Neonatal Brachial

Plexus Palsy:

Current Knowledge

Michele J. Grimm, Ph.D.

Department of Biomedical Engineering

© Michele Grimm, 2015

NBPP and Litigation

• It is permanent NBPP that may result in

litigation

• Cases are not filed based on a shoulder

dystocia alone

• Our understanding of the mechanisms of

injury comes from temporary and

permanent injuries

NBPP and Shoulder

Dystocia • Dystocia – abnormal, slow, or difficult child birth

process

• Shoulder Dystocia – delay in delivery of the infant

involving the shoulders

• Only a shoulder dystocia involving the anterior

shoulder will be observable

• NBPP can occur

• With a shoulder dystocia involving the

affected (anterior) limb

• With a shoulder dystocia involving the

contralateral limb (anterior shoulder in SD,

posterior arm NBPP)

• Without any shoulder dystocia (either arm)

Current Scientific Agreement on NBPP

• The primary force that injures the brachial plexus during the birth process is tension (pulling) on the nerve

• Injuries can happen in the absence of clinician-applied traction

• Stretch to the brachial plexus occurs during deliveries as a result of maternal forces alone

• BPI can occur to anterior or posterior shoulders and with or without a clinical shoulder dystocia

Current Debates on NBPP

• Is the pulling of the nerve that causes injury primarily due to clinician-applied traction or due to maternal forces?

• If maternal forces can stretch the brachial plexus when a shoulder is restrained by the mother’s pelvis, how will even normal traction add to that stretch?

• Can permanent injuries be caused by maternal forces alone?

• What is the injury threshold for the infant brachial plexus?

How do Maternal Forces

Stretch the Brachial Plexus?

• Spinal loading: driving force from the rear

• Loading to the infant’s bottom through the

uterus will continue up spine

• Spine in compression acts as a solid rod

• Will transmit force through to cervical spine,

continuing to move head forward

• If shoulder stuck, force will still try to move

spine/neck/head forward and will widen angle

between shoulder and neck

Important Concepts

Related to Nerve Injury

• Nerves can be injured through compression

(crushing) or tension (pulling)

• Tension or traction of nerves does not

necessarily require pulling on the human

body

• A combination of compression and tension

is more likely to cause injury than one of

these alone

Important Concepts

Related to Nerve Injury

• Nerve injuries occur along a continuum of severity

• Neuropraxia: sustained “falling asleep” of a limb

• Partial rupture: some nerve fibers are still connected, but amount of innervation of muscles is reduced

• Complete rupture: no axons remain connected, chance of spontaneous healing is minimal

• Avulsion: rupture at the connection of the nerve to the spinal cord, which does not provide any nerve to which a graft can be connected

Important Concepts

Related to Nerve Injury

• Nerves are a biological tissue

• There is no single value for nerve strength

• Whether a nerve will fail under a given force or stretch depends on many variables related to the individual in question

• Anatomy

• Tissue properties

• Where a nerve fails depends on the weak point of that nerve (rupture vs avulsion)

• The same injury in two individuals does not mean that the same force was applied to both nerves

Maternal Delivery Forces –

Clinical Estimates

• Calculated based on clinical measurements of

intrauterine pressure

• Varies based on intrauterine pressure

• Up to 120 mmHg

• Depends on the cross-sectional area of

baby’s torso

• For a 50th percentile male

• 30 – 40 lbf during the 2nd stage of labor

• The level of maternal forces cannot be

compared directly to clinician-applied forces to

estimate injury risk – the key factor is how the

force stretches the brachial plexus

ACOG, 2014

Applied Delivery Forces –

Clinical Measurements

• Force sensors on hands*

• Normal: 3.9 – 12.3 lbf

• “Difficult”: 11 – 16 lbf

• Shoulder Dystocia: 11 – 22.5 lbf

• Force plate under feet**

• Normal: 3.26 – 12.2 lbf

• Approximately 75- 100 deliveries

• 3 clinicians

• 4 shoulder dystocias

• 1 temporary BPP

*Allen, O&G, 1991; Poggi, AJOG, 2004; Poggi, AJOG, 2005;

