WO1991009566A1

WO1991009566A1 - Sensory delineator

Info

Publication number: WO1991009566A1
Application number: PCT/GB1990/001991
Authority: WO
Inventors: Martin John Howell; Geoffrey John Payne
Original assignee: Martin John Howell; Geoffrey John Payne
Priority date: 1989-12-21
Filing date: 1990-12-20
Publication date: 1991-07-11
Also published as: GB8928905D0; AU7044891A; EP0506811A1

Abstract

A sensory delineator comprises a common probe (5) applicable to a healthy area of the patient's skin, a test probe (6) applicable to an area of skin suspected to be subject to nerve damage, a reference probe (7) applicable to an area of similar skin suspected to be healthy, a resistance comparison circuit (20) for comparing the resistance present between the common probe (5) and the reference probe (7) (the reference resistance) with the resistance present between the common probe (5) and the test probe (6) (the test resistance), and output means (10, 11, 12) for indicating whether the test resistance (5, 6) exceeds the reference resistance (5, 7).

Description

SENSORY DELINEATOR

Background of The Invention

The present invention relates to a sensory delineator, that is to say a device for sensing the condition of nerves associated with an area of skin and in particular sensing areas of nerve damage..

Injury can cause damage to the nerves in certain areas of the body. Treatment of the injury is facilitated if the extent of the nerve damage is known. The traditional means of surveying extent of nerve damage is primitive, such as exploring those areas in which the patient can feel a pin prick and those in which he/she cannot. This is at once time consuming and virtually impossible unless the patient is at least conscious. The Invention

The object of the present invention is to provide a sensory delineator.

The sensory delineator of the invention comprises: a common probe applicable to a healthy area of the patient's ski , a test probe applicable to an area of skin suspected to be subject to nerve damage, a reference probe applicable to an area of similar skin suspected to be healthy, a resistance comparison circuit for comparing the resistance present between the common probe and the reference probe (the reference resistance) with the resistance present between the common probe and the test probe (the test resistance) , and output means for indicating whether the test resistance differs from that of the reference resistance.

When an area of skin is nerve damaged there is comparatively less blood flow in blood vessels associated with the skin. Accordingly, the skin's electrical resistance increases; that is to say, the resistance between a probe applied externally to the skin and the flesh internal of the skin which is well supplied with blood and hence of low electrical resistance. The sensor places the common skin resistance (at the common probe) in each body element of the bridge in series (in the element) with the skin resistance local to the test probe and the reference probe, respectively. When these latter are both on healthy skin the resistance of the two body elements is substantially the same. When the search probe is at a nerve damaged skin, the local resistance will be higher and this condition is indicated by the output means. We believe however that the junction region between healthy and damaged skin has contrastingly less skin resistance than normal. Preferably, the resistance comparison circuit is an analogue arithmetic circuit providing to the output means a voltage output indicative of an arithmetic relationship between the reference resistance and the test resistance. Alternatively the resistance comparison circuit can be a digital arithmetic circuit providing to the output means a voltage output indicative of an arithmetic relationship between the reference resistance and the test resistance.

In one embodiment, the resistance comparison circuit is a Wheatstone bridge circuit in which two of the usual four elements are provided in the resistance comparison circuit and two will be provided in use by parts of the body, the three probes being connected in the Wheatstone bridge circuit such that in use the body resistance between the common probe and the test probe forms one element of the bridge and the body resistance between the common probe and the reference probe forms another element of the bridge and the imbalance voltage in the bridge is indicative of the extent to which the ratio of the test and reference resistances differs from the ratio of the other two elements of the bridge. In the preferred Wheatstone bridge embodiment, the resistance comparison circuit includes a comparator connected to the output means, the common probe is connected to one voltage rail, the test probe is connected to another voltage rail via one of the other two elements and to one input of the comparator, the reference probe is connected to the other voltage rail via the other of the two elements and to another input of the comparator, the comparator being arranged to give an output if the test resistance exceeds the reference resistance by the ratio of the resistances of the other two elements of the bridge.

