Normal physiology

Normal intracranial pressure (ICP) is 6–18 cmH₂O. It is the product of the intracranial contents mainly blood, brain and cerebrospinal fluid (CSF) and the resistance of the cranial vault. Normal ICP has a diurnal cycle that is higher in the early hours of the morning when one is normally supine during sleep. Therefore the symptoms of raised ICP, such as headache and vomiting, are usually worse in the morning. Intracranial pressure may be raised by anything that can cause an increase in the volume of its contents or a decrease in the size of the cranial cavity (Table 8.2.1). This section will not cover the diagnosis and management of raised intracranial pressure associated with trauma. Those issues are covered in Chapter 3.2.

Most CSF is made by the choroid plexus in the lateral third and fourth ventricles. A normal child makes approximately 20 mL hr^–1 and the total volume is 50 mL in an infant, rising to 150 mL in an adult. It flows through the foramen of Monro into the third ventricle, then down the aqueduct of Sylvius, which is usually only 2 mm wide, and 3 mm long in a child. This leads into the fourth ventricle and from there via the foramina of Luschka and Magendie to the basal cisterns. From there the CSF flows over the surface of the cerebellar and cerebral hemispheres to be reabsorbed through the arachnoid villi on the superior sagittal sinus. CSF can also be reabsorbed through several other channels, including a small amount through the choroid plexus and via lymphatics. Intracranial pressure will be raised whenever there is obstruction to the flow or reabsorption of CSF or in the rare circumstance where CSF production is increased.

Measurement of ICP

Intracranial pressure is usually estimated from lumbar puncture (LP) manometry, when performed on a child lying in the lateral decubitus position in a relaxed posture. Measuring CSF pressure in a screaming child is likely to be inaccurate because of the temporary rise caused by high venous pressure. Similarly, the person holding the child in the fetal position for the LP should relax their grasp and allow the child to stretch out temporarily while the pressure is being measured. If the LP is being done under a general anaesthetic, attempts should be made to normalise the pCO₂, so that the pressure reflects awake ICP. If a lumbar puncture is performed on a child in a sitting posture measurement of hydrostatic pressure is likely to be elevated compared to ICP. Adjustment of the sitting posture LP pressure to reflect intracranial pressure is difficult because multiple factors affect the relationship, including; the height of the child, their posture and their emotional state. Nevertheless, if the child is very co-operative, pressure measurement can still be attempted by carefully laying the child on their side straight after the needle has entered the subarachnoid space.

Infants

An infant’s inability to communicate and smaller repertoire of behaviours makes raised intracranial pressure more difficult to diagnose and one needs to have a high index of suspicion as the presentation may be subtle.

The infant does not have a rigid cranial vault until fusion of the cranial sutures. This is a gradual process occurring throughout childhood; therefore ICP can cause diastasis of the cranial sutures in children, however, this is rare in children over the age of 7 years and even rarer now that computerised tomography (CT) scans and magnetic resonance imaging (MRI) allow for earlier diagnosis. Because of this flexibility, increasing intracranial contents in the infant will cause a lesser increase in the ICP and a greater increase in the head circumference than it would in older children or adults. This is one of the reasons why acute increases in ICP in infants are often not easily detected by the infant’s subtle change in behaviour patterns.

The measurement of head circumference (occipitofrontal) is a useful means of detecting intracranial pathology. The head circumference should be plotted on centile charts, using prior measurements, if available, to determine if there has been a trend to cross percentiles. The child’s length and weight should be plotted concurrently, to evaluate if the head is disproportionately large or small. When the cranial sutures separate due to expansion, percussion of the skull makes a sound similar to that of a ‘cracked pot’. This is known as Macewen’s sign.

Conversely, the open anterior fontanelle allows direct palpation of intracranial pressure up to the age of 9 to 18 months. It is highly recommended that one closely observes the fontanelles of normal infants, to help one identify abnormalities in clinical practice. The normal fontanelle will bulge slightly when the infant is lying down. It will become depressed when the child is sat up. It will bulge more prominently when the infant is crying or straining. The normal fontanelle will have an arterial pulsation more apparent when the infant is upright.

The fontanelle also provides access for emergency procedures such as the draining of a traumatic subdural or a ventricular tap. Fortunately the need for these procedures in the emergency department (ED) rarely arises. When it does, however, it should be done by a neurosurgical specialist (or trainee if sufficiently experienced). In centres where such help is not rapidly available, over-the-phone advice from a neurosurgeon may help with both the decision to do the procedure and the technique.