What is the trigeminal nerve?
The trigeminal nerve is one of the major nerves supplying the head. It has two important jobs:
- It controls the muscles used for chewing.
- It provides feeling (sensation) to the face, nose, mouth and eyes.
The trigeminal nerve is different from the facial nerve. While the facial nerve controls facial expression (such as blinking and moving the lips and ears), the trigeminal nerve is responsible for chewing and facial sensation.
When the trigeminal nerve becomes diseased, the signs depend on whether one side or both sides are affected.
Bilateral trigeminal neuropathy (both sides)
When both trigeminal nerves are affected, dogs usually develop a dropped jaw because the muscles that normally close the mouth become weak. They may struggle to pick up food or drink normally and often drool more than usual.
The good news is that this is most commonly an idiopathic condition, meaning that despite thorough investigations, no underlying cause can be identified. In these dogs, the outlook is generally excellent and most recover over several weeks, although temporary supportive feeding may sometimes be needed.
Less commonly, bilateral trigeminal neuropathy can occur as a result of inflammatory diseases affecting the trigeminal nerves, such as immune-mediated inflammation (including pachymeningitis), lymphoma or other rare neurological disorders. Because these conditions can sometimes affect neighbouring nerves, some dogs may also develop signs such as facial weakness, Horner’s syndrome (a drooping eyelid with a smaller pupil) or balance problems.
Unilateral trigeminal neuropathy (one side)
When only one trigeminal nerve is affected, dogs usually do not develop a dropped jaw because the normal side is still able to hold the mouth closed.
Instead, owners usually notice a gradual loss of the chewing muscles on one side of the head, particularly the temporal muscle, giving the head an uneven or sunken appearance. The muscle loss is not usually painful.
Unlike bilateral disease, unilateral trigeminal neuropathy is more likely to be caused by an underlying disorder affecting the nerve, such as inflammation or a tumour. Some dogs may also have reduced sensation on the affected side of the face or eye.
How is trigeminal neuropathy diagnosed?
A neurological examination can usually identify that the trigeminal nerve is affected, but further tests are often needed to determine the underlying cause.
The most important investigation is an MRI scan of the head, which allows the trigeminal nerves and surrounding brain structures to be examined in detail. MRI can help determine whether the nerve appears normal (supporting an idiopathic diagnosis) or whether there is evidence of inflammation, a tumour or another disease affecting the nerve.
In some dogs, a sample of cerebrospinal fluid (CSF) is also collected at the same time as the MRI to look for evidence of inflammation affecting the nervous system.
Treatment
Treatment depends on the underlying cause.
Dogs with idiopathic bilateral trigeminal neuropathy usually require only supportive care while the nerves recover naturally. This may include offering softened food, hand feeding or raising food and water bowls if eating is difficult.
If MRI identifies an underlying inflammatory disease or tumour, treatment will be directed at that condition.
What is the outlook?
The prognosis depends on the underlying cause.
Dogs with idiopathic bilateral trigeminal neuropathy usually recover well over several weeks and most regain normal jaw function.
The outlook for dogs with unilateral trigeminal neuropathy is more variable because it depends on the underlying diagnosis. Some inflammatory conditions respond well to treatment, whereas tumours generally carry a more guarded prognosis.