Measurement of Fontanelle Pressure (part I ) A New Instrument for Non-invasive Measurement of Intracranial Pressure via the Anterior Fontanelle

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1 THE KURUME MEDICAL JOURNAL Vol.31, p , 1984 Measurement Fontanelle Pressure (part I ) A New Instrument for Non-invasive Measurement Intracranial Pressure via Anterior Fontanelle EIICHIRO HONDA Department Neurosurgery, Kurume University School Medicine, Kurume 830, Japan Received for publication July 27, 1984 Summary: Intracranial pressure (ICP) is one most important parameters for evaluating intracranial condins. The skulls newborns infants, unlike tightly sutured skull adults, provide non-invasive access for measurement ICPs via anterior fontanelle. A new transducer has been developed which is convenient for non-invasive, continuous measurement intracranial pressure through anterior fontanelle. The new transducer is thinner lighter than APT-16 type transducer (Hewlett-Packard). The principles measurement TOP with this new transducer, results basic experiments, results obtained from clinical applications are deseribed in this report. Key words: Intracranial pressure \ anterior fontanelle \ adhesive strain gauge transducer \ hydrocephalus \ infant Introduction Intracranial diseases newborn infant, especially hydrocephalus, involve some disturbance cerebrospinal fluid circulation which leads abnormal dilatation ventricle an increase intracranial pressure (ICP) as main clinical signs. Today RI cisternography, metrizamide CT cisternography, or techniques have been established observe circulation absorption cerebrospinal fluid. CT scanners reveal intracranial morphology safely easily are widely used. The remaining problem in diagnosis followup hydrocepalus is develope a method safely easily measure ICP. Since ICP is a dynamic parameter that changes with or biological changes in body, such as arousal, sleep, tension, crying, body fluid change, a simple technique like lumbar puncture may be insufficient. Invasive techniques like ventricular puncture or epidural pressure moniring are not suitable for routine studies. Wealthall Smallwood (1974) reported a method measuring ICP via anterior f ontanelle with an APT-16 transducer using applanation method (manufactured by Hewlett- Packard Company). We have frequently used this method on infants. Although APT-16 transducr has sufficient accuracy, it is difficult fix because its weight shape. After reexamining advantages disadvantages APT-16 transducer, a new apparatus applanation type developed measure intracranial pressure with a paper strain gauge based on principle an adhesive resistance strain gauge (Fontanelle pressure sensor: F. P. sensor, manufactured by Sanei-Sokki Inc.). This apparatus has been demonstrated in 235

2 236 HONDA present study be useful not only for a single measurement, but also for continuous measurement ICP over many hours. Principle 1. Principle Apparatus Measurement measurement The applanation method described by Welthall Smallwood (1974) is used for this apparatus. This principle has already been applied an ophthalmic nometer. When an area (A) is pressed flat with a force (W), tension (T) scalp over anterior fontanelle is exerted in a tangential direction (X axis), refore does not become a counter-force (W) in direction Y axis. It is a vecr unrelated intracranial pressure. Furr if volume displaced by applanation surface (A) is sufficiently small, increase intracranial pressure (P) is negligible. Then Imbert-Fick equation, W/A=P (pressure per unit area), is applicable providing objects are relatively elastic spherical, such as eyeball infantile cranium (Fig. 1). Fig. 1. The ICP measured via anterior fontanelle by applanation method on basis Imbert-Fick equation. Pressure (P) equals force (W) per unit area (A). If a segment a flexible sphere is flattened by a plane surface, pressure in sphere acting on flattened area is in balance with an applied force. 2. Apparatus The apparatus for measuring ICP consists a transducer a pressure indicar. The transducer has a foot plate with an out ring a central plunger. It is important that foot plate plunger are on same plane (Shojima, 1980). The inner structure transducer is shown in Fig. 2. A thin spring plate phosphorus bronze is connected plunger paper strain gauges (Fig. 2). The mode pressure transmission is as follows. Pressure on central plunger is transmitted spring plate. The strain in direction Y axis is detected by 4 paper strain gauges amplified. The thickness spring plate can be changed arbitrarily between mm. In a preliminary study decide most suitable thickness spring plate, mechanical spring thickness has been plate 0.25 actual ments. for confirmed for plates firmed tionship a within tracranial lost sitivity above amplitude 600 high, sudden plunger has 0 point body its reached own thinner were con- linear rela- clinical range linearity Since excessive (overshooting) during mm, however mmh2o. an hard inappropriate sufficiently pressure, 0.15 spring o measure- Conversely, 0.10 have seemed 0.25) The pressure it use. each 0.20, 3). thickness Therefore practical 0.15, (Fig. mm intracranial spring appropriateness (0.10, sen- increase observed movements. weight, in- Since deviation a maximum k max-

