Nerve Fiber type and function

Erlanger and Gasser divided mammalian nerve fibers into A, B, and C groups.

 

Spurling's Maneuver

This is also called the foraminal compression test.

This test done in upper limbs is considered equivalent to SLR in lower limbs.

This is done to test cervical nerve root irritability in neck.

Standing behind the patient, head is bent backwards and flexed laterally to the symptomatic side.

This posture may elicit pain or paresthesia in the involved root.

If pain or paresthesia are not seen then a downward compression over the vertex is given to aggravate the foraminal compression so as to elicit the pain and paresthesia 

Hypoglossal Canal

Hypoglossal canal is the Bony canal which transmits the Hypoglossal nerve.

It is also called anterior condyloid foramen.

It is present in the Occipital Bone.

It lies in the epiphyseal junction between the basiocciput and the jugular process of the occipital bone.

This location corresponds to 10'O clock position on the left and 2'O clock position  in relation to the Foramen magnum.

The canal is superiorly surrounded by Jugular tubercle, Superio laterally by the Jugular foramen, laterally by the sigmoid sinus and inferiorly by the occipital condyle.

PTA In CP Angle tumors

In CP angle tumor evaluation audiometry is used to plan strategy for surgery depending weather hearing is serviceable or not.

Non-servicable hearing is typcically >50 dB audiogram and spech discrimation of <50% 

Gardner Robertson scale is used for hearing evaluation.

Gardner Robertson Scale

Grade I (good-excellent) 

pure tone audiogram (dB) : 0-30

speech discrimination (%) : 70-100

Grade II (serviceable)

pure tone audiogram (dB) : 31-50

speech discrimination (%) : 50-69

Grade III (non-serviceable)

pure tone audiogram (dB) : 51-90

speech discrimination (%) : 5-49

Grade IV (poor)

pure tone audiogram (dB) : 91-max

speech discrimination (%) : 1-4

Grade V (none)

pure tone audiogram (dB) : not testable

speech discrimination (%) : 0

Pure Tone Audiometry

Pure tone audiometry (PTA) is a hearing test used to identify hearing threshold levels or hearing sensitivity of an individual, enabling determination of the degree, type and configuration of a hearing loss.

Hearing sensitivity is plotted on an audiogram, which is a graph displaying intensity as a function of frequency.

The human ear can detect frequencies from 20-20,000 Hz.

The audiogram is a chart of hearing sensitivity with frequency charted on the X Coordinate (abscissa) and intensity on the Y Coordinate (ordinate).

Intensity is the level of sound power measured in decibels.

Hearing:

Normal hearing (0-25 dB): At this level, hearing is within normal limits.

Mild hearing loss (26-40 dB)

Moderate hearing loss (41-55 dB)

Moderate-severe hearing loss (56-70 dB)

Severe hearing loss (71-90 dB)

Profound hearing loss (>90 dB)

Burr Hole points for Shunt

1. Keen's Point : Parietal Burr hole - 3 cm above and 3 cm behind the external auditory meatus.

Direction of canula - Perpendicular to the cortex and slightly cephalic

Length of insertion about 4 to 5 cm. 

2. Kocher's Point : Frontal Burr hole - 2 to 3 cm lateral to mid line and 1 cm anterior to coronal suture in  the mid pupilaary line.

Direction of canula - Coronal plane towards ipsilaterl inner canthus and AP plane towards EAM. 

Length of insertion about 5 to 6 cm. 

3. Dandy's point : Occipital burrhole - 2 cm lateral to the midline and 3 cm above the inoin. In infants this usually corresponds with the lambdoid suture in the mid pupillary line. 

Direction of canula - Perpendicular to the cortex and slightly cephalic. 

Length of insertion about 4 to 5 cm. 

4. Frazier's point : 3 to 4 cm lateral to the midline and 6 cm above the inion.

Direction of canula - Perpendicular to the cortex. 

Length of insertion about 4 to 5 cm. 

Papez Circuit

James Papez described the circuit which is involved in the cortical control of emotion in 1937.

It is also important in storing the Memory

Papez discovered the circuit after injecting Rabies virus into a cat's hippocampus and monitoring its progression through the brain.

The initial pathway described by Papez include

Hippocampal formation (Subiculum) → Fornix → Mammillary bodies → Mammillothalamic tract → Anterior thalamic nucleus → Genu of the internal capsule → Cingulate gyrus→ Cingulum → Parahippocampal gyrus → Entorhinal cortex → Perforant pathway → Hippocampus.

The Perforant pathway provides a connectional route from the entorhinal cortex to all fields of the hippocampal formation, including the dentate gyrus, all CA fields (including CA1),and the subiculum.

