Advances and instrumentation in diagnosis and treatment of trigger points in human myofascial pain: veterinary implications

B.1. DIAGNOSTIC INSTRUMENTS TO ASSESS HUMAN MYOFASCIAL PAIN

The instrumentation consists of four different analogue force spring gauges. Each probe has a discoid rubber tip 1 cm sq (diameter 1.12 cm ). Gauges 1 and 2 record the pressure (in kg/cm sq) required to elicit pain threshold and tolerance in the muscle being investigated. Gauge 3 measures depth of penetration of a probe in muscle at precise pressures applied. Gauge 4 measures the force (kg) needed to overcome muscle power. Each instrument has a maximum "hold clutch" to register the maximum force applied. It is released by pressing a zero button.

These four gauges can document the location and severity of muscle and TP pain and weakness in humans and the outcome of treatment for medico-legal purposes. Accurate, objective documentation of TP locations and pain is very relevant to clinical assessment, diagnosis and prognosis. It also has most important medico-legal implications, especially in the differentiation of genuine pain patients from malingerers in insurance/compensation claims.

If pain is bilateral, a nearby normal area can be used as a reference. A decrease of pressure threshold to 3 kg/cm sq is considered abnormal (Fischer 1988).

1. PRESSURE THRESHOLD MEASUREMENT OF MUSCLE AND TPs: The instrument (Figure 1) is a spring gauge from 0-11 kg applied force, calibrated in steps of 0.1 kg. It is used to quantify the sensitivity of tender areas, such as sprains, strains, arthritic joints or TPs located by clinical examination/ palpation and to demonstrate the difference between thresholds of tender and healthy muscle. It also can demonstrate the disappearance of TP tenderness as the case is treated successfully. A rubber-protected pressure probe, 1 cm sq is used to apply vertical pressure to the tender areas. Pressure is increased steadily at 1 kg/sec. To prevent the probe from slipping, it is steadied by the thumb and index fingers of the left hand in right-handed operators. The patient is asked to indicate the first appearance of pain/discomfort to the applied pressure, at which time, the gauge is read and the pressure recorded in kg/cm sq. The "hold clutch" is released for the next measurement. Readings at TPs can be compared with those in healthy muscle on the opposite side, which has pressure thresholds 2-4 kg/cm sq higher than those recorded at TPs.

The gauge can be used to show the immediate improvement which occurs after correct injection of TP and to prove that the whole TP has been injected. Pressure threshold values increase typically by c. 4 kg/cm sq immediately after correct TPinjection (Fischer 1986). Clinical improvement in pain is associated with increased pressure threshold values in patients whose pain is caused by TPs (Fischer 1984, Jimenez 1985).

Pressure algometry is highly reproducible (Merskey et al 1962, 1964). Pressure threshold values reliably locate TPs and quantify their sensitivity. The repeatability of the measurements within and between operators was confirmed by Reeves et al(1986) and Tunks et al (1988). Makela and Pontinen (1988) confirmed the repeatability of pressure threshold measurements at latent TPs in healthy volunteers and also in pain patients. They confirmed the value of the method in demonstrating the success ofTP therapy by lasers and TENS methods of point therapy (Airaksinen et al 1988; Pontinen 1987).

2. PRESSURE TOLERANCE OF MUSCLE: The instrument (Figure 2) is a spring gauge from 0-17 kg applied force, calibrated in steps of 0.2 kg. It is used to quantify the upper level of sensitivity to pressure pain, i.e. the maximum pressure which can be tolerated when applied to standard sites (lower medial tibia (shin bone) and medial deltoid muscles), exploring non-tender areas and avoiding tender ones. (The deltoids seldom have TPs). A rubber-protected pressure probe, 1 cm sq is used to apply vertical pressure, increasing steadily at 1 kg/sec to the selected points. To prevent the probe from slipping, it is steadied by the thumb and index fingers of the left hand in right-handed operators. The patient is asked to indicate when the pressure pain becomes intolerable (very painful), at which time, the gauge is read and the pressure is recorded in kg/cm sq. The "hold clutch" is released for the next reading.

The purpose of pressure tolerance measurement is to quantify pain sensitivity and aid the diagnosis of abnormal muscle tenderness in relation to bone. In healthy subjects, muscle has higher pressure tolerance values than shinbone and values are very similar on both sides of the body. Fit athletes have higher pressure tolerance than unfit subjects. Men have slightly higher pressure tolerance than women. In patients with high pressure tolerance over muscle and bone, pressure thresholds at TPs may be higher than is usual.

Functional disorders show lower pressure tolerance values than those in organic disease.

If pressure tolerance of muscle is less than that of shinbone, generalised myopathy (a common cause of chronic pain and treatable by hormone therapy) is considered. It may arise in hypo- or hyper- thyroid disorders and in oestrogen deficiency (Fischer 1988).

If pressure tolerance is low at many measurement points over muscle and bone, it indicates an hyperalgesic syndrome, sometimes with psychiatric implications. The prognosis is poor and special therapies are needed (Fischer 1984, 1986, 1986, 1987, 1988). If there is a narrow gap between pressure threshold and pressure tolerance, hysterical personality is likely.

3. TISSUE COMPLIANCE MEASUREMENTS: The tissue compliance meter (Figure 3) is a spring gauge, force range from 0-5 kg in steps of 0.05 kg, with a special 5 cm diameter displacement head set flush at right angles to the probe. The rubber-protected pressure probe, 1 cm sq is used to apply vertical pressure. The displacement of the head (in mm) from the zero position is read off a scale at exactly 1, 2, 3, 4 and 5 kg applied pressure over the muscle being investigated. The principle is that a probe applied at a given pressure sinks deeper into healthy muscle than into tense or spastic muscle. When the displacement curves of normal and tense or spastic muscle are compared, the degree of tension can be demonstrated objectively. Especially in the range 2-5 kg/cm sq, penetration in tense or spastic muscle is 2-7 mm less than in healthy muscle. The meter can quantify displacement in other types of soft tissue pathology also: haematoma, oedema, scarring, tumours, inflammation, flaccidity etc (Fischer 1987). It can also be used to monitor the response to physical and other therapies (Fischer 1988).

4. DYNAMOMETRY OF JOINTS AND MUSCLE: The instrument (Figure 4) is a spring gauge from 0-27 kg applied force, calibrated in steps of 0.5 kg. It quantifies the current state of muscle power or weakness and the speed and extent of recovery of strength in cases treated successfully. It is used to assess the muscle power (flexion, extension, adduction, abduction) of individual joints or limbs. A rubber-protected contact plate, 14 cm sq is used to spread the force applied vertically to the part under investigation.

B.2. DIAGNOSTIC INSTRUMENTS TO ASSESS ANIMAL MYOFASCIAL PAIN

Similar instruments to those described in B.1. (above) could be adapted for veterinary use, especially to quantify TP tenderness and muscle spasm in dogs and horses. Adapted gauges would be especially useful in the investigation of lameness or poor racing performance, where muscle pain (paravertebral, shoulder, thigh etc) is suspected as the cause.

Although pressure meters are being tested by a few veterinarians at present, this area of research is wide open, as there appears to be nothing published on the topic yet.