P. Bjerring

Sensory and pain threshold characteristics to laser stimuli.

The clinical applications of thermal sensory and pain thresholds have been very limited due to large intra-individual variations. In the present paper CO2 and argon lasers were used as thermal stimulators, and the different factors (stimulus parameters and skin conditions) affecting the thresholds are described. The intra-individual variations obtained in sensory (9.3%) and pain (4.3%) thresholds were very low, which suggests that the method can be applied for clinical purposes.

Argon laser induced single cortical responses: a new method to quantify pre-pain and pain perceptions.

The shape (amplitude and latency) of single cortical responses to argon laser stimulation was found to match six perceptual classes: three pre-pain and three pain. The amplitude of the pain related single cortical responses correlated with the perceived feeling of pain. Easy detectable responses were obtained because habituation to the stimuli was reduced and a high degree of attention was given to each stimulus. Single cortical responses to argon laser stimuli are suggested as a new quantitative technique with application in the assessment of function in the thermal and nociceptive pathways.

Reaction times to painless and painful CO2 and argon laser stimulation

SummaryThe introduction of lasers in pain research has made it possible to activate the nociceptive system without activating mechanosensitive afferents. In the present study the reaction times to painless and painful laser stimuli were studied to investigate if the reaction time to experimental pain is reproduceable. CO2 and argon lasers were used for stimulation, and the influence of stimulus (intensity and duration) and skin parameters (temperature, thickness, and reflectance) on reaction time were investigated. When these parameters were controlled the reaction times to painful CO2 and argon laser stimulation were within the same range (350–450 ms), and the intra-individual variability minimal (6.9%). The reaction time was used to estimate peripheral conduction velocity (10 m · s−1) for the activated fibre population when distinct pain was perceived. Determination of reaction times to non-painful and painful stimuli may be suitable ways to assess the functioning of thermal and nociceptive pathways.

Onset and duration of hypoalgesia of lidocaine spray applied to oral mucosa--a dose response study.

Lidocaine is often used as a topical analgesic prior to painful procedures performed in the oral cavity and upper airways. In this study the optimal time interval for performance of painful procedures in the oral cavity and the upper airways was determined by spraying lidocaine solution on the mucous membranes of the mouth with subsequent measurements of pain thresholds induced by argon‐laser stimulation. Two different dosages (30 mg and 60 mg) of lidocaine spray were administered to the oral mucosa of the lower lip in healthy volunteers. Repeated measurements were performed until normal sensitivity returned after 15 min. Pain thresholds increased 62% after 30 mg lidocaine and 50% after 60 mg lidocaine (a non‐significant difference). Thus repeated applications were found to be without any additional hypoalgesic effect. Maximal hypoalgesia was reached after 4 to 5 min. Complete analgesia was not obtained. The hypoalgesic effect lasted until 14 min, but painful procedures should be performed in the time interval 3–8 min after application.

Hypoalgesia following epidural morphine: a controlled quantitative experimental study

The efficacy, duration, and spread of epidural morphine hypoalgesia were assessed by an experimentally induced pricking pain evoked by laser stimulation. Four mg of plain morphine was injected epidurally in 7 volunteers at the L2‐L3 interspace. Thresholds to warmth and pain perception, and pain‐evoked potentials were measured. In the first experiment, hypoalgesia was monitored each hour for 7 h at various dermatomes. Hypoalgesia was detected at SI dermatome after 2 h, but 3 h elapsed before hypoalgesia could be detected at the LI, T12, T10, T8, and T6 dermatomes. No effect was found at C7. No conduction delay was found along the pain pathway during hypoalgesia. Hypoalgesia lasted more than 7 h at S1, whereas hypoalgesia could not be detected after 5 h at other dermatomes. In the second experiment, naloxone (0.8 mg i.v.) was injected 230 min after injection of epidural morphine, and the subsequent recording 10 min later showed that hypoalgesia had been partly reversed. The onset and duration of hypoalgesia are different for experimentally induced pain and clinical pain. Experimentally laser‐induced pain has the advantage of being quantitative, and is, as such, useful to assess hypoalgesia, and to test the potency of narcotics.

Onset and duration of hypoalgesia following application of lidocaine spray on genital mucosa.

Lidocaine spray is used as a topical analgesic prior to minor surgical procedures performed on genital mucosa. In this study onset and duration of hypoalgesia on the genital mucosa was investigated by spraying lidocaine solution on the mucous membranes of female genitals with subsequent measurements of pin prick pain thresholds induced by argon laser stimulation. A dosage of 50 mg of lidocaine spray was administered to the genital mucosa of the inside of the labiae minora. Repeated measurements of pain threshold were performed until normal sensitivity was reached. Analgesia to 1.5 W laser stimulation was obtained in all volunteers after an average of 2.7 min±1.3 min (s.d.) (Range: 1–4 min). The effect lasted 29.7 min± 8.9 min (s.d.)(Range 12–45 min). The application of lidocaine spray produced a painful, smarting sensation which, however, disappeared again after 15–30 seconds. The study suggests that minor surgery should not be performed earlier than 4 min after application of lidocaine spray and it is recommended that it should be remembered that the duration of analgesia varies between individuals and thus may be very short in some patients.

A quantitative double-blind evaluation of the antinociceptive effects of perineurally administered morphine compared with lidocaine.

In two double‐blind, placebo‐controlled investigations, morphine and lidocaine were administered perineurally to the ulnar nerve. Thresholds (warmth and pain) and pain‐evoked brain potentials (amplitude and latency) to argon laser stimulation were measured up to 120 min after the injection. Hypalgesia to laser pain was detected 15 min after the injection of morphine and 5 min after the injection of lidocaine. The duration of hypalgesia and analgesia was less than 15 min for morphine and 85 min for the lidocaine injection. Both morphine and lidocaine increased the latency of the brain potentials, which indicates that the same blocking mechanisms could be involved. Pin‐prick analgesia was obtained 5 min after the injection of lidocaine, but 15–30 min elapsed before the laser pain was inhibited maximally. Laser pulses can activate larger skin areas than needle pricks, indicating that a central summation of the activity from many cutaneous nociceptors is important in order to obtain a reliable indicator of adequate analgesia.