With newer inventions the Prefrontal cortex (PFC), Amygdala, and Septum are also included in the Papez circuit with the PFC and amygdala considered as the  key components.

The nuclei in the septum play a role in reward and reinforcement with the nucleus accumbens identified to have a key role as a pleasure center

The Limbic System

Paul D. MacLean first introduced the term "limbic system" in a paper in 1952.

It is alo called the "old mammalian system" or the "mammalian brain"in the  Triune brain model

It is the source of emotions and instincts ( Feeding, Fleeing, Fighting and Sexual behaviour- 4 F's)

The components of limbic system  are 

1. Amygdala,

2. Hippocampus

3. Cingulate gyrus

4. Fornicate gyrus

5. Hypothalamus

6. Mammillary body

7. Epithalamus

8. Nucleus accumbens (the "pleasure center")

9. Orbitofrontal cortex

10. Parahippocampal gyrus,

11. Thalamus

Triune brain

The Triune Brain is a model proposed by Paul D. MacLean to explain the function of traces of evolution existing in the structure of the human brain.

In this model, the brain is broken down into three separate brains that have their own special intelligence, subjectivity, sense of time and space, and memory.

The three parts are 

1. The R-complex

2. The limbic system

3. The neocortex

The R-complex, also known as the "Reptilian brain", includes the brain stem and cerebellum.

Posterior fossa meningiomas

An accepted classification of posterior fossa meningiomas was given by Sekhar and Wright.

here are sx types of posterior fossa meningiomas

Type        Location                             Anatomical Extension

I               Cerebellar convexity,         Tentorium, Transverse 

                 Lateral tentorial                    and Sigmoid Sinusus                

II             CP Angle                                    Petrous ridge, IAC

III           Jugular Foramaen                Cerebello Medullary Angle, 

                                                                        IJ Vein,  Extracranial  

IV            Petro Clival                             Upper 2/3rd Clivus,

                                                                      Cavernous Sinus,                                                                                                                                                                                               Meckles cave, Petrous Ridge     

V             Foramem Magnum              Lower 1/3rd Clivus

                                                                       and C1, C2 Area

VI           Unclassified                            Entire clivus,

                                                                       Mid and lower clivus 

                                                                       and other  types   

Normal visual field

The normal visula field  is

60 degree Superiorly

60 Deegree Nasally

75 degree Inferiorly and

110 degree temporally

In proptosis the visual field of the patient is more than the Examiners by Confrontation. 

Visual Field Testing

Confrontation is the bedside procedure for visual field testing

Perimetry is the test for confirmation of Field of vision though it can test the peripheral field best( Excluding the central 30 Degree of vision).

Depending wether the stimulus moves or not the perimetry is of 2 types.

1. Static permetry - Eg: Humphry field analyzer

2. Kinetic perimetry - Eg: Goldmann's perimetry

The field of vision is plotted by plotting points along circles known as Isopters 

Bjerrum screen is used for testing the central field of vision (about 50º in diameter)

It consists of dull black cloth or other material perpendicular to the line of sight and placed usually 1 m away from the subject (2 m gives more accuracy).  In the centre of the screen is a white spot (white 1 mm pinhead)  that provides a fixation point and a series of radial and circumferential lines are sewn or drawn to facilitate the localization of the stimulus

Imaging in Neurocysticercosis

4 Stages in neurocysticercosis

1. Viable or Vesicular stage

2. Colloid satage

3. Nodular - Granular stage

4. Cacified cyst stage

CT Findings

Vesicular stage : Small and rounded low-density areas that are well demarcated from the surrounding brain parenchyma. These lack edema as there is supression of host's immune response by the living tenia in the cyst. the scolex in the cyst appears as an eccentric hyperdense nodule

Colloid satage: (So-called ‘acute encephalitic phase’ of neurocysticercosis ). Seen as ill defined lesions surrounded by edema. ( Dead Tenia)  iniitiating host response to form edema. Ring enhancement with contrast.

Nodular - Granular stage: Nodular hyperdense lesions surrounded edema after contrast. 

Calcified Stage : Hyperdense areas without any edema surrounding

MRI Characteristics :

Vesicular stage : Cysts with csf intensity in both T1 and T2 images and scolex in the cyst as a high intensity in the cyst eccentrically. ( Hole with dot imaging)

Colloid Stage: The wall of the cyst becomes thick and hypointense and there is marked perile-
sional edema; these findings are better visualized on T2-weighted images

Nodular - Granular stage: Granular cysticerci are visualized as areas of signal void on both T1- and T2-weighted images surrounded by edema or gliosis with hyperintense rims around the area of signal void

Calcified Stage : These lesions are usually not visualized with MRI ( Usually hypointnse on all the sequences)