Musculus depressor labii inferioris
The depressor of the lower lip (depressor labii inferioris) originates along the alveolar border of the mandible and inserts into the lower lip and chin. Its action is to depress and evert the lower lip. It is innervated by the ventral buccal branch of the facial nerve (CN VII). In a relaxed horse the lower lip often hangs slightly; this is normal. Excessive or asymmetric drooping, especially with other facial signs, can indicate sedation, facial-nerve paralysis, or other neurologic issues. The muscle contributes to fine control of the lower lip during grazing and eating.
Origin: Alveolar border of the mandible
Insertion: Lower lip and chin
Action: Depresses and everts the lower lip
Innervation: Ventral buccal branch of facial nerve (CN VII)
Facial-nerve paralysis commonly produces lower-lip droop on the affected side. Isolated muscle injury is rare. Asymmetry should prompt a neurologic and dental examination.
Lower lip paralysis results from damage to the ventral buccal branch of the facial nerve, causing flaccid paralysis of the lower lip. The lip hangs open, leading to difficulty eating and drinking, dro
## Overview Lower-lip relaxation is used to gauge sedation depth. Excessive or prolonged lip drooping beyond the expected recovery time, persistent drooling after dental work, or asymmetric droop may
A suddenly droopy or asymmetric lower lip, especially if the horse is also having trouble eating or shows other facial weakness, needs veterinary evaluation to rule out nerve damage or dental pain.
Lower lip paralysis results from damage to the ventral buccal branch of the facial nerve, causing flaccid paralysis of the lower lip. The lip hangs open, leading to difficulty eating and drinking, drooling, and an increased risk of dehydration and weight loss. ## What it is The facial nerve (cranial nerve VII) provides motor innervation to the muscles of facial expression. The ventral buccal branch specifically innervates the muscles of the lower lip. Trauma, inflammation, or compression of this nerve branch leads to unilateral or bilateral paralysis. Common causes include trauma from halters or bits, kicks, or iatrogenic damage during surgery or injections in the region. ## Signs and symptoms - Permanently drooping or flaccid lower lip on the affected side - Food and water falling from the mouth - Excessive drooling - Difficulty prehending and chewing food efficiently - Weight loss or dehydration in severe or bilateral cases - Inability to drink normally from buckets or streams ## Causes and risk factors Lower lip paralysis is most often caused by direct trauma to the facial nerve or its branches (kicks, halter injuries, tight nosebands). It can also occur secondary to inflammation, infection, or iatrogenic damage during dental work or injections. Risk is higher in horses that are handled roughly or have poor-fitting tack. ## How it is diagnosed Diagnosis is based on the characteristic clinical appearance of a drooping lower lip. A full neurologic examination helps determine whether other branches of the facial nerve or other cranial nerves are affected. In some cases, imaging or electromyography may be used to assess the extent of nerve damage. ## Treatment and management Treatment depends on the cause and severity. If the nerve is only bruised or inflamed, function may return over weeks to months with supportive care. Management includes offering soft, easily prehended feed (mashes, soaked hay), ensuring easy access to water, and monitoring body condition and hydration. In permanent cases, horses often adapt well to modified feeding methods. Surgical options are rarely used. ## When to call the vet Contact your veterinarian if your horse develops a sudden drooping lip, especially if accompanied by other facial paralysis or neurologic signs. Early evaluation helps determine the cause and prognosis for recovery. ## Prevention Prevention focuses on careful handling, proper-fitting tack (especially halters and nosebands), and avoiding trauma to the head and facial region. Good horsemanship and regular equipment checks reduce risk. ## Frequently asked questions **Will my horse recover from lower lip paralysis?** Recovery depends on the extent of nerve damage. Bruising or inflammation often resolves over time, while complete nerve transection may be permanent. **Can a horse with lower lip paralysis still eat normally?** Many horses adapt well with modified feeding (soaked feeds, lower buckets, frequent small meals), though efficiency is reduced. **Is lower lip paralysis painful?** The paralysis itself is not painful, but affected horses may become frustrated or lose condition if feeding is difficult. This information is for educational purposes only. Always consult with a licensed veterinarian for diagnosis and treatment of your horse. Signs include: Permanently drooping or flaccid lower lip on the affected side; food and water falling from the mouth; excessive drooling; difficulty prehending and chewing food efficiently; weight loss or dehydration in severe or bilateral cases; inability to drink normally from buckets or streams
## Overview Lower-lip relaxation is used to gauge sedation depth. Excessive or prolonged lip drooping beyond the expected recovery time, persistent drooling after dental work, or asymmetric droop may indicate nerve compression during anesthesia rather than normal sedation effects. ## Causes Normal sedation effect versus pathologic nerve compression. ## Diagnosis Observation of lip tone during and after sedation. Asymmetry or persistence suggests nerve involvement. ## Treatment None for normal sedation recovery. Supportive care if nerve compression is suspected. ## Recovery and Prognosis Normal sedation effects resolve quickly. Nerve compression may take longer. ## Prevention Careful positioning during procedures. Signs include: excessive lip drooping beyond normal recovery; persistent drooling after dental procedures; asymmetric droop suggesting nerve damage
Facial-nerve paralysis commonly produces lower-lip droop on the affected side. Isolated muscle injury is rare. Asymmetry should prompt a neurologic and dental examination.
A suddenly droopy or asymmetric lower lip, especially if the horse is also having trouble eating or shows other facial weakness, needs veterinary evaluation to rule out nerve damage or dental pain.
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