** Peisner, AJOG, 2011.

Delivery Forces and NBPP

– Clinical Measurements

• Single, large scale prospective study by Mollberg

• 31,000 deliveries -- 18 permanent BPI

• Clinicians asked to mark on a scale from 0 to 100

• 0 – no force

• 100 – ”greatest force you would apply”

• No attempt to equate with actual force

• Permanent BPI

• More likely to have force greater than 50% of

“greatest force you would apply”

• 17 of 18 permanent BPI had fundal pressure

applied after the head delivered

• 18 of 18 permanent BPI had at least 3

attempts at pushing after head delivered

Computer Modeling:

BP Stretch and Delivery Force Predictions

0

2

4

6

8

10

12

14

16

18

20

0

5

10

15

20

25

30

MaternalForces -StandardPosition

MaternalForces -

McRobertsPosition

MaternalForces -

LithotomyPosition

(NoDelivery)

ClinicianForces -StandardPosition(Axial)

ClinicianForces -StandardPosition

(Bending)

Re

su

ltin

g B

rac

hia

l P

lex

us

Str

etc

h (

%)

Ap

pli

ed

De

live

ry F

orc

e (

lbf)

Effect of Delivery Forces in a Shoulder Dystocia

DeliveryForce

BrachialPlexusStretch

Gonik, AJOG, 189:1168, 2003

Computer Modeling:

BP Stretch and Delivery Force Predictions

Gonik, AJOG, 2003 & 2010

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16

18

ClinicanForces -

LithotomyPosition

ClinicanForces -

McRoberts(30 deg)

ClinicanForces -

McRoberts(20 deg)

ClinicanForces - 80 NSuprapubicPressure

(Lithotomy)

ClinicanForces -Oblique

Positioning

ClinicanForces -

Posterior ArmDelivery

Re

su

ltin

g B

rac

hia

l P

lex

us

Str

etc

h (

%)

Ap

pli

ed

Fo

rce

(lb

f)

Effect of Clinican Maneuvers in a Shoulder Dystocia

DeliveryForce

BrachialPlexusStretch

Physical Modeling:

BP Stretch and Delivery Force Predictions

0

5

10

15

20

25

30

35

0

2

4

6

8

10

12

14

16

18

ClinicianForces -

McRobertsPosition

ClinicianForces -PosteriorRubins

ClinicianForces -AnteriorRubins

MaternalForces - NoSD (BD =11.9 cm)

MaternalForces -

Unilateral SD(BD = 12.4

cm)

MaternalForces -

Bilateral SD(BD = 12.9

cm)

Bra

ch

ial P

lex

us

Str

etc

h

De

live

ry F

orc

e (

lbf)

Effect of Force Type and Maneuvers

DeliveryForce (N)

Anterior BPStretch (mm)

Posterior BPStretch (mm)

Anterior BPStretch (%)

Posterior BPStretch (%)

AJOG: Gurewitsch, 2005; Allen, 2007

Physical Modeling: Delivery

Force Predictions

• Delivery forces measured in clinical simulations

of shoulder dystocia before any new training

• Delivery of posterior arm required to relieve

SD

• 113 clinicians

• Maximum traction applied if no delivery:

• 1.35 – 53 lbf

• 10 – 260 seconds after start of sim

• Maximum traction applied if delivered:

• 10.3 – 56 lbf

• 50 – 250 seconds after start of sim

Crofts, AJOG, 2007

Forces and BP Stretch -

Summary

• Maximum clinician-applied force measured in a

clinical delivery: 22.5 lbf

• Maximum clinician-applied force measured in a

simulator that would not deliver without a

tertiary maneuver: 56 lbf

• Maximum stretch predicted due to clinician-

applied forces

• 16 lbf traction in McR – 30% (phys model)

• Early physical model

• 18 lbf bending – 18.2% (computer model)

