Cadaveric Anatomy

 Brachial Plexus     Femoral Nerve at Groin      Sciatic Nerve      Popleteal Fossa     Ankle Block
Dr.Anitha M.S(Anatomy)

Candaveric Anatomy of Brachial Plexus
Formation of the Plexus
Roots:The plexus is formed by the anterior primary rami of C5 to C8 nerves along with the bulk of T1 nerve. Occasionally there may be a contribution from C4 or T2 nerves leading to the formation of pre-fixed or post-fixed plexus. The roots emerge from the respective intervertebral foramina to enter the perivascular shealth.
Trunk:Sandwiched between the scalenus anterior and medius muscle, the roots combine to form the trunks. The C5 and C6 roots combine to form the upper trunk, C7 continues
as the middle trunk and C8 and T1 combines to form the lower trunk
Divisions: Behind the clavicle the trunks divide into a anterior and posterior divisions and stream into the axilla.
Cords: In the upper part of the axilla the six divisions combine to form lateral, medial and posterior cords.
The lateral cord is formed by the union of anterior division of the upper and middle trunks.
The medial cord is the continuation of the anterior division of the lower trunk.
The posterior cord is due to union of the posterior divisions of all the three trunks
Terminal branches: Lower down in the axilla the cords give rise to terminal branches namely the ulnar, median and radial nerves.
Relationship of the brachial plexus:
Roots: These lie between the scalenus anterior and medius muscles. It lies above the second part of the subclavian artery. The classical interscalene approach to the brachial plexus blocks is at the root level.
Trunks: These lie in close relationship to the subclavian artery above the clavicle. Here also they are sandwiched between the scalene muscles. The trunks extend upto the lateral border of the first rib. The subclavian perivascular approach blocks the plexus at this level.
Divisions: they start at thelateral border of the first rib and lie behind the clavicle. The rib hitching technique causes blockade at this level.
Cords: The divisions unite to form cords at the upper part of the axilla. They remain grouped around the axillary artery. The infraclavicular approach causes blockade at the junction of cords and divisions.
Terminal Branches: They are formed lower down in the axilla. The reorganization of the cords to form the terminal branches occurs at the lateral border of the pectoralis minor muscle. The axillary approach causes blockade at this level
The anatomic factors which determine the success and complications of the brachial plexus blockade are
The perivascular sheath
The vertical arrangement of the cervical roots
The interconnections – This is due to combining, dividing, recombining and redividing of the original five cervical roots.
The relationship of the site of needle entry to vital structures.
Perivascular Sheath – Its Importance
The perivascular sheath is a fibrous sheath covering the brachial Plexus in its entirety. It extends from the origin of the scalene muscles down to middle of upper arm. The potential space formed by this sheath can hold upto 80-100 ml of local anaesthetic.
This sheath gives a classical ‘Pop Off ‘ feeling when pierced by the needle. This sheath is the single most important factor in determining the success of bracial plexus blockade. The plexus can be blocked by introducing a needle at any point along the sheath. But the site of needle entry determines which components arepreferentially blocked and which compenents are spared. However this can be overcome to a certain extent by increasing the volume of the local anaesthetic and by applying proximal or distal digital pressure.
The suggestion that the covering is discontinuous with septa subdividing the space into separate compartments that clinically prevent the spread of local anaesthetic
However these septal divisions are more prominent in the axilla than above. This probably is the reason for frequent sparing of the radial and musculocutaneous nerves during axillary blockade.As the septa are more prominent in the lower part, there may be more sparing in the infraclavicular approach than in the supraclavicular approach. The perivascular sheath may also be discontinuous leading to spillage of drug out side thesheath.
Vertical arrangement of roots:
This arrangement of the brachial plexus assumes significance in the classical interscalene approach. Here the needle is applied close to the C5 and C6 nerve roots. As the roots are vertically arranged the local anaesthetic will have to travel caudally to reach C8 and T1 level. If the caudal travel of the local anaesthetic is deficient then these roots may be spared leading to poor analgesia in the ulnar nerve distribution. In the infraclavicular approach sparing of the ulnar nerve is rarely seen.
The interconnections:
The interconnections between the original five cervical roots means that the cutaneous distribution of the individual nerve terrioratories differs from the myotomal and detematomal pattern and that the muscular and other deep structures do not underlie the sensory distribution of that nerve. For example blockade of the ulnar nerve at the elbow, produces sensory loss on the ulnar side of the hand but motor loss of the flexor muscles on the anterior aspect.
Site of needle entry and complications:
If site of needle entry is at the C6 level, r=the chances of epidural and subarachnoid injections are more. The chances of vertebral artery puncture, phrenic and recurrent laryngeal nerve paralysis are also higher.
If the site of needle entry is close to the clavicle the chances of pneumothorax and subclavian artery puncture are more.
If the site of needle entry is below the clavicle as in the infraclavicular approach, complication rate is much less than in the above routes but the chances of incomplete blockade are more.
In the axillary site apart from accidental artery puncture other complications are less but chances of incomplete blockade increases

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