Relationship between intracranial pressure and intracranial volume in craniosynostosis

H. Fok, B. M. Jones, D. G. Gault, U. Andar and R. Hayward

Craniofacial Centre, Hospital for Sick Children, Great Ormond Street, London

SUMMARY. Premature fusion of cranial sutures in craniosynostosis has been thought to lead to craniostenosis, which in turn may lead to increased intracranial pressures. In 41 consecutive patients with craniosynostosis, intracranial pressure and intracranial volume were measured. Of the 41 patients, 38 (92.6%) had raised intracranial pressure but only 4 (9.7%) had a decreased skull volume. In the present study, there is no correlation between intracranial volume and intracranial pressure. This study confirms that the measurement of intracranial volume, a non invasive procedure, cannot be used to assess intracranial pressure and to avoid an invasive procedure.

The premature fusion of cranial sutures in cranio- synostosis is frequently thought to restrict skull de- velopment and produce associated skull and facial deformities leading to functional impairment of the brain, eyes, air passages and jaws (Tessier, 1971). The exact pathogenesis and natural history of this process remains unclear. Management of these patients, especially in infancy, has been based on the precept that suture synostosis alters skull growth, resulting in decreased skull volume or craniostenosis which leads to increased intracranial pressure as the enclosed brain grows (David et al., 1982).

Increased intracranial pressure has certainly been recorded in craniosynostosis and it tends to occur more frequently when several sutures are involved. There is also a significant correlation between raised intracranial pressure and decreased intelligence quo- tient (Renier et al., 1978). Thus the detection and treatment of raised intracranial pressure in children with craniosynostosis has important functional imph- cations.

Clinical signs of raised intracranial pressure are unreliable guides since they develop late in the pro- gression of the condition. Although various direct and indirect methods of intracranial pressure monitoring have been described, accurate measurement requires an intracranial sensor, a procedure which is invasive and usually requires a general anaesthetic.

If there is a correlation between raised intracranial pressure and decreased skull volume, then the measurement of skull volumes by a non-invasive method may avoid the need for an invasive procedure. The purpose of this study is:

1. to determine the relationship between intra- cranial volume and intracranial pressure in cranio- synostosis.

2. to determine the incidence of raised intracranial pressure in our series.

Patients and methods

Forty-one consecutive cases of craniosynostosis treated at the Hospital For Sick Children, Great Ormond Street, London were studied. There were 16 females and 25 males. Ages ranged from 1.5-122 months with a mean of 27 months. The study popu- lation included those with both multiple and single suture synostosis (Table I).

As part of their routine craniofacial assessment, each patient had his or her intracranial pressure and intracranial volume measured during the same ad- mission.

The intracranial pressure was measured using an OLM intracranial pressure monitoring unit made by Camino Laboratories, USA. This system involves placement of a subdural pressure sensitive transducer through a small hole made by a twist drill. Pressure alterations are transmitted along a fibreoptic coupling to an external recorder and this eliminates the prob- lems inherent in a fluid filled system. It gives a fast frequency response and reflects the true wave form of the intracranial pressure. The atmospheric pressure is used as the reference pressure in this system and the system is calibrated to the atmospheric pressure prior to the insertion of the transducer. Each patient was monitored overnight for a period of at least 12 h. Baseline intracranial pressures during sleep and peak

Table 1

Distribution of cases

Unicoronal synostosis 7 Bicoronal synostosis 7 Sagittal synostosis 6 Metopic synostosis 7 Lambdoid synostosis 2 Multiple synostosis 3 Crouzon’s 7 Apert’s 5 Saethre Chotzen 2

Total 41

394

Intracranial Pressure and Volume in Craniosynostosis 395

0 10 20 30 40 50 60 IO a0 90 100 110 120

Age (months)

/ + Normal ICP 1

Fig. I

Figure I-The intracranial pressure of 41 children with craniosynostosis. The line indicates the normal intracranial pressure.

Suture(s) involved

Unicoronal

Bicoronal

Sagittal

Metopic

Lambdoid

.:

Multiple *

Crowon’s

Apert’s

S-Chotzen

-8 -6 -4 -2 0 2 4

Volume Z-score

+ Raised ICP a Normal ICP

Fig. 2

Figure 2-The volume distribution of the study population. The symbols indicate the status of their intracranial pressure.

pressures during activity were noted. The mean in- tracranial pressure is the average of the waveform values one-third up the waveform.

The normal intracranial pressure varies with age. In the neonate, it is normally about 2 mmHg (Philip et al., 1981), at 12 months 5 mmHg, at 7 years 6-13 mmHg and in older children up to 15 mmHg (Minns, 1979, 1984). The neonatal intracranial pressure obtained by Philip et al. was through sequential measurement of 120 full-term and preterm infants using a pressure activated fibreoptic placed over the anterior fontanelle. The other values suggested by Minns resulted from his survey of the literature on intracranial pressure in

various age groups. The results were obtained by various methods including lumbar puncture and direct intraventricular measurements.