The other element of the bridge having the test probe connected to it can comprise a chain of resistors and a plurality of comparators is provided and respectively connected to the common points of pairs of the resistors in the chain, the arrangement being such that the ones of the comparators which give an output to the output means are determined by the ratio of the test resistance to the reference resistance. In a second embodiment, the resistance comparison circuit includes a feed-back amplifier circuit having an amplifier and a source of substantially constant voltage, the common probe being connected to one input terminal of the amplifier, the test probe being connected to an output terminal of the amplifier for arrangement in use of the reference resistance as a feed back resistance between the output terminal and the input terminal and the test probe being connected to an output terminal of the voltage source for arrangement in use of the test resistance between the output terminal of the voltage source and the said input terminal of the amplifier, the output of the amplifier being proportional to the ratio of the test resistance to the reference resistance.

In a third embodiment, the resistance comparison circuit includes a voltage follower circuit having a feed-back amplifier arranged for unity gain with a direct connection between an output terminal and an input terminal, the common probe being connected to the input terminal, the reference probe being connected to one supply rail and the test probe being connected to another supply rail, the arrangement being such that in use the reference and test resistances are connected in series to divide the potential between the references in proportion to their values and feed this proportion to the input terminal of the amplifier, the output of the amplifier being equal to its input and to the ratio of the potential across one of the test and reference resistances to the potential cross both of these.

In the second or third embodiments, the resistance comparison circuit further includes a comparator connected to the output means and a potential divider, the comparator being arranged to compared the output of the amplifier with divided potential from the potential divider and cause the output means to give an output if the test resistance exceeds the reference resistance by a predetermined factor. There can be provided a plurality of comparators and and a chain of resistors in the potential divider, the comparators being respectively connected to the common points of pairs of the resistors in the chain and giving, the arrangement being such that the ones of the comparators which give an output to the output means are determined by the ratio of the test resistance to the reference resistance.

In any embodiment of the invention, the output means can include at least two optical indicators, one of one colour - preferably green - and the or each other of another colour - preferably red, the resistance comparison circuit being adapted to cause the output means to illuminate the one indicator if the ratio of the test resistance to the reference resistance is less than a predetermined ratio, preferably 2.7 to 1.0, and to illuminate at least one of the other indicators if the ratio is greater than the predetermined ratio. This is because we have discovered that on average the resistance of nerve damaged skin at the test probe is such that it exhibits a resistance of 2.7 times the resistance between the test probe and the reference probe when the former is at healthy skin. The predetermined, 2.7 to 1.0 ratio may be adjustable.

The output of the comparator (the first comparator) is preferably connected to one input of a second comparator, whilst the other input of the second comparator is connected to the common point of a voltage divider, a switching transistor connects the two optical indicators to a voltage rail via a series resistor, the base of the switching transistor is connected to a second transistor which applies the rail voltage to the base of the switching transistor under control of two distributed AND gate comparators, the one input of both of these comparators are connected to the common point of a second voltage divider, the other input of one of these comparators is connected to one input of the first comparator, whilst the other input of these comparators is connected to the other input of the first comparator.

Preferably, the output means includes an audio alarm for indicating if the ratio of the test resistance to the reference resistance is less than a predetermined ratio, preferably 2.7 to 1.0.

Preferably, the test and reference probes are each provided with a terminal mounted, ' preferably resiliently, within an insulated hollow spigot to normally protrude by a small amount whereby a determined contact force is exerted when the end of the spigot is pressed against the skin. In one embodiment, the terminal is a resilient sphere or diaphragm.

