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Positioning and Holding

For the infant, positioning and proper holding is important for a successful spinal tap. The holder secures the knees to the chest by wrapping her arm around the knees and placing the baby in the flexed position. The head should be flexed forward but pay special attention to the airway and do not occlude the airway by over flexion of the neck. Make sure the back is parallel with the edge of the mat and the hips and shoulders remain perpendicular to the mat. For an older child who is too young to cooperate and too big to hold adequately, sedation will be necessary; otherwise there will be too much movement and the spinal tap is unlikely to be successful.

Local Anesthesia

The patient will be more comfortable and resist less if the skin is anesthetized first with an eutectic mixture of local anesthetics such as EMLA cream (lidocaine 2.5% and prilocaine 2.5%). Apply the cream in a glob over the site for the spinal tap. Apply the occlusive plastic dressing over the glob of cream and smooth down the edges. Ideally allow the cream 60 minutes to work but if the tap must be done on a more immediate basis then 20-30 minutes will provide some anesthesia. After the waiting time, wipe off the cream and proceed with the prepping and draping for the spinal tap.

Sedation

If the patient is too young to cooperate and cannot be held still for the spinal tap then sedation is necessary. Choral hydrate is commonly used for nonpainful procedures but it is inadequate for an LP. Conscious sedation using one of the short acting benzodiazepines such as midazolam or other agents such as propofol is commonly used. Use of conscious sedation should be in a setting where there is the ability to respond to airway management needs and vital signs and oxygenation can be adequately monitored.

Depth of Needle

The depth that the spinal needle needs to be inserted to reach the subarachnoid space is proportional to body size. In an infant less then 3 months of age the depth of the needle is usually about 1-1.5 cm. For older children there are multiple formulas that have been published to estimate the depth of the needle. One that is easy to use is: Estimated depth of needle in cm = 1.3 + 0.07(weight in kg). Another that yields values very close to this formula is: Estimated depth of needle in cm = 0.77 + 2.56(m²). Both of these formulas give only an approximate estimate of the depth of the needle but they have a value when the operator is wondering if he has gone too deep or not deep enough with the needle. For most children under 12 years of age, a 1 ½ inch 22 gauge spinal needle is adequate. For children 12 years or older, a 2 ½ to 3 ½ inch 22 gauge spinal needle is used depending on the size of the child and the estimated depth of the needle required for that size of individual. It is better to have a needle that is longer than necessary than to have a needle that is not long enough.

CSF Lab Values

There are some differences in the CSF normal lab values in the infant compared to the older child and adult. The following differences and observations are important to be aware of:
1. The opening pressure for an infant is usually 6-11 cm of water. For the older child and adult the upper limit of normal is 18-20 cm of water.
2. Newborn CSF glucose values are similar to adult values.
3. CSF protein in the newborn is usually about 90 mg/dl with the normal range up to 150 mg/dl. By 3 months of age the protein level is within the 20-40 mg/dl range.
4. Red Blood Cell Count- typically the red blood cell count is similar to adults with 0 RBCs found. On the other hand there are reports of RBC counts of up to 100/mm³ or higher in normal newborns. These counts may reflect mild asymptomatic subarachnoid bleeding during the birth process.
5. White Blood Cell Count- The WBC count in the newborn is usually 9 cells/mm³ or less but the range can be up to 32 cells/mm³ and still be normal. By the time the infant is one month old the WBC count is 5 cells or less.