Skull volumes were calculated using the technique described by Gault et al. The head was scanned horizontally from the vertex to the foramen magnum

at 5 mm intervals. The intracranial area on each scan was measured using a planimeter. The total area multiplied by the scan interval gave the skull volume. This technique has a mean percentage error of only 1.13% (Gault et al., 1988).

The volume obtained was then compared to a normal population. Skull volume data of normal population by Lichtenberg (1960) has been found to be most acceptable and is used in this study. As intra- cranial volume varies with age, standard deviation scores or Z-scores were calculated to enable com- parison between groups (Gault et al., 1990).

Results

Of the 41 patients with craniosynostosis, 38 patients (92.6%) had raised intracranial pressure. Only 3

396 British Journal of‘ Plastic Surgery

patients had normal or near normal intracranial pressure (Fig. 1). These 3 patients had unicoronal synostosis, bicoronal synostosis and multiple suture synostosis respectively.

When intracranial volumes were measured, 29 patients (70.7 %) had a normal skull volume ( f 2 S.D.) Only 4 patients had an intracranial volume that was less than 2 standard deviations below the mean for their age, while 8 patients (19.5%) had a skull volume that was greater than 2 standard deviations above the mean for their age (Fig. 2). All these 8 patients had normal ventricular size with no evidence of hydro- cephalus. The enlarged skull volume was filled with brain tissue rather than cerebrospinal fluid.

When comparing the intracranial volumes of those patients with single suture synostosis (plagiocephaly, brachycephaly, scaphocephaly, lambdoid synostosis) to those with multiple suture synostosis, the single suture synostosis patients had significantly larger volumes (p < 0.05).

Of the 38 patients with raised intracranial pressure, only three had decreased intracranial volume. The remaining 35 patients had either normal (73.6%) or enlarged (18.4%) intracranial volumes (Fig. 2).

Discussion

The traditional management of infants with cranio- synostosis has been based on the concept that prema- ture fusion of skull sutures may lead to craniostenosis and the push of the growing brain within the restricted skull vault may in turn give rise to raised intracranial pressure. Surgical procedures have therefore been designed to release the calvarial sutures, thus allowing the skull volume to expand and reduce the intracranial pressure.

For our unit, measurement of intracranial pressures has become an important component in the assessment of patients with craniosynostosis and the results provide a significant factor in the decision on whether to operate and which procedure to carry out. When the intracranial pressure is raised, the indications for surgery are obvious. If the intracranial pressure is normal, surgery may still be indicated for reasons of cosmesis.

A major difficulty remains in that normal intra- cranial pressure has not been accurately determined, especially in infants and young children. This is the group of patients whose development is most at risk

and also the group likely to be the most responsive to early treatment.

It is generally agreed that an intracranial pressure of less than 15 mmHg would be normal for an adult. However, various studies have indicated that normal pressures in children are at a much lower level. Table 2 shows the range of cerebrospinal fluid pressures in infants and children from various studies. The values used in our study were in the range suggested by Table L.

Of the patients in our series. 92.7 % had raised intra- cranial pressures. This result differs markedly from

other series (Renier et cd., 1987; Gault et cd.. 1992) because the values for normal pressures applied were different. These studies applied the value of 15 mmHg as the normal intracranial pressure in all age groups. However, if the “normal ” intracranial pressure in these studies is adjusted downwards for children and infants according to the criteria we have used, the results would be very similar to the present study.

The large proportion of patients having raised intracranial pressure emphasises the need to measure intracranial pressure in every patient with cranio- synostosis. Even in patients with normal pressure, there may be a case for repeating the intracranial pressure measurements after a suitable interval. Firstly, when pressures are measured early in infancy. the growth spurt of the underlying brain may not have advanced sufficiently to raise the intracranial pressures. Secondly, we have noted the phenomenon of creeping synostosis, whereby there is progressive involvement of other sutures with a subsequent rise in intracranial pressure.

Contrary to the traditional concept of cranio- stenosis, our study demonstrates that only 9.7 % of our patients with craniosynostosis have diminished skull volumes i.e. less than 2 standard deviations below the mean for their age. The remaining patients had either normal or even enlarged skull volumes, most of the latter having only single suture synostosis. This result is consistent with that of a similar joint study between our centre and that at Hopital des Enfants Malades, Paris (Gault et al., 1992) and suggests that sufficient compensatory overgrowth occurs both from the un- involved sutures and presumably from the calvarial bone itself to accommodate the expanding brain.