3 MEASUREMENT OF FONTANELLE PRESURE 237 Fig. 2. A drawing F. P. sensor. A thin metal board (spring plate) is connected a plunger four gauges. A change in ICP is transmitted strain gauges through plunger strain is electronically converted a voltage signal. imum deviation 0 point measured when transducer just above sample (0 ) or just below sample (180 )), 45 mmh2o (0.10 mm) depending on angle transducer. In practice, angle is limited from immediately above (0 ) a lateral position (90 ), deviation 0 point in this clinical situation is less than 15 mm H2O with 0.10 mm spring plate. The 0.2 mm spring plate has largest deviation, 25 mmh2o. The basis for selecting a spring plate Fig. 3. Relationship between weight on plunger signal output (mm pen deflection) with spring plates several thicknesses. There a linear relationship with spring plates, 0.15, 0.20, mm thick. : 1) amplification capability, 2) linearity, 3) relative stability. The 0.20 mm plate seemed most suitable. Next, size (diameter) plunger is discussed. To obtain an accurate measure ICP, applanation area should be as small as possible a linear relationship should also exist, even when

4 238 HONDA anterior fontanelle is 1 cm in diameter. Plungers, 3, 6, 9 mm in diameter, were produced, examined in regard balance between size plunger thickness spring plate as well as linearity. A linear relationship not observed when a 3 mm plunger used. On contrary, a linear relationship found when a 6 mm plunger used with any spring plate. With 9 mm plunger, only a spring plate 0.20 mm thickness used. Because gain is narrow with amplifier, sufficient adjustment could not be made. The baseline pressure 140 mmh2o rar than conventional pressure 100 mmh2o, but orwise linearity satisfacry (Fig. 4). It ascertained that sufficient linearity sensitivity were obtained when plungers with a diameter 6 mm or more were used. In addition, non-linear relationship with 3 mm plunger Fig. 4a. The pressure in a 1 liter flask raised by 100 mmh2o increments from mmh2o compare values recorded from an artificial "fontanelle" with a transducer having a 9 mm plunger a 0.20 mm spring plate. The results demonstrate that an inner pressure 100 mmh2o, recorded as 140 mmh2o by transducer. A linear relationship obtained as inner pressure increased. Fig. 4b. Results using a transducer with a 6 mm plunger a 0.20 mm spring plate. The pressure were identical.

5 MEASUREMENT OF FONTANELLE TABLE Transducer *Tested at full-scale pressure considered area pressure a 6 mm plunger which gave used had 4 paper were core, center or two were plate. construct an parallel a bridge amplifier. system with 4 increase in F. P. drift characteristics marized Table were back used used gauge increase each The 2.7 connected strain from this sensitivities in on temperature. sensor Two side plate, 120ƒ ) each PSGs reaction P. sen- type. multiple PSGs study. F. spring which The pressure minimize 4 best (PSG; mounted The 0.2 on not corrected in PSG with The apparatus are sum- with a skull model 1. A 1 liter results flask filled with Fig. 5. A hole (3 cm diameter) made in p a 1 liter round flask which filled with water. The hole covered by a membrane, such as a surgical glove with a mouse skin. This served as a model for scalp underlying fontanelle. The internal pressure (P) adusted by changing level water in reservoir (H). sensitivity V/g/cm2. Results 1. Experimental with applied plunger. present gauges temperature spring strain mounted in structure (FSD) small would were deflection strain because plate sor spring internal characteristics sample, pressure transferred 1 contact unstable, The an small Therefore balance, intracranial accurately mm plunger mmh2o transmission change due gauge be be between PRESURE water used as a model for skull. A round opening made in p flask, covered with a rubber membrane imitate "anterior fontanelle". The inner flask pressure changed by changing fluid level in a connecting reservoir (Fig. 5). The properties this "fontanelle" model system were studied. Several ma-

