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Ovid: Clinical Neuroanatomy

Authors: Snell, Richard S. Title: Clinical Neuroanatomy, 7th Edition Copyright ©2010 Lippincott Williams & Wilkins > Back of Book > Appendix Appendix Important Neuroanatomical Data of Clinical Significance Baseline of the Skull The baseline of the skull extends from the lower margin of the orbit backward through the upper margin of the external auditory meatus. The cerebrum lies entirely above the line, and the cerebellum lies in the posterior cranial fossa below the posterior third of the line (Fig. A-1). Falx Cerebri, Superior Sagittal Sinus, and the Longitudinal Cerebral Fissure Between the Cerebral Hemispheres The position of the falx cerebri, superior sagittal sinus, and the longitudinal cerebral fissure between the cerebral hemispheres can be indicated by passing a line over the vertex of the skull in the sagittal plane that joins the root of the nose to the external occipital protuberance. Parietal Eminence The parietal eminence is a raised area on the lateral surface of the parietal bone that can be felt about 2 inches (5 cm) above the auricle. It lies close to the lower end of the central cerebral sulcus of the brain (Fig. A-1). Pterion The pterion is the point where the greater wing of the sphenoid bone meets the anteroinferior angle of the parietal bone. Lying 1-1/2 inches (4 cm) above the midpoint of the zygomatic arch (Fig. A-1), it is not marked by an eminence or a depression, but it is important since the anterior branches of the middle meningeal artery and vein lie beneath it. Clinical Neuroanatomy of Techniques for Treating Intracranial Hematomas Burr Holes Indications for Burr Holes Cranial decompression is performed in a patient with a history of progressive neurologic deterioration and signs of brain herniation, despite adequate medical treatment. The presence of a hematoma should be confirmed by a computed tomography scan, if possible. Anatomy of the Technique for a Temporal Burr Hole

  • The patient is placed in a supine position with the head rotated so that the side for the burr hole is uppermost. For example, in a patient with a right-sided fixed and dilated pupil, indicating herniation of the right uncus with pressure on the right oculomotor nerve, a hematoma on the right side must be presumed, and a burr hole is placed on the right side.
  • The temporal skin is shaved and prepared for surgery in the usual way.
  • A 3-cm vertical skin incision is made two fingerbreadths anterior to the tragus of the ear and three fingerbreadths above this level (Fig. A-2).
  • The following structures are then incised:
    • Skin.
    • Superficial fascia containing small branches of the superficial temporal artery.
    • Deep fascia covering the outer surface of the temporalis muscle.
    • The temporalis muscle is then incised vertically down to the periosteum of the squamous part of the temporal bone (Fig. A-2).
    • The temporalis muscle is elevated from its attachment to the skull, and a retractor is positioned (some muscular bleeding will be encountered).
    • A small hole is then drilled through the outer and inner tables of the skull at right angles to the skull P.522 surface, and the hole is enlarged with a burr (unless a blood clot is present between the inner table and the endosteal layer of dura).
      Figure A-1 Surface landmarks on the right side of the head. The relation of the middle meningeal artery and the brain to the surface of the skull is shown.
    • The white meningeal layer of dura is flexible and gives slightly on gentle pressure.
    • The hole may be enlarged with a curette, and bleeding from the diploe may be controlled with bone wax.

