1. What is actually happening
Arteries carry blood to tissue. If filler is injected directly into an artery, or if enough filler is placed around one to compress it, blood stops reaching the area that artery supplies. Deprived of oxygen, the tissue becomes painful, then discoloured, and if the supply is not restored it dies. Dead skin becomes a scar.
In a small number of cases, filler injected under pressure can travel backwards along an artery and reach vessels supplying the eye. That is the mechanism behind the rare reports of visual loss after facial filler, and it is why any visual symptom is treated as an emergency without waiting to see whether it settles.
2. What it looks and feels like
Pain. Severe, or increasing after the procedure, or clearly disproportionate. Sometimes described as burning. Note that pain is not always present, particularly if local anaesthetic is in the product.
Blanching. An area that goes white, immediately or shortly after injection. This can be subtle and can be mistaken for normal pressure effects.
Livedo. A mottled, net like, dusky purple pattern in the skin over the following hours. This is the sign most people would recognise as clearly wrong.
Later. Blistering, then dark discolouration and crusting over days as tissue dies.
Visual symptoms. Blurring, a shadow or field loss, double vision, pain in or behind the eye, or sudden loss of vision. This is an immediate emergency.
3. Why hours matter
The tissue is surviving on whatever collateral supply exists. Every hour that passes without restoring flow increases the amount that will not recover. Treatment given early can produce complete resolution; treatment given late produces a scar in the shape of the artery's territory.
This is why the response cannot be a message on Monday. It is also why clinics that manage this well have a protocol, hold the medicine, and answer the phone.
4. What treatment involves
For hyaluronic acid filler, the treatment is hyaluronidase, an enzyme that breaks down the gel. It is injected into and around the affected area, often in substantial doses, and frequently repeated over hours until the circulation is restored. Warm compresses, massage and other measures may be used alongside it, and management is guided by the response.
Where the filler is not hyaluronic acid, there is no equivalent dissolving agent. That is one of the more significant practical reasons to be cautious about permanent and semi permanent fillers, particularly in higher risk areas.
For visual symptoms, the response is immediate transfer to emergency care, ideally with ophthalmology involvement. Time is measured in minutes rather than hours.
5. Where the risk is highest
The nose is the highest risk area on the face, because the vessels are small, the space is tight and the connections towards the ocular circulation are direct. Non surgical rhinoplasty is a high risk procedure routinely marketed as a quick treatment.
The glabella, between the eyebrows, is a classic high risk site. So is the nasolabial fold, and the under eye area where the vessels are close and the skin is thin. The lips carry a genuine risk too, through the labial arteries.
Risk does not mean these areas cannot be treated. It means the injector's anatomical knowledge, technique and preparedness matter more there than anywhere else.
6. What reduces the risk
Aspiration, slow injection, low pressure, small aliquots, moving the needle, using cannulas in certain areas, and detailed anatomical knowledge all reduce risk. None of them eliminates it. Occlusion has been reported by careful, experienced injectors using correct technique.
Because it cannot be eliminated, preparedness is the thing to select for. A clinic that says this never happens here has told you either that they have been fortunate or that they are not looking.
7. What to do if you think it is happening
Contact the treating practitioner immediately, by phone, and say clearly that you are concerned about a vascular occlusion. That phrase gets a different response from I have a sore lip.
If you cannot reach them within a short period, call NHS 24 on 111, and attend an emergency department if the symptoms are severe or if you have any visual symptom at all. Do not wait for a call back before acting on visual symptoms.
Take photographs as you go. Note the times. Bring the details of what was injected if you have them.
8. The questions to ask before any filler treatment
Do you hold hyaluronidase on the premises. Who can prescribe it for me, and how quickly out of hours. What is your protocol for suspected vascular occlusion. Have you managed one, and what happened. What number do I call at nine on a Sunday evening, and who answers it.
Those five questions take a minute. They are the most useful minute in the whole process, and they are set out alongside the rest in the questions to ask before any injectable treatment. The underlying regulatory reason they matter is in who can prescribe injectable cosmetic medicines.