Preferably, the common probe is of flexble material adapted to conform to the skin for low contact resistance with the sub-cutaneous body mass. In the preferred embodiment, the common probe is of one and preferably two orders of magnitude greater in contact surface-area than the contact surface area of the test and reference probes. To help understanding of the invention, four specific embodiments thereof will now be described, with reference to the accompanying drawings in which:- The Drawings

Figure 1 is a perspective view of a sensory delineator or nerve damage sensor of the invention in use;

Figure 2 is a circuit diagram of the sensor of Figure i;

Figure 3 is a circuit diagram of a variant of the nerve damage sensor of Figure 2; Figure 4 is a cross-sectional view of a probe of the embodiment of Figure 1;

Figure 5 is a circuit diagram of another sensor of the invention;

Figure 6 is a circuit diagram of a third sensor of the invention; and

Figure 7 is a block diagram of a fourth sensor of the invention. First Preferred Embodiment

Referring first to Figures 1 to 4, the sensor of Figure 1 has a housing 1 for a circuit to be described and batteries. Extending from the housing are three wires 2, 3, 4 terminating in a common probe 5, a test probe 6 and a reference probe 7. The common probe is a square pad of conductive, elastomeric material having a surface area of 2500 mm². In use it is attached to the patient by self-adhesive tape 5' with the interposition of conductive gel (not shown) . This arrangement provides low resistance of a few ohms from the common probe to the sub-cutaneous body mass. It is envisaged that a smaller pad could be used, but below a square of 625mm², the the contact resistance may increase significantly.

The test and reference probes as shown in Figure 1 comprise a hollow rod 8 with a terminal 9 resiliently, or possibly fixedly, mounted within an insulated hollow spigot 8' to normally protrude by a small amount, typically 2mm, whereby a determined contact force is exerted when the end of the spigot is pressed against the skin. The spigot is typically of 2mm diameter, giving a contact surface area at its end of 3.14mm². Thus the contact surface area of the common probe is two orders of magnitude greater than that of the test and reference probes. An alternative probe is described in more detail below with reference to Figure 4. The sensor has a "healthy" green LED indicator 10, a "damaged" red LED indicator 11 and an audio output 12 changing according to which of the LEDs is illuminated.

In use with the common probe in position and a hand for instance being investigated, the test probe is applied to successive points on the hand being investigated. The reference probe is applied to exactly corresponding points on the other, healthy hand. When the test probe is at a position on the hand at which there is healthy skin, the Wheatstone bridge circuit to be described is in one condition and the green, "healthy" LED 10 is illuminated; when the test probe is at damaged skin, the condition of the bridge changes and the red, damaged LED 11 is illuminated. Referring now' to Figure 2, the electrical circuit of the sensor comprises a Wheatstone bridge 20, a comparator 21 for controlling the LEDs 10, 11, a distributed AND gate circuit 22 for powering the switched LEDs when the probes are positioned and an audio drive circuit 23.

In the Wheatstone bridge circuit 20, the sensor provides two elements Rl, a 2.7 Mohm resistor, and R2, a 1.0 Moh resistor, in the two arms of the bridge. The skin/body resistance between the test probe 6 and the earthed, common probe 5 and between the reference probe 7 and the common probe form the other elements of the bridge in the respective arms. The mid-points of the arms, namely the test probe 6 and the reference probe 7 are connected to the respective inputs 26, 27 of the comparator IClA which is one stage of an LM339 chip. The output 28 of IClA is connected to one input 27 of comparator IC1B, whilst the other input 30 is connected to the common point of the voltage divider R3, R4, respectively 220 kohm and 479 kohm. A switching transistor TRl connects LEDs 10, 11 to the positive rail Vcc via a 300 ohm series resistor R5. The base of TRl is connected to transistor TR2 which applies the rail voltage to the base of TRl under control of the distributed AND gate comparators IC1C, IC1D. The one input 31, 32 of both of these comparators are connected to the common point of the voltage divider R6, R7 respectively 10 ohms and 2.2 Mohms. The other input 36 of IC1C is connected to the test probe 6; whilst the other input 37 of IC1D is connected to the reference probe 7.

