Neuron, Development of Brain

A single neuron is the basic functional unit of the nervous system. It is like a brick in a house. As house is made up of many bricks similarly brain is made of a billion neurons. As cement is needed to provide strength , similarly the neurons are strengthened by neuroglia cells.

A single neuron has has s body (which is also known as soma) with nucleus. Inside  the body there are Nissl granules. From the body arise many projections which carry impulses to the body of the neuron, these tiny projections are called Dendrites ( look like branches of a tree). From the3 body or soma of the neuron a long Axon emerges. The junction of the body and axon is called Axon hillock. Signal and material from the body travel through the axon. Most of the axons are myelinated , i.e., axons are usually covered by a sheath of myelin. In the peripheral nervous sytem myelin is formed by Schwann cells and in the brain it is formed by Oligodendrocytes ( glia cells). This myelin provides white colour and the area of the brain where large number of myelinated fibres are present is called white matter. There are gaps in the myelin sheath along the entire length of axon, these gaps are known as Nodes of Renveer, which allows fast movement of electrical impulses ( action potentials) along the axon. At distal end axon synapses with other neuron or muscle fibre.

This image is a simple depiction to show the development of the brain. 

The brain arises from a tube which is known as neural tube in an embryo. 

In the early embryonic stage of development there are three layers , endoderm, mesoderm and ectoderm. 

Ectoderm is the outer layer of the embryo. Some of the ectoderm cells which are present in the center become destined to form  neural ectoderm. These cells invaginate and convert into a tube which is now, known as Neural tube which is open at both the ends, also known as anterior and posterioral neuropore.

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After 3rd week of embryo development cranial opening of the neural tube which is also known as anterior neuropore , closes. Any abnormality of the closure of the this end may lead to congenital birth anomaly of the brain like anencephaly.

The posterior or caudal neuropore closes at the end of 4th week of embryonic development. Abnormality of the closure of posterior neuropore may lead to spina bifida, split cord malformation. This fact is very significant because when a female becomes aware that she is pregnant most of the neural tube is already developed . A flic acid or folate deficiency has been known to cause neural tube defects, that is why, folate supplementation is advocated in females who plan a pregnancy.

To repeat it again,  brain arises from a tube which is known as neural tube in an embryo. A tube is shown in the diagram mentioned above.

Then the anterior or cranial end of this tube is expanded into three parts which form the brain and caudal or elongated part forms the spinal cord. The three segments of the cranial part of the neural tube are destined to form three major part of the brain. Forebrain, midbrain and hindbrain.

Three cranial expansions are : Prosencephalon ( Forebrain), Mesencehalon (Midbrain) and Rhombencephalon ( Hindbrain). Remember nemonic for 3 sementation stage: PMR. Cavities inside these structures form the ventricles.

Later there are further subdivisions and it becomes of five segments.

Prosencephalon divides into two ;  Telencephalon and Diencephalon. Telencephalon later transforms into two cerebral hemispheres and its cavities become lateral ventricles. Diencephalon may be simply imagined to be like a box , the cavity iside becomes Third ventricle. he lateral walls of this box become 2 thalami, floor becomes hypothalamus, roof becomes epithalamus and part becomes subthalamus.

Mesencephalon changes into midbrain.

Rhombenecphalon subdivides into two subdivision; Metencephalon and Myelencephalon. From Metencephalon Pons and Cerebellum develop. Myelencephalon transforms into medulla oblongata. The distal part of the neural tube gives rise to spinal cord.

HEADACHE

Pain in head is called headache. Almost everybody experiences headache. One episode of mild headache may be due to exertion, stress or some causes which may not be alarming and you can afford to neglect.
Headache is a very common symptom. Almost everyone experiences headache at some stage of life. Despite of being so common it becomes sometimes it becomes a matter of concern.
So, one should not panic and should analyze the severity of the problem and proceed further for seeking the medical attention & investigations.
Lot of literature is available on the causes of headache but I would like to overly simplify this topic so that one can have an overview.