6 240 HONDA terials were used, including a condom, a surgical rubber glove, a rubber sheet with little elasticity, a rubber glove with mouse skin, a rubber glove with infant's back skin. The flask pressure measured with a F. P. sensor via "fontanelle". The results are shown in Fig. 6. Even with a rubber sheet mouse skin, which is far from ideal, Y equal 1.006X this nearly corresponds a straight line, Y=X. The correlation With a relatively elastic condom or surgical glove, error measurement negligible (Fig. 6). From se results, when human anterior fontanelle scalp has a certain degree elasticity, measurement pressure with F. P. sensor over scalp probably reflects absolute ICP. The size round window in flask changed from 3 cm 5 cm. In both cases, a rubber sheet with little elasticity used for "fontanelle". The measurement very accurate (Fig. 7). This suggested that a difference in size anterior fontanelle is unlikely cause an error in measurement. Next, inner pressure measured simultaneously, with fontanelle pressure. The fontanelle pressure measured with APT-16 transducer or F. P.sensor, inner pressure measured directly with a Statham P-36 pressure transducer. Both APT-16 transdcer F. P. sensor produced same pressure values as Statham P-36. However, when a sudden change pressure occurred, APT-16 F. P. sensor tended generate slightly higher amplitudes than Statham P-36 transducer (Fig. 8). This is probably attributable a difference in frequency-response characteristics. 1) A decrease in frequency-response system, including compliance at Hz, occurred with Statham P-36 transducer, 2) an enhancement frequency-response system at approximately 100 Hz occurred due high resonance frequency with APT-16 or F. P. sensor. Fundamental experiment (I ) Correlation in two artificial fontanelle materials Correlation Fig. flask F. P. glove 6. Relationship pressure sensor ( œ) with two a mouse between output from membranes, skin a ( ). Fundamental inner surgical in Fig. ficial in 7. fontanelles, diameter. two experiment sizes Pressure ( U) artificial relationship 3cm ( ) fontanelle for 5cm arti( )

7 MEASUREMENT OF FONTANELLE PRESURE 241 Fig. 8. Simultaneous recording with an F. P. sensor (above) a Statham pressure transducer (below). The F. P. sensor detects inner flask pressure pressure fluctuations more accurately. Fig. 9. Simultaneous recording with a needle transducer (above) an F. P. sensor (below) in a hydrocephalic patient, 1 month age. Note that F.P.sensor detects ventricular pressure pressure fluctuations more accurately. Fig. 10. Tracings from a 13 month-old patient with hydrocephalus using an F. P. sensor (above) an intraventricular needle connected a Statham transducer (below). Note excellent reproduction intraventricular tracing with F. P. sensor.

8 242 HONDA 2. Results simultaneous measurement ventricular pressure To evaluate reliability F. P. sensor clinically, ventricular pressure measured with a Statham P-36 transducer by ventricular puncture ICP simultaneously determined with an F. P. sensor via anterior fontanelle in a patient with hydrocephalus. The subjects were hydrocephalic patients, 1 13 months age. There were differences in elasticity scalp over anterior Fig. 11. Polygraphic recordings ICPs measured with an F. P. sensor during non-rem (above) REM periods (below) sleep in a patient with meningomyelocele hydrocephalus. The height amplitude ICP incresed during REM sleep.

9 MEASUREMENT OF FONTANELLE PRESURE 243 fontanelle in size fontanelle. The anterior fontanelle relatively hard 4 cm ~3.3 cm in 13 month old patient. The size 3 cm ~2.2 cm in one month old patient. In both cases baseline pressure nearly same with Stath am P-36 F. P. sensor. However, during a sudden change ICP accompanying a body movement, e. g. cough, amplitude each pulse tended be slightly higher with F. P, sensor than with Statham P-36, similar model experiment (especially with 0.15 mm spring plate in F. P. sensor). In older infant with hard anterior fontanelle, baseline pressure measured with an F. P. sensor via anterior fontanelle compared with intraventricular pressure from a needle tap. The difference in baseline pressure between two systems reached a maximum 60 mmh2o depending on degree force needed f ix F. P, sensor on anterior f antanelle. Furr each pulse had a shorter duration, approximately 0.1 cosec, with F. P, sensor than with Statham P-36. This is probably due differences in sizes transducer transmission area, in medium for transmission, in frequencyresponse characteristics, in specific facrs each measurement system (Fig. 9,10). 3. Clinical application ICPs were recorded with a polygraph from 14 infants with hydrocephalus, 8 normal infants (less than 2 months) newborns, 25 infants with or types intracranial diseases. During sleep, especially during REM non-rem periods, ICP } mmh2o }19.1 mmh2o in hydrocephalus, 88 } 16.1 mmh2o 76.1 } 23.0 mm H2O in normal infants newborns, respectively. Fig. 11 is a polygraphic recording REM non-rem periods from a patient with meningomyelocele complicated by hydrocephalus. Since this recorder has a range beyond 550 mmh2o, 1/2 gain used (Fig. 11). Discussion Many measurements ICP via anterior fontanelle or scalp defect have been described, methods measurement are summarized in Table 2. The characteristics have been already reported in detail by Hayashi (1975). Wentzler (1922) measured anterior fontanelle pressure by improving SChiOtzs nometer. This method is simple, but is limited use in sitting position, which leads extremely low incorrect values. The method Purin (1964), using Marey's tambour, is oretically accurate can be applied clinically, but some training is necessary for measurement. Also, anterior fontanelle must be relatively large, continuous measurement for long durations is impossible, even though necessary determine ICP. The reproducibility is poor. Weathall Smallwood (1974) used APT-16 transducer, an applanation transducer based on Imbert- TABLE 2 Classification ICP measurement via fontanelle

