Abstract
Introduction
Ever since the first description, SUNCT has been regarded as a neuralgiform headache. The abruptness, and brevity of attacks, the localization of pain, i.e. confined to the territory of the ophthalmic branch (V-1) of the trigeminal nerve, and the precipitating mechanisms—mostly acting on trigeminal innervated structures—are reminiscent of neuralgic pain (1, 2). Naturally, V-1 trigeminal neuralgia is also brought to mind whenever encountering such an array of clinical features.
In SUNCT, many features are however, at variance with V-1 tic. Namely, substantially longer duration of attacks (3), absence of extension over time of pain towards V-2 and V-3 territories (4), autonomic accompaniments such marked lacrimation and conjunctival injection, and lack of convincing response to carbamazepine—and other anti-neuralgic drugs—are characteristics of SUNCT that clearly differentiate it from V-1 tic (5, 6).
With regard to the triggering of attacks, SUNCT also behaves differently from trigeminal neuralgia in that the appropriate stimuli may need to be repetitive or may have to reach a certain level of pressure in order to trigger an attack (7). It has also been observed that in some patients there is no refractory period, since consecutive attacks can be elicited one immediately after the other, or they can even be precipitated during the decrescendo phase of an ongoing attack (2, 7).
More knowledge on this topic is added by the present report of a SUNCT patient in whom the duration of attacks proved to be influenced by the persistence of some precipitating stimuli. Indeed, it has been convincingly proven that in this very patient there were no refractory periods.
Case report
This 63-year-old female had a history of hypertension, bacterial pneumonia and osteoarthritis. She had no previous personal or family history of headache. Since age 55 she had suffered from right-sided pain in the forehead. These pain attacks had an abrupt onset and reached maximum intensity instantaneously. The pain was moderate to severe in intensity and piercing in character. There was complete resolution of pain in between attacks. Marked ipsilateral conjunctival injection and lacrimation accompanied each pain attack. These accompaniments appeared 2–5 s after pain onset and vanished around 30 s after the pain was over. There were neither rhinorrhoea nor nasal stuffiness. Nausea, vomiting, photophobia and photophobia were no part of the attacks. There were no changes in pupil size, extraocular motility or ptosis during attacks.
Although most attacks were triggered by some precipitating factors, there were spontaneous attacks too. The main trigger factor was opening the mouth in a forceful way. There were many other stimuli such as neck flexion or extension, light stimuli on the ipsilateral forehead, nose and lip, bright light, mastication, swallowing and talking.
In her spontaneous attacks, the duration of the pain was about 30 s. The frequency of the episodes was about 10–20 attacks per day, but it varied largely in close relationship with the presence of precipitating stimuli. Most attacks presented during daytime with a slight morning and afternoon preponderance. Nocturnal attacks were seldom reported.
The temporal pattern at the onset of symptoms was characterized by symptomatic periods lasting 1–2 months, alternating with spontaneous remission lasting from several days to several months—a year at the most. Five years since onset, the pattern showed a tendency to chronicity in that the last symptomatic period lasted 2 years.
Neurological examination was normal. Temporomandibular joints and masticatory muscles were considered normal after a careful specific examination. Ophthalmological examination demonstrated an intraocular pressure (IOP) of 17 mm on the symptomatic side and 19 mm on the non-symptomatic side. Routine blood analyses as well and erythrocyte sedimentation rate (ESR) were normal. Both computed tomography (CT) and magnetic resonance imaging (MRI) of the brain were normal.
Experimental setting
During one symptomatic period, we recorded many attacks in a high-quality videotape thus, allowing all three investigators to analyse carefully the characteristics of the attacks. The patient was informed of the procedure and easily agreed to participate. She was seated in front of a video camera and was asked to trigger attacks herself by forceful opening of the mouth. Then, the attacks immediately ensued and we could easily demonstrate that the duration of successive attacks could be progressively prolonged in tight relationship with the rather stepwise increased duration of the stimulus from 30 s to up 3–4 min. The precipitated stretched attacks were otherwise typical in terms of localization, intensity of pain, and accompaniments
Discussion
Atypical duration of attacks has previously been reported as rarely occurring in some SUNCT patients (3, 8). Absence of refractory periods has also been communicated (2, 7–10) in some SUNCT patients in whom attacks could be triggered consecutively one immediately after the other, even if the triggering stimulus was applied within the decrescendo phase of an attack. Not in all reports however, is it mentioned whether SUNCT attacks were followed by refractory periods. This could be due to a relative unawareness of such a feature, or it may have not been systematically paid attention to. Whatever the reason, from now on the presence or absence of refractory periods should be scrutinized—and explicitly mentioned in the literature–in every SUNCT patient, since this seems to be a putatively essential feature of SUNCT.
Both the duration of attacks, dependent on the persistence of the stimuli, and the absence of refractory period can generally be explained by a shortcoming of the inhibitory mechanism. However, stimulation of tonic receptors, which have a slow adaptation, could theoretically account for the stretching of attacks. The discharge from a tonic receptor is relatively persistent, since adaptation may take many minutes. In our patient, this could be feasible when a sustained open mouth triggered the attacks, since both capsular and muscular activated receptors are of the tonic type. However, other stimuli such as touching the tip of the nose, or touching the external canthus of eyelids were as appropriate in producing overlasting attacks, provided such stimuli were persistently applied. All these latter precipitating stimuli do activate mechanoreceptors of the phasic type, i.e. with a fast adaptation period. Therefore, in SUNCT, output from tonic receptors does not seem to have a decisive influence on the duration of the attacks. That militates against considering that the stretched attacks observed in our patient were due to a particular stimulus acting on tonic receptors. We therefore consider that shortcoming of inhibitory mechanisms may have accounted for both the absence of refractory periods and the modulation of the duration of attacks.
In trigeminal neuralgia many attacks, if not the majority, are precisely triggered by mastication and other oral movements, thus activating both the temporomandibular joint and masticatory muscles, which are both provided with tonic receptors, and even under such circumstances refractory periods invariable ensue (11). Therefore, considering the same precipitating mechanisms, the pain modulating system may seem to behave differently in SUNCT compared with trigeminal neuralgia.
In trigeminal neuralgia, both the cessation of the attack and inhibition of the next attack during the refractory period have to do with the inactivation of the pain generator process owing to strong central modulation. Also, local mechanisms such as hyperpolarization, possibly related to the status of cationic channels, and ATP depletion may account for the temporal pause in the firing of an abnormally functioning neuronal foci that would behave as pain generator. Whether the pain generator is located in gasserian neurones or within the trigeminospinal tract has been discussed on other grounds, but this matter and the pathophysiological details are beyond the scope of this study (12, 13).
Nevertheless, with the evidence at hand, SUNCT and trigeminal neuralgia may essentially differ in either the localization of the origin of attacks or the functioning and modulation of the pain generator. With the present and previous observations (2, 7), it seems to be substantiated that in SUNCT, there may be a possible absence of refractory periods that may be part of the make up of at least some patients. This is a remarkable feature which is in clear contradistinction to what is generally encountered in trigeminal neuralgia—a condition in which the presence of refractory periods is not only characteristic but constant. It is again noted that in SUNCT the term neuralgiform has been wisely chosen.