If headache is associated with exertion at the end of the working day and is over the vertex, frontal or occipital region of the skull and is relieved on taking rest or head massage, one should not worry.
if someone is very stressed and there is obvious stress then there can be psychogenic headache. That person should adopt the less stressful lifestyle and practice relaxation exercise , yoga  , and very rarely psychotherapy or some medicines may be required.


Maxillary or frontal sinusitis also causes headache.
Redness of eyes, frequent sneezing, cold , allergy may also cause headache.Cluster headache occurs in clusters.


In older persons headache may occur due to hypertension and temporal arteritis.



Refractive errors like myopia and hypermetropia may also cause periorbital pain and headache especially in young children going to school. So vision examination should also be done in patients complaining of headache.
If a child complains of headache it may due to refraction error in vision. Common cause is myopia when someone complains of difficulty in seeing distant things. child may not read the letters written on blackboard in a class and there may be decrease in scholastic performance in school. Another refractive error is hypermetropia which is difficulty in reading the small letters. Although, this is a common problem in people over 40 years of age, it is becoming more common in childrenm because of too much indulgence in mobile games, computer games and less outdoor play activities.
So common cause of Persistent headache is refractive error of vision.

Other common cause is sinusitis. Such patients will have frequent history of nasal infections, pain over the bony air sinuses in skull. Maxillary sinusitis, frontal air sinusitis may be diagnosed by tenderness and X-Ray Skull. Sometimes CT scan may be needed. Most of the patients are treated by antibiotics.

Common cause of headache is tension headache. In this type is headache pain is over the vertex, i.e, top of the head. Since the muscles are under continuous tension , such headache will diminish if head massage is given.

Hypertension, anemia may also cause headache.

Migraine is very common and typically paroxysm of throbbing type of headache on irregular intervals, unilateral, associated with vomiting. Migraine is more common in females. There are many variants of migraine.Migraine is the diagnosis of exclusion. Before labelling a ptient with this diagnosis all other causes must be ruled out, like anemia, brain tumor, hemorrhage, infection. CT scan or MRI of the brain rules out any intracranial mass occupying lesion.  It is normally hemicranial,i.e., involes one half of the head. It is usually throbbing headache and associated with nauses and vomiting. Migraine is recurrent and gradually the duration between episodes become less.


What should not be missed?
Any physician or person should not ignore headache due to brain tumor and subarachnoid hemorrhage due intracranial aneurysm rupture.
How to recognize headache due to brain tumor? Usually progressive, associated with vomiting, temporarily relieved after vomiting, may be associated with blurring of vision ( due to papilloedema), or other neurological deficit.
How to recognize headache due to subarachnoid hemorrhage caused by rupture of intracranial aneurysm? It is sudden onset severe headache ( bolt from blue, thunderclap headache which a patient in the age group of 4th - 5th decade, says that he or she had never experienced such type of headache in life time. Sometimes headache may not be so severe and it is called warning leak. There may be associated neck rigidity.
What is the most valuable investigation in the management of headache??
If  Visual examination of the patient is performed  and  Non contrast CT scan of the head is advised one will never be guilty of missing a life threatening brain condition like SAH ( subarachnoid hemorrhage and brain tumor) and it will guide the further course of treatment.

Sudden onset severe headache in a person of about 40 to 50 years of age which is so intense as patient describes that he or she had never experienced such headache in his or her life time,"Bolt from Blue," is typical of spontaneous subarachnoid hemorrhage ( SAH) due to rupture of intracranial aneurysm.

Head trauma, cervical spondylosis may also cause headache.

Headache associated with seizures is alarming. CT scan or MRI of the brain must be done.

Cranio-Vertebral Junction ( CV Junction)

Cranio-Vertebral Junction is the bony junction of the skull and vertebral column. It is formed by Clivus, Occipital bone and C1 and C2 vertebra.

This area is important as any abnormality in this area may lead to compression over the cervico- medullary junction, which is the junction of the cervical region of the spinal cord qnd medulla oblongata.