When either or both probe 6, 7 is out of skin contact, it is at rail voltage Vcc. This results in the respective inverting input 36, 37 being more positive than the non-inverting input 31, 32 to IC1C, IC1D respectively and its output 38, 39 being in grounded condition. TR2 is then switched OFF. When however, both probes 6, 7 are in position making skin contact, their voltage falls below that of the non-inverting inputs 31, 32 and the outputs 38, 39 are in effect open circuit. This results in Vcc being applied to the base of TR2 via the 220 kohm bias resistor R8. TR2 switches ON, switching ON TRl which applies Vcc to Dl and D2 via series resistor R5. One or other of the diodes illuminates in accordance with the relative voltages of the probes 6, 7.

When the test probe 6 is on healthy skin the resistance between it and earth - the common probe 5 - and the resistance between the reference probe 7 and earth is equal. Thus the test probe 6 is at a lower voltage than the reference probe, the associated resistance Rl being higher; and the inverting input 26 is less positive than the non-inverting input 27 of IClA. The result is that the output 28 appears to be open circuit. Therefore current cannot flow through the red Dl LED 11. The voltage at the output 28 rises above the voltage of the common point of the voltage divider R3, R4; therefore the inverting input 30 of IC1B rises above the non-inverting input 29 and the output 40 is earthed. Green D2 LED 10 switches ON - indicating that the skin at the test probe 6 has comparatively low resistance and is healthy.

When the test probe 6 is on nerve-damaged, high resistance skin, the situation is reversed. The voltage of the test probe 6 is no longer lower than that of the reference probe; and the inverting input is no longer lower than the non-inverting input with the result that the output 28 of IClA is grounded. Current is able to flow through the red Dl LED 11 indicating that the skin local to the test probe has been nerve damaged, with the result that the blood vessels have contracted raising its resistance. The inverting input 30 to IClB falls below the non-inverting input 29 and the output 40 opens circuit so that the green D2 LED 10 does not illuminate. The audio drive circuit 23 comprises a first buffer stage 23A, a 7Hz oscillator stage 23B, a second buffer stage 23C and a 700Hz oscillator 23D. Each stage employs a stage of a second LM339 integrated circuit. In so far as the arrangement of the circuit 23 will be understood by a man skilled in the art from Figure 2 and the values of the components shown thereon, it will be described only briefly. IC2A has its non-inverting input 41 held at a steady potential by the voltage divider R9, R10, whilst the inverting input 42 has the output from IClA applied to it. The result is that when healthy skin is probed the output 43 is IC2A is grounded. This prevents the capacitor Cl from charging and the inverting input C19 of IC2B is grounded. The voltage divider R15, R14+R12 controls the non-inverting input 45 of IC2C and the voltage divider R16, R17 controls the inverting input 46 with the result that the output is open circuit. Capacitor C2 is allowed to charge and the stage 23D oscillates at 700Hz, causing TR3 to cycle and a tone to be sounded by the speaker LSI.

When damaged skin is probed, the output 43 of IC2A is open circuit allowing stage 23B to oscillate at 7HZ. This modulates stage 23D and a chopped tone is produced. IC2 itself is switched ON when TR2 is ON, with the result that no tone at all is produced if either of the probes 6, 7 is not in skin contact. Variant of the First Embodiment

Turning now to the variant described in Figure 3, the Wheatstone bridge is provided in the test probe arm sensor element with four resistors in series R101, R103 R104, R105, having the values 4.11 Mohm, .47 Mohm, .47 Mohm, .47 Mohm; whilst the resistance R102 in the other arm is 1.0 Mohm.