On the basis of this study, we are unable to find a relationship between intracranial volumes and intra- cranial pressures. The measurement of intracranial volume cannot be used to predict the status of the intracranial pressure, nor to select out a group of patients with craniosynostosis for intracranial pressure monitoring. With a significant proportion of patients having raised intracranial pressures and yet having normal or enlarged skull volumes. the original concept of craniostenosis giving rise to raised intracranial pressures is not tenable. If this is so, the question that immediately poses itself is, “Why is the intracranial pressure raised in craniosynostosis?“. It is possible to define some of the abnormalities which could be responsible :

1. Presence of other factors such as hydrocephalus. None of the patients in our study had hydrocephalus.

2. Chronic airway obstruction occurs frequently in severely affected children. Our policy is now to combine intracranial pressure monitoring with res- piratory studies during sleep in order to define the relationship between the two factors. There is in- sufficient data at present to draw any conclusions.

3. The bony abnormality of the skull base may interfere with intracranial venous drainage and con- tribute to a rise in intracranial pressure.

4. The intracranial pressure measurements that are assumed to be normal may be incorrect. None of the normal results have been obtained from known normal children for obvious reasons but have been inferred

Intracranial Pressure and Volume in Craniosvnostosis 397

Table 2 Range of CSF pressures

Authors. year Series Pressure (mmHg)

Gerlach et al.. 1969 Newborns 0.7-I .o Welch, 1980 Infants 2.63.7 Levinson, 1928 Infants 1.45.1 Levinson, 1928 Children 2.9-5.9 Lups and Haan, 1954 Children 3.c7.5 Gerlach et al.. 1969 Children 2.9-7.3

from lumbar puncture studies or indirect methods of intracranial pressure studies such as aplanation trans- ducers. None of these methods present the accuracy that can be obtained from the overnight intracranial pressure study which is the hallmark of our method.

References

David, D. J., Poswillo, D. and Simpson, D. A. (1982). The cranio- s,vnostoses: causes, natural history and management. Berlin. Springer Verlag.

The Authors

Gaul& D., Brunelle, F., Renier, D. and Marchac, D. (1988). The calculation of intracranial volume using CT scans. Chikfs Nervous System, 4. 271.

H. Fok, FRCS(Ed), Senior Registrar in Plastic Surgery B. M. Jones, MS, FRCS, Consultant Plastic and Reconstructive

Surgeon

Gaul& D., Renier, D., Marchac, D., Ackland, F. M. and Jones, B. M. (1990). Intracranial volume in children with craniosynostoses. Journal of Craniofacial Surgery. 1, I.

Gault, D., Renier, D., Marchac, D. and Jones, B. M. (1992). Intracranial pressure and intracranial volume in children with craniosynostoses. Plastic and Reconstructive Surgery. In Press.

Gerlach, J., Jensen, H. P. and Koos, W. (1969). Paediatrische Neurochirurgie mit klinischer Diagnostik und Dlflerentialdiag- nostik in Paediatrie und Neurologie. Stuttgart. Georg Thieme Verlag, p. 139.

D. G. Gaul& FRCS, Consultant Plastic Surgeon U. Andar, MBBS, MS, Senior Registrar in Neurosurgery R. Hayward, FRCS, Consultant Neurosurgeon

Craniofacial Centre. Hospital for Sick Children. Great Ormond Street, London WCIN 3JH.

Requests for reprints to: Mr H. Fok, Department of Plastic Surgery, Singapore General Hospital. Outram Road, Singapore 0316.

Levinson, A. (1918). Cerebrospinal fluid in infants and in children. Paper received 30 December 1991, American Journal qf Diseases in Children. 36. 799. Accepted 20 January 1992. after revision.

Lichtenberg, R. (1960). Radiographie du crlne de 226 enfants normaux de la naissance ZYI 8 ans. Impressions digtiformes. capacitt ; angles et indices. Thesis. University of Paris.

Lups, S. and Haan, A. M. F. H. (1954). The cerebrospinal fluid. Amsterdam, Elsevier.

Minns, R. A. (1979). The monitoring of intracranial pressure in infants and children. Ph.D. Thesis. University of Edinburgh.

Minns, R. A. (1984). Intracranial pressure monitoring. Archives of Disease in Childhood. 59, 486.

Philip, A. G. S., Long, J. G. and Donn, S. M. (1981). Intracranial pressure. Sequential measurements in full-term and preterm infants. American Journal of Diseases in Children. 135. 521.

Renier, D., Sainte Rose, C. and Marchac, D. (1987). Intracranial pressure in craniostenoses. 302 recordings. In Marchac, D. (Ed). Craniofacial Surgery. Berlin, Springer Verlag, p. 110.

Tessier, P. (1971). Relationship of craniostenosis to craniofacial dystoses and to faciostenosis. Plastic and Reronstructive Surgery, 48. 3.

Welch, K. (1980). The intracranial pressure in infants. Journal of Neurosurgery, 52, 693.