The abnormality at this region could be congeital anomaly or acquired.

This is simple diagram of the sagittal view shows location of CV Junction. TOO is abbreviation for transoral odontoidectomy . This surgery is done through oral cavity . Anterior arch of Atlas vertebra ( first cervical vertebra)  lies just anterior to the odontoid process of Axis vertebra ( 2nd cervical vertebra). Any dislocation of Atlanto-Axial Joint ( Atlanto axial dislocation -AAD) may compress over the medulla Oblongata, which lies just posterior to the Dens of the C2 vertebra.

Stability of the CV junction is mainly due to ligaments. 

The anterior atlanto-occipital membrane is the cranial extension of the anterior longitudinal ligament. It extends from anterior margin of foramen magnum to anterior arch of C1.

Posterior Atlanto-occipital membrane connects posterior margin of the foramen magnum to the posterior arch of Atlas.

Tectorial Membrane is the upward continuation of posterior longitudinal ligament. It connects the dorsal surface of the Dens of the Odontoid process of C2 vertebra anterior lip of the Foramen magnum.

Apical odontoid Ligament connects the tip of Dens to the anterior lip of Foramen Magnum.

Alar Ligament  connects side of the Dens to occipital condyles.

Transverse Ligament is the horizontal component of the Cruciate ligament which traps or straps or binds the Dens anteriorly against the C1. It is the strongest ligament.

Cerebral Angiography, Digital Substraction Angiography ( DSA)

Invention of Cerebral Angiography is an important historical landmark in evolution of Neuroradiology and Neurosurgery. 

In 1927, Dr Egas Moniz developed a technique of injecting a contrast material into the arteries . The dye or contrast used was Thorotrast. 

When a contrast or dye which is soluble in blood is injected into the common carotid artery the blood vessels become visible and look opaque on background of X-ray image of skull.  This technique of imaging the blood vessels of the brain is known as cerebral angiography. Rapid sequences of the X-ray images are obtained after injecting contrast into common carotid artery. Similar images could also be obtained when contrast is injected into the femoral artery.  Cerebral angiography allows visualization of blood vessels in and around the brain.

Before 1927, may be called  X-ray era, i.e., in the begining of 20th century, neurosurgery was possible by localization of the lesion by neurological examination and using the X-ray techniques. Application of X-rays in investigations like ventriculogram, encephalography, myelogram were used to localize lesions. 
Roentgen invented application of x-rays imaging in 1896.
Walter Dandy described pneumoencephalography and pneumoventriculography in 1918.
In 1921, myelography was introduced when jean Sicard a French clinician and his pupil Jacques Forestier injected analgesics into the spine of a patient suffering from low back pain and subsequently found that the oil they used as a carrier for the analgesic, lipiodol, was radio-opaque.
The invention of myelography encouraged the Portuguese neurologist Egas Moniz to develop ' Arterial Encephalography'.

After 1927, the dignostic ability of the physician changed dramatically. Using the technique of cerebral angiography, Norman Dott, in 1931,  demostrated cerebral aneurysm on cerebral angiography and later, Walter Dandy was able to operate a case of intracranial aneurysm, in 1933.

Cerebral angiography depicts the vessels along with X-ray of the skull. 

Later, with the use of computer, the background of skull image could be substracted. So, the artery or vein is visualized better. So, the technique of substracting the bony details  is called Digital Substraction  Angiography  ( DSA ). But, if somebody wants to see relation of the aneurysm against the bony landmark of the skull, it is still possible in DSA because the bone image was substracted digitally and computer again can depict the same.  Thus, DSA is gold standard investigation for detection and surgical planning for intracranial aneurysms.

                                                     Image of DSA , here the internal carotid artery is seen , the image is lateral view of the skull but the bony details have been digitally substracted so that one can focus on the ICA and its branches. ICA is seen as in neck, carotid siphon as a loop and intracranial branches. The first intracranial branch of ICA   which runs anteriorly is ophthalmic artery. Second intracranial branch which runs poseriorly is Posterior communicating artery. Anterior Choroidal artery is another intrcranial branch of ICA which is seen to traverse posteriorly on lateral view of DSA.