10 244 HONDA Fick's principle, measure ICP noninvasively via anterior fontanelle infant. This method is simple, can measure ICP accurately. Furrmore it can measure ICP continuously. Robinson et al. (1977) used similar methods in clinical cases, mainly with newborns. Shojima (1930) Salmon et al. (1977) have reported in detail on accuracy APT-16 transducer. An APT-16 transducer is an electromagnetic type transducer in which an, AC current is passed through primary coil produce a magnetic field. A metallic core is connected plunger responds pressure by moving through magnetic field, inducing an electric current. The movement is used calculate ICP, based on a proportional relationship with electric current. The disadvantages this transducer are height coil required produce magnetic field, an unstable fixation transducer on anterior fontanelle due weight coil, also possibility or resultant forces, such as strain on X or Y axes, by central core. The F. P. sensor is similar APT-16 transducer in principle operation accuracy, however F. P. sensor is 12 mm high weighs 15 g. This is less than half height weight APT-16 transducer (24 mm 36 g). Thus it is easily fixed on anterior fontanelle seems most suitable for continuous measurement TOP over several hours. Also it is less expensive than APT-16 transducer. Wealthall Smallwood (1974) cornpared TOP via anterior fontanelle intraventricular pressure by ventricular puncture. Only a small difference noted which expressed by Y= 0.97X+7.6 (p0.001). This small error in measurement should not be a problem. A small pressure from fixing transducer on anterior fontanelle could produce this error. Measurements extradural TOP, utilizing coplanar measurement ory by Imbert-Fick's principle, also involves a problem at point measurement, apart from difference between scalp dura mater. Major et al. (1972) described relationship between nometer depth TOP from a model experiment in which re 3 stages, i.e. (1) aflattened membrane surface, (2) point coplanar attachment, (3) region producing an increase inner pressure with increas - ing depth nometer. It decided that point coplanar attachment most suitable point for measurement. Ikeyama et al. (1976) stated that artificial increase TOP not due pressure buffer system blood vessels or cerebrospinal fluid cavity, if depth compression by a nometer is small. Therefore measurement is possible even in region 3 as defined by Major et al. (1972). Schettini (1975) determined relationship between depth nometer ICP in animals, regarded measurement possible in region producing a sudden increase pressure with relatively stable amplitude. The F. P, sensor fixed anterior fontanelle by tension a rubber ring. This should correspond stage 2 or 3. This method fixation does not result in an increase ICP by movements made during measurement. However, if ICP is greatly increased, buffering actions intracranial vein cerebrospinal fluid cavity are small, fixation sensor by compression may increase TOP artificially. In clinical practice, pressure fixation has little influence in newborns with elastic anterior fontanelles. As anterior fontanelle hardens with age, or as TOP is increased markedly, elasticity anterior fontanelle is lost, area applanated by transducer is irregular, conditions are no longer appropriate for use applanation method.

11 MEASUREMENT Fig. 12a. View F. P. sensor OF FONTANELLE 245 from side. Fig. Fig. 12b. View F. P. sensor from below. The foot plate F. P. sensor is stationary. When sensor is placed on PRESURE anterior fontanelle, plunger is dis- 12d. This picture demonstrates placement F. P. sensor on anterior fontanelle. The guide ring is fixed by cyanoacrylate directly scalp over anterior fontanelle, sensor is easily attached flanged spring. guide ring with a three- placed Fig. above. 12c. View F. P. sensor from Consequently, intracranial pressure is markedly influenced by fixation pressure. The error in steady state pressure may be as large as 60 mmh2o. Measurement ICPs in older infants infants with expremely high pressures must be investigated furr in future. The applanation method fixation may be improved by using different adhesives (Fig. 12). In addition, height weight transducer have been reduced as much as possible make it suitable for continuous recording ICP. Instead electromagnetic type APT-16 transducer, an F. P. sensor using strain gauges has been developed, but