Forces and BP Stretch -

Summary

• Maximum stretch predicted due to maternal

forces during shoulder impaction

• 22.5 lbf in Lithotomy (shoulder remains

stuck) – 18% (computer model)

• 28 lbf in Lithotomy (shoulder cleared

spontaneously) – 15.7% (computer model)

• Lithotomy (anterior SD – physical model)

• Anterior: 10.0 +/- 3.3%

• Posterior: 14.5 +/- 4.5%

• Lithotomy (bilateral SD – physical model)

• Anterior: 10.4 +/- 6.6%

• Posterior: 15.3 +/- 3.4%

Susceptibility to Brachial

Plexus Injuries

• How much an a neonatal brachial plexus

stretch before it is injured?

• Not directly measured in infant BP

• Surrogate studies are required

Susceptibility to Brachial

Plexus Injuries

• Kalmin (1995)*:

• Russian study of elastic and failure

properties of neonatal/fetal C3 and C4

nerves

• Measured up to 50% stretch before

failure

• C3 and C4 responded differently

• Did not follow modern practices for

measuring stretch in the nerves

Kalmin, Morfologiia, 111:39, 1997

Susceptibility to Brachial

Plexus Injuries

Original

Length

New Length 1

Stretch of the

Nerve

New Length 2

Slip of Nerve

in Grips

Susceptibility to Brachial

Plexus Injuries

• Singh et al. (2006)*:

• Spinal nerve roots of rats fail at a wide range of

strains

• 29+/- 9% failure strain - what does that mean?

• 2/3 of nerves in the population will fail

between 20 and 38% stretch

• 1/6 of nerves in the population will fail

between 11 and 20% stretch

Singh, J Biomech, 39:1669, 2006

Why Don’t More Injuries

Occur? • To be injured, an infant’s shoulders must

be restrained as forces move the infant’s

head and neck forward

• Nominally 1-2% of deliveries

• Out of those 1-2% of infants, what is the

overlap with the population that is most

susceptible to injury?

Population Statistics

High injury risk

Shoulder dystocia

All births

No PBPP due to

maternal forces

Population Statistics

High injury risk

Shoulder dystocia

All births

10-13% of SD

result in PBPP

due to maternal

forces

Population Statistics

High injury risk

Shoulder dystocia

All births

Some portion of

SD result in

PBPP due to

maternal forces

Susceptibility to Brachial Plexus Injuries

• What makes one baby more susceptible

to injuries than another?

• $64,000 question

• NOTE: All of these are generalities!

Susceptibility to Brachial Plexus Injuries

• Surrounding Tissue Properties: the less stiff the shoulder and neck, the greater the amount of force and stretch that will be experienced by the nerve

• As muscle tone goes down, the stiffness of the shoulder and neck will go down

• 1 minute Apgar score lower than 7 significantly increases risk of injury*

• Increased risk even higher if 5 minute Apgar is less than 7

McFarland, Obst Gyn, 68:784, 1986

Susceptibility to Brachial Plexus Injuries

• Anatomy: smaller babies will experience more stretch to their tissues (including BP) for the same amount of applied force

• Smaller structures are less stiff and will stretch more

• Force Applied: within the same infant a larger applied force will cause a larger stretch and increase the risk of injury

Susceptibility to Brachial Plexus Injuries

• Properties of the nerve itself

• Stiffness - less stiff nerves will stretch more, but will not necessarily fail earlier

• Failure strength or strain - how much force or stretch the nerve can take before it fails

• These will all vary between individuals and may depend on some factors in yet unknown ways

• Effect of diabetes?

• Effect of in utero positioning, compression, or development?

Conclusions • Infant brachial plexus can be stretched

significantly

• Due to maternal forces

• Due to clinician applied forces

• In both anterior and posterior shoulder

• Stretch due to bending is greatest

• Stretch due to maternal forces is higher than

caused by axial traction in lithotomy or during

maneuvers

• Most recent data on nerve injury thresholds

indicates that some infants will sustain a

permanent injury at stretch levels that occur due

to maternal forces

• All infants are different – the pattern of injury

cannot be used to determine the amount of force

applied