Anatomy, Head and Neck, Procerus Muscle

The procerus muscle works with other muscles part of the glabellar complex. The glabellar complex includes bilateral fibers of the procerus, frontalis, corrugator supercilii, depressor supercilii, and orbicularis oculi.[1]

The exact size and position of the procerus muscle may vary between individuals. Neurovascular innervation may also differ. As mentioned previously, the precise branches of the facial nerve innervating the procerus are not exact but may include the buccal, temporal, and/or zygomatic branches.[4][5] Likewise, variations in the facial artery may exist.

The arterial supply of the procerus and glabellar complex is vital for surgical procedures, including botulinum injections and injectable fillers. Disruption of the normal vasculature can cause necrosis of the supplied tissues. Similarly, disruption of the normal nervous innervation of the procerus muscle and glabellar complex may cause unwanted facial drooping of the medial eyebrow region. 

The procerus muscle is of importance in cosmetic and surgical procedures. The procerus and the larger glabellar complex are a frequent target for botulinum toxin injection to ease or prevent skin wrinkles. Botulinum toxin is a protein that inhibits the release of acetylcholine into the neuromuscular junction, thus preventing muscle contraction. Therefore, injection of botulinum toxin into the glabella (including the procerus muscle) will paralyze its function, preventing cutaneous furrows and associated wrinkling. The anatomy of the facial arterial supply is of greater importance regarding the use of fillers. Fillers are injectable products comprising hyaluronic acid, poly-L lactic acid, calcium hydroxylapatite, or polymethylmethacrylate that help fill in wrinkles or "plump up" skin for cosmetic purposes. The injection of these fillers directly into an artery can cause injection necrosis. Injection necrosis is uncommon, but the glabella is one of the most common sites where it will occur.[6] Blepharoplasty is a surgical procedure used to treat eyebrow ptosis or forehead wrinkling. Knowledge of the structure and function of the procerus and associated glabellar complex muscles is vital for a surgeon to correct eyebrow ptosis or forehead wrinkling associated with the medial eyebrow depressors.[7]

The function of the procerus muscle and associated glabellar complex is a useful tool during a routine physical exam. Physicians can test the muscles of facial expression to ensure the facial nerve is grossly intact. Instructing the patient to pull their eyebrows down as if making an expression of anger will show intact facial nerve innervation and procerus muscle function. In pediatrics, examination of the glabella can help to recognize dysmorphic facies, such as those seen in Wolf-Hirschhorn syndrome. Wolf-Hirschhorn syndrome is a developmental disorder involving chromosome 4 with dysmorphic facial features described as a "Greek warrior helmet" that includes a prominent glabella.[8][9] In neurology, the "procerus sign" is a term used as a clinical tool to diagnose early progressive supranuclear palsy. Progressive supranuclear palsy is an uncommon neurodegenerative disorder that is often misdiagnosed as Parkinson's disease. Researchers argue the procerus sign is due to a dystonic reaction causing horizontal forehead wrinkling. However, the "procerus sign" may be a misnomer as the corrugator supercilii muscles may be the primary muscles involved.[10] The clinical significance of the procerus sign is controversial as many patients may have physiologic forehead wrinkling associated with age. The glabellar tap sign, or glabellar reflex, is another examination tool that involves repetitive tapping of the glabella to elicit a blink response. A normal response includes blinking only after the first few taps. Abnormal responses involve continued blinking with repetitive tapping. Clinicians once used this diagnostic tool to diagnose Parkinson's disease, but researchers have found the glabella reflex associated with other neurodegenerative disorders and normal aging.[11][12]