neonatal brachial plexus palsy: current knowledge · 2018-12-13 · susceptibility to brachial...
TRANSCRIPT
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