The surgical wound is closed in layers with interrupted sutures placed in the temporalis muscle, the deep fascia covering the temporalis muscle, and the scalp. Burr Hole for Epidural Hematoma Once the inner table of the squamous part of the temporal bone (or the anterior inferior angle of the parietal bone) is pierced with a small bit and enlarged with a burr, the dark red clotted blood beneath the endosteal layer of dura is usually easily recognized. However, bright red liquid blood means that the middle meningeal artery or one of its branches is bleeding. The meningeal artery is located deep to the clot and between the endosteal layer of dura and the meningeal layer of dura or in the substance of the endosteal layer of dura; or it may lie in a tunnel of bone. Burr Hole for Subdural Hematoma When the squamous part of the temporal bone is penetrated, as described earlier, the endosteal layer of dura will be exposed. In this case, there is no blood clot between the endosteal layer of dura and the meningeal layer of dura, but both fused layers of dura will be dark bluish. The dura (endosteal and meningeal layers) is gently incised to enter the space between the meningeal layer of dura and the arachnoid mater. The subdural blood usually gushes out, leaving the unprotected brain covered only by arachnoid and pia mater in the depths of the hole. Clinical Neuroanatomy of the Technique of Ventriculostomy Indications for Ventriculostomy Ventriculostomy is indicated in acute hydrocephalus, in which there is a sudden obstruction to the flow of cerebrospinal fluid. Anatomy of the Technique of Ventriculostomy To perform a ventriculostomy, the needle is inserted into the lateral ventricle through either a frontal or parietal burr hole. The anatomy of these burr holes has been described previously. The needle is inserted through the burr hole using the following anatomical landmarks.

  • Frontal Approach. The needle is inserted through the frontal burr hole and is directed downward and forward in the direction of the inner canthus of the ipsilateral eye (Fig. A-3). P.523
    Figure A-2 A: Surface landmarks for a temporal burr hole. B: The vertical incision passes through the temporalis muscle down to bone. The middle meningeal artery lies between the endosteal and meningeal layers of dura and is embedded in the endosteal layer of dura or lies in a bony tunnel.


    Figure A-3 Ventriculostomy. Needles passing through frontal or parietal burr holes to enter the lateral ventricle area are shown. The needle is inserted to a depth of about 2 inches (5.5 cm) from the skull opening in order to enter the lateral ventricle.
  • Parietal Approach. The needle is inserted through the parietal burr hole and is directed downward and forward in the direction of the pupil of the ipsilateral eye (Fig. A-3).

The needle is inserted to a depth of about 2 inches (5.5 cm) from the skull opening; in cases of chronic hydrocephalus with gross dilatation of the ventricles, the depth of penetration to the ventricular cavity may be much less. Vertebral Numbers and Spinal Cord Segments Table A-1 relates which vertebral body is related to a particular spinal cord segment.

Table A-1
Vertebrae Spinal Segment
Cervical vertebrae Add 1
Upper thoracic vertebrae Add 2
Lower thoracic vertebrae (7–9) Add 3
Tenth thoracic vertebra L1-2 cord segments
Eleventh thoracic vertebra L3-4 cord segments
Twelfth thoracic vertebra L5 cord segment
First lumbar vertebra Sacral and coccygeal cord segments

Segmental Innervation of Muscles It is possible to test for the integrity of the segmental innervation of muscles by performing the following simple muscle reflexes on the patient.

  • Biceps brachii tendon reflex C5-6 (flexion of the elbow joint by tapping the biceps tendon).
  • Triceps tendon reflex C6-7 and 8 (extension of the elbow joint by tapping the triceps tendon).
  • Brachioradialis tendon reflex C5-6 and 7 (supination of the radioulnar joints by tapping the insertion of the brachioradialis tendon).
  • Abdominal superficial reflexes (contraction of underlying abdominal muscles by stroking the skin). Upper abdominal skin T6-7; middle abdominal skin T8-9; lower abdominal skin T10–12.
  • Patellar tendon reflex (knee jerk) L2, 3, and 4 (extension of knee joint on tapping the patellar tendon).
  • Achilles tendon reflex (ankle jerk) S1 and 2 (plantar flexion of ankle joint on tapping the Achilles tendon–tendo calcaneus).