First investigation for investigating a suspected case of spontaneous subarachnoid hemorrhage is CT scan of the brain. Non-contrast CT scan reveals the site of the bleed, like blood in the anterior inter-hemispheiric space is suggestive of SAH due to rupture of anterior communicating artery. Then, the second investiation is to know the detail of the aneurysm, which is done by DSA.

Since, DSA is an invasive procedure, so the another option is to go for CT angioprphy. MR angiography ( MRA) is a noninvasive procedure and it does not require any injection. But MRA is less sensitive than CT angiography and DSA.

Digital Substraction Angiography ( DSA) involves passing a catheter in the femoral artery over the anterior aspect of the thigh in the femoral triangle and then advancing the catheter upward. Then, injection of a contrast material ( Dye) through a catheter. Once catheter is at the arch of aorta it needs to be negotiated to the carotid artery and vertebral artery. So, anatomy of arch of aorta is important.

                                         The contrast is injected into the Femoral artery on right side and then catheter is advanced upward into the arota under the fluroscopic guidance. At the arch of aorta the catheter is negotiated into the artery depending upon the need of DSA. On the right side there is first branch of arch of aorta, brachiocephalic trunk. Right common carotid artery is a branch of brachiocephalic trunk.
Left common carotid artery is a direct branch of arch of aorta. Left subclavian artery is also a direct branch of arch of aorta and vertebral artery is a second branh of subclavian artery.

Spontaneous subarachnoid hemorrhage due to ruture of intracranial aneurysm commonly occurs in the age group of 40-60 years with a peak incidence in the fifties.
Aneurysm bleed is the commonest cause of spontaneous SAH ( about 85%). 
Other common causes of spontaneous SAH are:  artriovenous malformations ( AVM), hemorrhage from tumor, pituitary apoplexy, vasculopathy ( like collagen vascular disease, amyloid angiopathy , arterial dissection) , haematological ( anticoagulant therapy, leukaemia, hepatic or renal disease induced coagulopathy ) and drugs like cocaine, amphetamine and ephedrine.

Subarachnoid hemorrhage (SAH) is a neurological emergency  characterized by hemorrhage into the subarachnoid space, and may present as sudden, severe headache ( as bolt from blue ) which patient may state that he or she may have never experienced before. Sentinel hemorrhage occurs in about 40% of patients with SAH. This is also known as " warning leak". Nuchal rigidity or meningismus is noted in 50% of patients due to meningeal irritation following SAH. Hemiparesis, focal neurological deficits including cranial nerve deficits are other common features. Fundus examination may reveal papilloedema and subhyaloid hemorrhage.

On the basis of GCS and Focal deficit, the severity of the clinical presentation of the patients may be graded into 5 grades , according to the World Federation of Neurological Surgeons  (WFNS).
In WFNS Grade 1 the patients are of GCS 15/15 and have no focal deficit. And, if patient's GCS is 13 or 14 then it is Grade 2. And patient has a focal deficit with a GCS of 13 or 14, his grade becomes Grade 3.   poor GCS of 7-12 makes a patient of grade 4 and if patients GCS is 6 or less then his garde becomes 5, irrespective of the presence or absence of focal deficit.

NCCT ( NECT) , i.e., Noncontrast or nonenhanced CT scan of the barin is the first investigation of choice. It shows hyperdensity in the subarachnoid space and may indicate the site of bleed. or example, anterior communicationg artery aneurysm bleed shows blood or hyperdensity in the anterior interhemispheric cisterm. the ruture of the Middle cerebral artery may present with hematoma in the temporal lobe or in the sylvian fissure of that side.

Fischer's grading of SAH on the basis of CT findings:
In Grade 1: there is no detectable blood on CT scan
Grade 2 : Diffuse thin  SAH  less than 1 mm thickness, & if thickness of clot is more than 1mm then it is labeled as Grade 3.
Grade 4: Intraventricular or intracerebral clot with diffuse or no subarachnoid hemorrhage

If  CT scan is normal and still there is strong suspicion of SAH, then the next  investigation is Lumbar Puncture, which reveals xanthochromia. 

