism detected in RET gene)

 - Neurofibromatosis type I (mechanism detected in NF1 gene)

 - Carney’s triad (unknown genetic mechanism, it may be familial)

3.2. Not associated with syndromes (unknown genetic mechanisms):

- Parathyroid adenoma

- Astrocytoma

- Thyroid carcinoma

- Pituitary tumour

4. Unknown associations: Thymoma