Posted: 21 Jan 2011 06:06 PM PST

Definition

Arnold-Chiari Malformations are rare diseases that denote to a spectrum of congenital malformation of the brain. It is often called Chiari Malformation or tonsillar herniation. In this disorder, congenital hindbrain abnormalities are present that affects the structural relationships between the cerebellum, brainstem, the upper cervical cord and the bony cranial bone.

History

The disorder is named after Hans Chiari who was an Austrian pathologist. Hans Chiari classified the Chiari malformations through types I to III in 1891. However, Cleland was the first person to describe the first case of Chiari malformation in 1833. Julius Arnold, Chiari's colleague, made additional information and contributions to the definition of Chiari II malformation. Because of this, student of Dr. Arnold later named type II malformation as Ranold-Chiari malformation in his honor. The type IV malformation was later added by other investigators.

Classification

1. Chiari type I malformation – this is the most common form of Chiari malformation. Aside from being common it is the least severe type and is often diagnosed in adulthood. In this type, generally, the individual is asymptomatic during childhood.
Chiari type II malformation – this type is less common but is more severe than type I. it is usually accompanied by a lumbar myelomenigocele which results to either a partial or complete paralysis below the spinal defect. It becomes symptomatic in infancy or early childhood.
Chiari Type III malformation – this is an exceedingly rare and generally incompatible with life. It causes severe neurological defects and is associated with an occipital encephalocele.
Chiari Type IV malformation – like type III, this is a very rare disease and is generally incompatible with life. It is characterized by a lack of cerebellar development.

Causes

Chiari Type I Malformation

Genetic basis –There are suggestions that occurrence of type I malformation is linked to chromosomes 9 and 15.
Disorder of para-axial mesoderm – results in the formation of a small posterior fossa. In return, the cerebellum development within the small compartment would lead to overcrowding of the posterior fossa, herniation of the cerbellar tonsils and the impaction of the foramen magnum.

Chiari Type II Maformation

CSF loss theory – because chiari II malformation is associated with meylomeningocele, fluids escape through the open area in the myelomeningocele that leads to the inadequate stimulus for condensation at the base of the skull. Because of this, inadequate and disordered growth of the fossa occurs that results to the upward herniation of the vermis and the downward herniation of the brainstem.

image from choa.org

Allen’s Test

Posted: 21 Jan 2011 05:45 PM PST

Definition

Allen's test is a used to test the blood supply in the hand that is performed before radial arterial blood sampling or cannulation. It is also called as Allen test.

Etymology

The test is named after Edgar Van Nuys Allen.

Purpose

This test is used to determine adequate collateral blood flow in the hand by evaluating the patency of the radial and ulnar arteries.
Allen test determines whether ulnar circulation is inadequate or nonexistence. Once inadequate or nonexistence of ulnar circulation is noted the radial artery supplying arterial blood to that hand should not be punctured once.

Procedure

Check the patient's identity and explain the procedure to ensure that correct client undergoes the procedure and promote coordination of the client.
Instruct the patient clench his/her fist, or if the patient is unable, you may close the hand tightly. This removes as much blood from the hand as possible.
Using your fingers, apply direct pressure to the radial and ulnar arteries to obstruct the arterial blood flow to the hand.
Have the client open his or her hand while applying occlusive pressure to both arteries. Fingers and hand should be pale and blanched which indicates lack of arterial blood flow.
Release the pressure over the ulnar artery. Observe color of fingers, thumbs and the hand. Fingers and hand should flush within 15 seconds.
Repeat procedures 2 to 5 with the other hand.

Interpretation of Results

Flushing is a positive Allen's test. If test is negative, no flushing, radial artery should be avoided. If collateral circulation to hand is present through ulnar arteries, the hand and the fingers should flush. Ulnar artery can supply blood flow if radial artery is damaged or becomes occluded during the procedure. When the hands and fingers flush within 15 seconds, a positive Allen's test, arterial puncture can be done.

image from texasheart.org

The Fetal Skull

Posted: 21 Jan 2011 05:16 PM PST

Importance of the fetal skull

Largest part of the fetal body.
Most frequent [resenting part of the fetus.
Least compressible of all fetal parts.

Anatomy of the Fetal Skull

Cranial Bones

The fetal skull is made up of six cranial bones which are the following:

Sphenoid
Ethmoid
Temporal
Frontal
Occipital
Parietal

The frontal, occipital and the parietal cranial bones could either be fetal presenting part if the presentation is vertex.

Membrane Spaces

During birth, bones move and overlap with each other to allow the fetal head to fit through the birth canal which is a process termed as molding. Molding is made possible because of the presence of the suture lines. Without these structures a fetus' head cannot pass through the birth canal. There are different types of sutures:

Sagittal suture line – joins the two parietal bones.
Coronal suture line – joins the frontal and the parietal bones.
Lambdoid suture line – joins the occiput and the parietal bones.

Fontanelles

Definition: Fontanelle is a membrane-covered space at the junction of a main suture line.

Types of Fontanelles:

Anterior fontanelle – diamond-shaped fontanelle. This fontanelle closes at about 12-18 months and is larger than the other.
Posterior fontanelle – triangular-shaped fontanelle. This fontanelle closes between 2-3 months of age and is smaller.

Measurements of the fetal skull

Transverse diameters of the fetal skull

Biparietal – 9.25 cm
Bitemporal – 8 cm
Bimastoid – 7 cm

Anteroposterior (AP) diameter

Suboocipitobregmatic – 9.5 cm (the narrowest AP diameter). This measurement is taken from below the occiput to the anterior fontanelle.
Occipitofrontal – 12 cm (from the occiput to the mid-frontal bone)
Occipitomental – 13.5 cm (the widest AP diameter). This measurement is taken from the occiput to the chin.

Which one of these diameters is presented at the birth canal depends on the degree of flexion, which is known as the ATTITUDE, the fetal head assumes prior to delivery.

image from dvpeds.com