Relationship Between Possible Intervertebral Disc Herniations and Spinal Nerve Roots It is useful to be able to relate possible nucleus pulposus herniations with spinal nerve roots. These are shown for the cervical and lumbar regions in Figure A-4. A correlation between the nerve roots involved, the pain dermatome, the muscle weakness, and the missing or diminished reflex is shown in Table A-2. P.525

Table A-2 Correlation Between Nerve Roots Involved, the Pain Dermatome, the Muscle Weakness, and the Missing or Diminished Reflex
Root Injury Dermatome Pain Muscles Supplied Movement Weakness Reflex Involved
C5 Lower lateral aspect of upper arm Deltoid and biceps Shoulder abduction, elbow flexion Biceps
C6 Lateral aspect of forearm Extensor carpi radialis longus and brevis Wrist extensors Brachioradialis
C7 Middle finger Triceps and flexor carpi radialis Extension of elbow and flexion of wrist Triceps
C8 Medial aspect of forearm Flexor digitorum superficialis and profundus Finger flexion None
L1 Groin Iliopsoas Hip flexion Cremaster
L2 Anterior aspect of thigh Iliopsoas, sartorius, hip adductors Hip flexion, hip adduction Cremaster
L3 Medial aspect of knee Iliopsoas, sartorius, quadriceps, hip adductors Hip flexion, knee extension, hip adduction Patellar
L4 Medial aspect of calf Tibialis anterior, quadriceps Foot inversion, knee extension Patellar
L5 Lateral part of lower leg and dorsum of foot Extensor hallucis longus, extensor digitorum longus Toe extension, ankle dorsiflexion None
S1 Lateral edge of foot Gastrocnemius, soleus Ankle plantar flexion Ankle jerk
S2 Posterior part of thigh Flexor digitorum longus, flexor hallucis longus Ankle plantar flexion, toe flexion None
Figure A-4 A, B: Posterior views of vertebral bodies in the cervical and lumbar regions showing the relationship that might exist between herniated nucleus pulposus (pink) and spinal nerve roots. Note that there are eight cervical nerves and only seven cervical vertebrae. In the lumbar region, for example, the emerging L4 nerve roots pass out laterally close to the pedicles of the fourth lumbar vertebra and are not related to the intervertebral disc between the fourth and the fifth lumbar vertebrae. Pressure on the L5 motor nerve root produces weakness of plantar flexion of the ankle joint.

P.526 Surface Landmarks for Performing a Spinal Tap To perform a spinal tap, the patient is placed in the lateral prone position or in the upright sitting position. The trunk is then bent well forward to open up to the maximum the space between adjoining laminae in the lumbar region. A groove runs down the middle of the back over the tips of the spines of the thoracic and the upper four lumbar vertebrae. The spines are made more prominent when the vertebral column is flexed. An imaginary line joining the highest points on the iliac crests passes over the fourth lumbar spine. With a careful aseptic technique and under local anesthesia, the spinal tap needle, fitted with a stylet, is passed into the vertebral canal above or below the fourth lumbar spine.

Figure A-5 A: Structures penetrated by the spinal tap needle before it reaches the dura mater. B: Important anatomic landmarks when performing a spinal tap. Although this is usually performed with the patient in a lateral recumbent position with the vertebral column well flexed, the patient may be placed in the sitting position and bent well forward.

Structures Pierced by the Spinal Tap Needle The following structures are pierced by the needle before it enters the subarachnoid space (Fig. A-5):

  • Skin
  • Superficial fascia
  • Supraspinous ligament
  • P.527

  • Interspinous ligament
  • Ligamentum flavum
  • Areolar tissue containing the internal vertebral venous plexus in the epidural space
  • Dura mater
  • Arachnoid mater
Table A-3 The Physical Characteristics and Composition of the Cerebrospinal Fluid
Appearance Clear and colorless
Volume c. 150 mL
Rate of production 0.5 mL/minute
Pressure (lumbar puncture) 60–150 mm of water (with patient in lateral recumbent position)
   Protein 15–45 mg/100 mL
   Glucose 50–85 mg/100 mL
   Chloride 720–750 mg/100 mL
Number of cells 0–3 lymphocytes/cumm

The depth to which the needle will have to pass will vary from an inch or less in children to as much as 4 inches (10 cm) in obese adults. The pressure of the cerebrospinal fluid in the lateral recumbent position is normally about 60 to 150 mm of water. See Table A-3 for physical characteristics and composition of the cerebrospinal fluid.

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