Common locations of intracranial aneurysms include Anterior communicating artery ( 30%), the junction of the ICA and Pcom ( 25% ), MCA bifurcation ( 20% ), ICA bifurcation ( 7.5%). Around 7% arise from the basilar bifurcation and 3% arise from the PICA, a branch of vertebral artery.

Digital Substraction Angiography ( DSA) or CT Angiography ( CTA) is the investigation of choice. MR angiography ( MRA) does not use any contrast and it is a good non invasive screening investigation.

Source: Wikipedia

Understanding of CNS Tuberculosis- Three Landmark Papers

It is amazing and inspiring to see the excellent contributions of founders of medicine. While working on CNS tuberculosis I found that our present understanding of the tuberculosis is based on the works of physicians who existed about a century ago. Their contributions are immense and very encouraging to us.  The following three landmark papers helped in the understanding of aetiology , pathogenesis and treatment of tuberculosis of the central nervous system.

1. Green PH. Tubercular meningitis. Lancet 1836; II: 232–35.

2. Koch R. Die aetiologie der tuberculosis.
Berlin Klinische Wochenschrift 1882; 19: 221–30.

3. Rich AR, McCordock HA. The pathogenesis of tuberculous
meningitis. Bull John Hopkins Hosp 1933; 52: 5–37.

Before the demonstration of a tuberculous bacteria by Robert Koch, Green PH had published a paper in Lancet in 1836, describing a new diagnosis called tubercular meningitis.

May 14, 1836

Volume 26
Number 663
p225-256

Originally published as Volume 2, Issue 663

Green PH. Tubercular meningitis. Lancet 1836; II: 232–35.

In this paper Green P.H., President of the Society of Foreign Physicians, Paris, France, coined the new term in medicine " Tubercular Meningitis. In this paper there is description of clinical presentation of tubercular meningitis and acute hydrocephalus and post mortem findings of the patients showing caseation and exudates.


Koch R. Die aetiologie der tuberculosis. Berlin Klinische Wochenschrift 1882; 19: 221–30.

Robert Koch (1843-1910) was a German master of medicine. He got Nobel Prize in Medicine. he is regarded as Founder of science of Bacteriology. In 1876, he demonstrated Anthrax bacillus and in 1882 he demontrated tubercle bacillus.

          Robert Koch: German physician & Pioneering Microbiologist ( demonstated bacteria causing Anthrax, Cholera ans tuberculosis)




Rich AR, McCordock HA. The pathogenesis of tuberculous meningitis. Bull John Hopkins Hosp 1933; 52: 5–37.

Arnold Rice Rich (1893-1968) was born in Birmingham, Alabama in USA on March 28, 1893. He studied at University of Virginia and John Hopkins. He worked as Professor of pathology at John Hopkins , Baltimore, USA. He was excellent teacher and researcher. His contribution in medical sciences is immense. His contributions include the understanding of pathology of jaundice, pathogenesis of tuberculosis.

                                                                       A R Rich
                                Source http://www.medicalarchives.jhmi.edu/images/rich.jpg

This paper by AR Rich is the outcome of clinical and animal experiments. this paper described the occurrence exudates in tuberculosis.



Sources:

http://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/rich-arnold.pdf

Robert Koch: centenary of the discovery of the tubercle bacillus, 1882
ALEX SAKULA, From Redhill General Hospital, Surrey
Thorax 1982;37:246-251

Wikipedia: Robert Koch

http://www.medicalarchives.jhmi.edu/images/rich.jpg

http://www.medicalarchives.jhmi.edu/papers/rich.html

Vladimir Bekhterev and Ludwig Puusepp

Vladimir Mikhailovich Bekhterev (1857-1927)

The first operating room at the neurology department of the Russian Military Medical Academy was established in 1897 by the famous Russian neurologist and psychiatrist Vladimir Bekhterev. According to Bekhterev, neurology should become a surgical speciality like gynaecology or opthalmology and neurologists will take a knife in their hands [1]. He was an outstanding Russian neurologist, psychiatrist, psychologist, morphologist, physiologist authored over 1000 scientific publications. He created a new multidimensional multidisciplinary scientific branch - psychoneurology. He also proposed "biopsychosocial" model in the interpretation of human diseases [2].