12 246HONDA temperature characteristics strain gauge is still a problem. Although this sensor has a 4 gauge system compensate for temperature drift, drift is still as large as 10 mmh2o/c. Since F. P. sensor is attached human body, drift due changes body temperature becomes a problem during continuous measurement for many hours. If body temperature changes by 0.3 Ž in an infant, a drift 3 mmh2o is produced, although this degree change is not a problem. When F. P. sensor is used for continuous measurement, it is necessary warm sensor up for minutes at body temperature subject before it is attached. Conclusion An APT-16 transducer used measure ICP via anterior fontanelle. The advantages disadvantages this applanation transducer were analyzed. A transducer (F. P, sensor) using paper strain gauges (PSG) manufactured found be useful for continuous measurement ICP via anterior fontanelle. Mechanical problems temperature characteristics F. P. sensor were discussed. Outlines this article were presented at 17 th Meeting Japanese ME Association (June 1978, Sapporo), 38th Congress Japanese Association for Neurosurgery (Ocber 1979, Tokyo), 25 th Meeting Premature Newborn Infants Seminar (November 1980, Tokyo), 9 th Meeting International Society for Paediatric Neurosurgery (July 1981, Budapest). Acknowledgments: The author expresses his thanks Pr. S. Kuramo Dr. T. Hayashi for ir constant interest guidance in this investigation. References BARASHNEV, Y. I. LEONTIV, A. F. (1970). Cerebrospinal fluid pressure in premature infants with without intracranial birth injury (Russian). Rediatriya, 44, 507. DAVIDOFF, L. M. CHAMLIN, M. (1959). The "Fontanometer" adaptatio n Schiotz nometer for determination intracranial pressure in neonatal early period infancy. Pediatrics, 24, DORSCH, N.M.C. SYMON, L. (1975). Apractical technique for moniring extradural pressure. J. Neurosurg. 42, EDWARDS, J. (1974). An intracranial pressure nometer for use on neonates preliminary report. Dev. Med. Child Neu ol. 16, 38. HAYASHI, T. (1975). Studies on early diagnosis infantile central nervous system disorder, especially, for hydrocephalic infant. The Journal Kurume Medical Association, 38, HONDA, E., HAYASHI, T., HIKITA, T., SHOJIMA, T. KURAMOTO, S. (1978). Clinical analysis chronic subdural effusion-relationship between CT findings ICP via fontanelle. Nervous System in Children, 3, IKEYAMA, J., MAEDA, N., NAGAI, H., KOGA, K., IGARASHI, I, INAGAKI, D. (1976). Measurement intracranial pressure - (5) - problems on epidural measurement intracranial pressure-. Brain Nerve, 28, MAJOR, R., SCHETTINI, A., MAHIG, J. NERIES, A. H. (1972). Intracranial pressure measured with coplanar pressure transducer. Med. Biol. Eng. 10, PICTON-WARLOW, C. G. Robinson, R. J. (1970). Evaluation a method indirect measurement intracranial pressure in infants. Dev. Med. Child Neurol. 12, 507. PURIN, V. R. (1964). Measurement cerebrospinal fluid pressure in infant without puncture. A New Method Pediatriya, 43, ROBINSON, R. O., ROLFE, P. SUTTON, P. (1977). Non-invasive method for measuring intracranial pressure in normal new born infant. Dev. Med. Child Neurol. 19, 305. SHOJIMA, T. (1980). Studies on non-invasive method intracranial pressure (ICP) measurement in infants. The Journal Kurume Medical Association, 43,

13 MEASUREMENT OF FONTANELLE PRESURE 247 SALMON, J. H., HAJIAR, W, RADA, H. S. (1977). Fongram; A non-invasive intracranial pressure monir. Pediatrics, 60, SCHETTINI, A, WALCH, E. (1975). Simulataneous pressure depth measurement intracranial system made epidurally, in Lundberg N, Ponten U, Brock M. Intracranial Pressure U. Berlin, Springer, WEALTHALL, S. R. SMALLWOOD, R. (1974). Method measuring intracranial pressure via fontanelle without puncture. J. Neurol. Neu rosurg. Psychiatry, 37, WENTZLER, P. (1922). Emn Apparat zur Messung des Schadel innenendruckeres an der Fontanelle des Sauglings. Vorl Mitteilung Arch Kinderhk, 70, 24.