The common points between the four resistors are connected to the inverting inputs of four LM339 stages ICllA, IC11B, IC11C, ICllD. The outputs are connected to switch LEDs Dll, D12, D13, D14 via series resistors R106, R107 R108, R109 and switched by two parallel transistors TRll, TR12. The arrangement is such that Dll illuminates for nerve damaged skin, with D12, D13, D14 illuminating at higher levels of damage or where the skin resistance is raised generally by low humidity. As in the first embodiment Dll illuminates at a resistance ratio of 2.7:1. Dll and D12 illuminates at a ratio of 3.7:1, D13 is illuminated at 4.7:1 and exceptionally all four of these red LEDs are illuminated at a ratio of 5.7:1. A green LED D15 is provided for normal skin indication. -li¬

lt is controlled by IC12A, which is analogous IClA. IC12B and IC12C are analogous to IC1C and IC1D. The audio circuit in this variant is analogous to that in the first embodiment and gives the same tone whatever state of damage is indicated by the LEDs.

Turning now to Figure 4, the structure of the probes is there shown. Each probe 6,7 has a hollow body 51 with a hollow terminal spigot 52 and a rim 53 close to the spigot for a user's fingers to abut against. Within the spigot is housed a hollow elastomeric sphere 54 with a conductive surface 55. Gas within the sphere is compressed to a pressure in excess of atmospheric pressure whereby the sphere has resilience. The housing 56 within the spigot for the sphere is so shaped that a small portion 57 of the sphere extends by a determined amount beyond the end 58 of the spigot. Due to the internal pressure in the sphere, the portion 57 is resiliently deformable inwards of the spigot; and in use, when the spigot is applied firmly against the skin, the conductive sphere's is held against the skin with a determined, reproducible force. The contact diameter will be of the order of 3mm and the contact area will be of the order of 7mm . The opposite side of the sphere is connected to a terminal lead for the probe, and thus the electrical contact of the probe with the skin is made in a constant manner. In a non-illustrated alternative, the sphere is replaced by a diaphragm sealed at the position of the portion 57, and the interior of the probe is pressurised. Second Preferred Embodiment

Turning now to Figure 5, the embodiment of sensor there shown in circuit diagram form includes a negative feed-back amplifier resistance comparison circuit 101, to which are connected common, test and reference probes 105,106,107. The circuit 101 includes a low impedance constant voltage source 102 to which the reference probe 107 is connected. The common probe 105 is connected to the negative feed-back input 110 of an operational amplifier 111 in a LMC660CN chip. The positive input 112 is grounded. The test probe is connected to the output terminal 113, which is also connected to the positive input 114 of the next stage 115 of the chip. This stage is connected as unity gain buffer. Via a 1.0 Mohm input resistor 116 connection is made to the third stage 117, which has a 1.0 Mohm negative feed back resistor 118. The output voltage from the third stage 117 is equal to the input voltage from the source 102 multiplied by the ratio of the test resistance - i.e. the resistance between the test probe and the common probe - to the reference resistance - i.e. the resistance between the reference probe and the common probe - in accordance with the following equation:

Rtest

V out -V. in reference

This output voltage is applied to a comparator 119 for comparison with voltage from a potential divider 120. The output from the latter is applied to an output circuit 121 the same as that of Figure 2, i.e. including red and green LEDs 122,123 controlled by further comparator stages not described. It should be noted that this embodiment is not suitable to be varied in accordance with the variant of Figure 3, because skin capacitance can disturb the above equation in accordance with the Miller effect. It is however suitable for Damage/No Damage analysis as is the embodiment of Figure 2. Third Embodiment

Turning now to Figure 6, the embodiment of sensor there shown in circuit diagram form includes a voltage follower resistance comparison circuit 201, to which are connected common, test and reference probes 205,206,207. The circuit 201 includes an operational amplifier 210 arranged for unity gain. The reference probe 207 is connected to a positive voltage rail V The common probe 205 is connected to the positive input terminal 211 of the amplifier 210. The test probe 206 is grounded. The unity gain amplifier has a high input impedance and thus acts as a buffer in not loading the common probe. The input voltage at the terminal 211 is equal to the output voltage at the output terminal 212 and to the ratio of the test resistance' to the sum of the test and reference resistances - in accordance with the following equation:

Rtest

Vout.

