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Facial Nerve Palsy Induced Blepharospasm
Robert S. Baker, M.D.
University of Kentucky
The cause of most cases of benign essential blepharospasm remains unknown. However, there are some clues that are leading us in the direction of concluding that blepharospasm is an out-of-control adaptation response of the neurocircuitry controlling the complex interaction of the orbicularis oculi (innervated by the seventh nerve) and the levator palpebrae superioris (innervated by the third cranial nerve). The underlying predisposition to this maladaptive neural response is probably a deficiency or imbalance in neuro transmitters involved in movement control. This imbalance in neuro transmitters could result from a premature decline in the number of cells producing neuro-tansmitters with age or a decline in the ability of the existing nerve cells to manufacture the neuro- transmitter. With this as the background setting then, normal demands made on the blink adaptation system may trigger an up-regulation of the blink mechanism, which then cannot be shut off, leading to a runaway neurocircuit reverberation that manifests itself in the uncontrolled squeezing together of the eyes. It has been postulated that in most instances, the normal stimulus to this runaway neurocircuit abnormality is simply dry eye or corneal breakdown because of poor tear film. However, we have recently discovered a number of patients who developed blepharospasm at varying times after the development of a Bell's palsy, leading us to the postulation that the weak closure of an eyelid, which definitely up-regulates the blink adaptation mechanisms could be the trigger, in susceptible individuals, to the maladaptive inability to shut off this up-regulation, even when the facial nerve palsy has mostly recovered.
There are several movement disorders, which can follow a facial nerve palsy, which must be clearly distinguished from blepharospasm in order to be sure that we are dealing with the mechanism noted above. Following a facial nerve palsy, patients may develop synkinetic movements from aberrant reinnervation, leading to unwanted associated movements of the mouth with blinking of the eyes, or partial closure of the eye, with smiling or other movements of facial expression. An experienced clinician can easily distinguish these synkinetic movements from truly involuntary movements. Other movements, including involuntary hemifacial spasm can occur following Bell's palsy. These are distinguished from blepharospasm by their complete unilaterality. In the patients that we have described with benign essential blepharospasm, we have looked carefully, clinically and with quantitative eyelid kinematic testing, to be sure we are not dealing with either of these other entities.
In previous studies, we have clearly shown that the blink adaptation mechanism is up regulated following facial nerve palsy, and that this up-regulation is bilateral. It appears that the blink adaptation circuitry is incapable, under normal circumstances, of a unilateral change in the eyelid kinematic parameters. Our testing shows that a hypometric blink on the affected side of a facial nerve palsy is accompanied by an exaggerated blink in terms of the size of blink and the speed of the blink on the other side. The typical patient with Bell's palsy will show an increase in the main sequence relationship that compares the amplitude of the blink with the peak velocity of the eyelid movement achieved during the blink. In contrast, patients with benign essential blepharospasm not caused by a facial nerve palsy, will show a decrease in this main sequence relationship, presumably as an attempt of one neural circuit to adapt to an abnormally low threshold for initiating a blink in another neural circuit.
Having studied both patients with pure Bell's palsy and patients with idiopathic benign essential blepharospasm, we can compare these data with both normal aging patients and with patients developing benign essential blepharospasm, following facial nerve palsy. We are able to see from these studies, that the patients with facial nerve palsy induced benign essential blepharospasm do not show a picture typical of either of the other two pathological groups, but rather, may show some features of each group. That is, the main sequence relationship may be either elevated or depressed in an individual patient. A graphic representation of the adaptation of the blink system is shown in Figure 1, while the adaptation to facial nerve palsy model is shown is Figure 2. Our concept of the two-stage blepharospasm model is shown in Figure 3. We feel that future work in this area could focus on understanding what feedback signals are used by the blink adaptive controller, by understanding which neuro transmitters regulate the blink adaptive controller, and trying to learn how we can exploit the properties of this adaptation response to suggest therapeutic intervention.
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