Introduction:
The extra-neurocranial anatomy is a challenging and complex area to master.
In the setting of head and neck malignancies it is very important to recognise specific signs and have knowledge of specific routes in order to scrutinise for possible perineural spread.
Knowledge of the imaging appearance of perineural spread is important,
for it can be very subte and easy to miss.
Also the majority of patients do not have clinical signs of perineural spread.
Perineural spread:
Perineural spread is spreading of tumorous cells along the perineurium and endoneurium of peripheral nerves.
Mostly it occurs from the periphery into the central nervous system,
but it can also occur in retrograde fashion.
It is very important to recognise this entity because it has a detrimental influence on the prognosis.
Moreover measures can be taken to minimise further spread,
usually by extending the radiation field.
![](https://epos.myesr.org/posterimage/esr/ecr2014/121287/media/523658?maxheight=300&maxwidth=300)
Fig. 1: Illustration of a peripheral nerve
References: smartdraw.com
It can be very challenging due to certain facts:
- only 30 % of patients report clinical signs of perineural spread
- detection of a second mass that has spread perineurally can be at another site as the primary tumor.
- the anatomy in the head and neck region of cranial nerves is very complex
- radiological signs can be very subtle.
Symptoms with which patients can present include:
sensibility disorders as hypesthesia,
pruritis and muscle weakness due to denervation.
As mentioned earlier,
only approximately 30 % of patients present with signs of perineural spread.
When there are multiple cranial nerves involved,
extensive spread could be the case and especially intracranial tumor spread should be explored.
There are certain histological types of tumor which are especially prone to perineural spread.
These are: basal cell carcinomas,
adenoid cystic carcinomas and sarcomas.
Also certain locations are subdue to perineural spread: nasopharynx,
parotid and sublingual glands and deep skin tumors.
Finally certain nerves show more perineural spread,
especially the facial and trigeminal nerve due to their location and abundant crossings.
To diagnose perineural spread,
it is important to use special imaging protocols.
First of all always include post gadolinium scans.
MR imaging is especially recomended,
as the threshold for detecting perineural spread is lower than with CT.
Thin slices should be used,
A T1 without fat sat and without contrast enhancement should be included in the protocol.
Also special care should be taken in choosing the right field of view,
with the entire face,
the skull base and the brainstem,
to include all anatomical sights.
With CT,
it is important to reconstruct in thin slices ( 2mm) and include bone as well as soft tissue filters and reconstruct in three directions.
Imaging findings of perineural spread as mentioned before can be very subtle.
These constitute of:
- Obliteration of fat planes (on CT and MRI)
- Enlargement of the affected nerve (on MRI)
- Excessive enhancement (on MRI)
- Enlargement of the affected foramen (on CT)