= Vcc X Rt.est. + Rref,erence

The output voltage is applied to a plurality of open collector output comparators 213, also connected to a resistance chain 214 and a plurality of LEDs 215. The individual resistors in the chain have the values given in Figure 6. The LEDs 215 illuminate as a bar graph in accordance with whether the resistance ratio reaches the value shown in Figure 6 against the individual LEDs. A unity gain, low pass filter 216, having a 28dB attenuation above 2Hz, is connected between the amplifier 210 and the comparator 213 to isolate them from any AC, mains frequency effects. The man skilled in the art will be able to modify the value of the resistors in the chain for detecting lower than normal resistance in the region of the junction between nerve damaged and normal skin. Fourth Embodiment

Turning now to Figure 7, the embodiment of sensor there shown in block diagram form includes a Wheatstone bridge circuit 301 as described in respect of the first embodiment, with fixed resistances 302,303 and common, test and reference probes 305,306,307 connected in the bridge in like manner to the probes 5,6,7. The test and reference probes 306,307 are connected to respective analogue-to-digital convertors 309,310. These in turn are connected to a digital processing system 311, which in turn outputs information indicative of the ratio of the test and reference resistances to an output indicator 312. The specification and programming of the units 309,310,311,312 will be within the competence of the man skilled in the art and will not be further described.

It should be noted that all the above embodiments apply DC voltage only to the three probes. This is because we believe that AC voltage would disguise skin resistance changes in that nerve damaged skin will have a higher dielectric effect and hence higher capacitance to the probes. Use of AC might not detect nerve damage as the total skin impedance may appear constant, ie as the resistance increases with damage its capacitive impedance reduces (as its capacitance increases) . Further probe size affects capacitance. We therefore prefer to use DC and measure resistance to provide results which are most easy to interpret.

Claims

CLAIMS :

1. A sensory delineator comprising: a common probe applicable to a healthy area of the patient's skin, a test probe applicable to an area of skin suspected to be subject to nerve damage, a reference probe applicable to an area of similar skin suspected to be healthy, a resistance comparison circuit for comparing the resistance present between the common probe and the reference probe (the reference resistance) with the resistance present between the common probe and the test probe (the test resistance), and output means for indicating whether the test resistance differs from that of the reference resistance.

2. A sensory delineator as claimed in claim 1, wherein the resistance comparison circuit is an analogue arithmetic circuit providing to the output means a voltage output indicative of an arithmetic relationship between the reference resistance and the test resistance.

3. A sensory delineator as claimed in claim 1, wherein the resistance comparison circuit is a digital arithmetic circuit providing to the output means a voltage output indicative of an arithmetic relationship between the reference resistance and the test resistance.

4. A sensory delineator as claimed in claim 2, wherein the resistance comparison circuit is a Wheatstone bridge circuit in which two of the usual four elements are provided in the resistance comparison circuit and two will be provided in use by parts of the body, the three probes being connected in the Wheatstone bridge circuit such that in use the body resistance between the common probe and the test probe forms one element of the bridge and the body resistance between the common probe and the reference probe forms another ele ent of the bridge and the imbalance voltage in the bridge is indicative of the extent to which the ratio of the test and reference resistances differs from the ratio of the other two elements of the bridge.

5. A sensory delineator as claimed in claim 4, wherein the resistance comparison circuit includes a comparator connected to the output means, the common probe is connected to one voltage rail, the test probe is connected to another voltage rail via one of the other two elements and to one input of the comparator, the reference probe is connected to the other voltage rail via the other of the two elements and to another input of the comparator, the comparator being arranged to give an output if the test resistance exceeds the reference resistance by the ratio of the resistances of the other two elements of the bridge.

6. A sensory delineator as claimed in claim as claimed in claim 5, wherein the other element of the bridge having the test probe connected to it comprises a chain of resistors and a plurality of comparators is provided and respectively connected to the common points of pairs of the resistors in the chain, the arrangement being such that the ones of the comparators which give an output to the output means are determined by the ratio of the test resistance to the reference resistance.