Both Ivan Pavlov and Bekhterev independently developed a theory of conditioned reflexes which describe automatic responses to the environment. What was called association reflex by Bekhterev is called the conditioned reflex by Pavlov, although the two theories are essentially the same [3].

Vladimir Bekhterev (source: wikipedia)

Bekhterev's pupil Ludwig Puusepp (1875-1942) became the first full-time Russian neurosurgeon ("surgical neurologist"). He headed the first university course in surgical neurology in the world organised in 1909 at Bekhterev's Psychoneurological Institutte in St. Petersburg and became professor of surgical neurology in 1910. The role of neurologist might be illustrated by the development of a sterotactic instrument named "encephalometer" designed by D. Zernov in 1889 and improved by G. Rossolimo in 1907 [1].

Ludwig Puusepp: world's first professor of Neurosurgery ( Source ; Wikipedia)

Sources: 

1.Lichterman BL. Roots and routes of Russian neurosurgery (from surgical neurology towards neurological surgery).J Hist Neurosci. 1998 Aug;7(2):125-35.

2.Akimenko MA.Vladimir Mikhailovich Bekhterev. J Hist Neurosci. 2007 Jan-Jun;16(1-2):100-9

3. Wikipedia

Cerebellopontine angle and Retromastoid Craniectomy

Retromastoid craniectomy is an important neurosurgical approach for operating lesions of the cerebellopontine angle ( CP angle).

Commonest CP angle tumor is acoustic schwannoma  Other common tumors of CP angle are CP angle meningioma and epidermoid. 

Retrosigmoid region is the part of the cranium situated just behind the sigmoid sinus and below the transverse sinus.

The lateral cerebellomedullary and cerebellopontine cisterns are encountered in this approach.

Three important arteries are encountered bilaterally in this region: 

Posterior inferior cerbellar artery (PICA),

Anterior inferior cerbellar artery (AICA), &

Superior cerebellar artery (SCA)

PICA arises from the vertebral artery and is seen in relation to the lower cranial nerves. Occasionally, it may reach high enough to come in contact with the lower limit of the VII/VIII complex.

AICA arises from the basilar artery and distal part is seen in relation to the VII/VIII complex. This nerve-related segment is divided into premeatal, meatal, and postmeatal parts. The meatal part forms a loop ( meatal loop) which travels along the VII/VIII complex for a variable distance toward the internal acoustic meatus. Several nerve-related branches of AICA exist: the internal auditory artery, recurrent perforating arteries, subarcuate artery , and cerebellosubarcuate artery. The internal auditory artery is the most constant branch of the AICA.

The SCA arises from the basilar artery quadrification courses back along the side of the mid brain and pons , and is seen in the cerebellopontine angle in front of the Vth cranial nerve.

The veins of the posterior fossa can be divided into superficial , deep , and bridging veins. Anterior hemispheric veins , veins of the cerebellopontine fissureand middle cerebellar peduncular veins, lateral medullary and lateral mesencephalic veins, and petrosal bridging veins.

Petrosal veins are divided into superior and inferior veins which drain into superior or inferior petrosal sinus.

The superior petrosal veins are the most prominent bridging veins of the posterior fossa. They drain blood from a large part of the cerebellar hemisphere and brainstem. Superior petrosal veins are divided into a lateral, intermediate or medial group based on their relation to the site of drainage into the superior petrosal sinus and IAM.

Cranial nerves V through XII are seen in this approach. CN V is a stout nerve arising from the upper and outer aspect of the anterior pons. Its motor component arises as a separate rootlet just anterior to the main nerve , and both quickly merge to form a single nerve. Loops of the SCA can be seen in front of the nerve at this level.The nerve courses anteriorly and outward to cross the petrous apex and enter into the Meckel,s cave.

Cranial nerve VI arise medially at the pontomedullary sulcus. It courses up the surface of the clivus exiting the posterior fossa through Dorello’s canal. It enters the posterior cavernous sinus on its way through the middle cranial fossa to the orbit.

The VII/VIII complex, with the nervus intermedius in between, arises from the pontomedullary junction surrounded by the same fascial sheath. They course backward, downward, and outward toward the internal acoustic meatus.

Cranial nerves IX, X, XI arise as a linear series of rootlets from just below the pontomedullary junction in the anterolateral medullary sulcus. After their origin, they converge outward and backward toward the jugular foramen. CN XII lies in close proximity to the vertebral artery-PICA junction. Its many rootlets exit though the hypoglossal canal.

Retromastoid craniectomy can be performed with the patient in sitting, park-bench, three-quarter prone position, lateral or supine position.
The description by Robert L. Martuza for surgery of acoustic neuroma in supine position is straight forward. The patient is positioned supine with one or two folded soft cotton blankets beneath the ipsilateral shoulder and padding beneath the knees and is secured with one or two belts to allow the table to be rolled if necessary during surgery. Too much turning of the body can place the shoulder in a position that interferes with surgery.
The head should be fixed in Mayfield or Sugita frame.
A retroauricular lazy 'S' or linear or C shaped incision is made in the scalp which is paramedian and about 5 mm medial to the mastoid notch.
Dr Robert L. Martuza suggests a linear incision approximately 3 cm behind the insertion of the pinna and going from approximately 1 cm above the tip of the pinna to 1 cm below the ear lobe. The muscles are then divided inferiorly and self retaining retractors are placed.
The sigmoid sinus lies beneath the mastoid groove.
The transverse sinus and sigmoid sinus generally curve around the asterion. Therefore, a burr hole is placed medial to the asterion.
Craniectomy is done after placing a burr hole.  After craniectomy the inferior edge of the transverse sinus and the medial edge of the sigmoid sinus should be defined with the help of rongeurs. The transverse sinus lies above the upper limit of craniectomy and sigmoid sinus is lateral to the craniectomy.typically a K-shaped incision or linear incision about 2 cm medial to the edge of the sigmoid sinus  is made in the dura with the edges being based on the sinus.
After retracting the cerebellar hemisphere medially and superiorly cisterna magna comes into the view. The arachnoid of this cistern is opened and CSF is drained and this maneuver relaxes the cerebellum, obviating the need for further retraction.

Middle neurovascular complex in CP angle includes AICA, pons and middle cerebellar peduncle. The AICA arises at the pontine level and courses in relationship to the abducens, facial & vestibulocochlear nerve to reach the surface of the middle cerebellar peduncle, where it courses along the cerebellopontine fissure and terminates by supplying the petrosal surface of the cerebellum. Common operations in this region are for removal of acoustic schwannomas and for the relief of hemifacial spasm.

Acoustic schwannomas, as they expand, may involve a majority of the cranial nerves, cerebellar arteries, and parts of the brain stem.

Lateral view of skull to show Asterion: Meeting point of Parietomastoid suture, Occipitomastoid suture and lambdoid suture.   

Sources:

1. Chapter: Suboccipital retrosigmoid surgical approach for vestibular schwannoma ( acoustic neuroma) by Robert L. Martuza in Schmidek & Sweet operative neurosurgical techniques- indications, methods & results. Alfredo Quinones- Hinojosa. 6^th ed. (Elsevier Saunders) 

2. Rhoton Cranial anatomy and surgical approaches, Neurosurgery, Albert L. Rhoton, Jr  

3. Oerative Neurosurgical Anatomy by Damirez T. Fosset and Anthony J. Caputy ( Thieme publication)
4, Wikipedia