7. A sensory delineator as claimed in claim 2, wherein the resistance comparison circuit includes a feed-back amplifier circuit having an amplifier and a source of substantially constant voltage, the common probe being connected to one input terminal of the amplifier, the test probe being connected to an output terminal of the amplifier for arrangement in use of the reference resistance as a feed back resistance between the output terminal and the input terminal and the test probe being connected to an output terminal of the voltage source for arrangement in use of the test resistance between the output terminal of the voltage source and the said input terminal of the amplifier, the output of the amplifier being proportional to the ratio of the test resistance to the reference resistance.

8. A sensory delineator as claimed in claim 2, wherein the resistance comparison circuit includes a voltage follower circuit having a feed-back amplifier arranged for unity gain with a direct connection between an output terminal and an input terminal, the common probe being connected to the input terminal, the reference probe being connected to one supply rail and the test probe being connected to another supply rail, the arrangement being such that in use the reference and test resistances are connected in series to divide the potential between the references in proportion to their values and feed this proportion to the input terminal of the amplifier, the output of the amplifier being equal to its input and to the ratio of the potential across one of the test and reference resistances to the potential cross both of these.

9. A sensory delineator as claimed in claim 7 or claim 8, wherein the resistance comparison circuit further includes a comparator connected to the output means and a potential divider, the comparator being arranged to compared the output of the amplifier with divided potential from the potential divider and cause the output means to give an output if the test resistance exceeds the reference resistance by a predetermined factor.

10. A sensory delineator as claimed in claim claim 9, wherein the comparison circuit includes a plurality of comparators and and a chain of resistors in the potential divider, the comparators being respectively connected to the common points of pairs of the resistors in the chain and giving, the arrangement being such that the ones of the comparators which give an output to the output means are determined by the ratio of the test resistance to the reference resistance.

11. A sensory delineator as claimed in any preceding claim wherein the output means includes at least two optical indicators, one of one colour - preferably green - and the or each other of another colour - preferably red, the resistance comparison circuit being adapted to cause the output means to illuminate the one indicator if the ratio of the test resistance to the reference resistance is less than a predetermined ratio, preferably 2.7 to 1.0, and to illuminate at least one of the other indicators if the ratio is greater than the predetermined ratio.

12. A sensory delineator as claimed in claim 11 as appendant to claim 5, claim 6 or claim 9, wherein the output of the comparator (the first comparator) is connected to one input of a second comparator, whilst the other input of the second comparator is connected to the common point of a voltage divider, a switching transistor connects the two optical indicators to a voltage rail via a series resistor, the base of the switching transistor is connected to a second transistor which applies the rail voltage to the base of the switching transistor under control of two distributed AND gate comparators, the one input of both of these comparators are connected to the common point of a second voltage divider, the other input of one of these comparators is connected to one input of the first comparator, whilst the other input of these comparators is connected to the other input of the first comparator.

13. A sensory delineator as claimed in any preceding claim, wherein the output means includes an audio alarm for indicating if the ratio of the test resistance to the reference resistance is less than a predetermined ratio, preferably 2.7 to 1.0.

14. A sensory delineator as claimed in any preceding claim, wherein the test and reference probes are each provided with a terminal mounted, preferably resiliently, within an insulated hollow spigot to normally protrude by a small amount whereby a determined contact force is exerted when the end of the spigot is pressed against the skin.

15. A sensory delineator as claimed in claim 14, wherein the terminal is a resilient sphere or diaphragm.

16. A sensory delineator as claimed in any preceding claim, wherein the common probe is of flexble material adapted to conform to the skin for low contact resistance with the sub-cutaneous body mass.

17. A sensory delineator as claimed in any preceding claim, wherein the common probe is of one and preferably two orders of magnitude greater in contact surface area than the contact surface area of the test and reference probes.