05 Roots, plexus and nerves: the map problem

Every limb is covered by three maps at once, and they don't agree with one another. The dermatome is the skin that a single dorsal root supplies, and the myotome is the set of muscles that a single ventral root supplies. The nerve territory is whatever one named peripheral nerve supplies after the plexus has shuffled the roots together. A root lesion stands apart from a plexus or nerve lesion because its signs follow the segments (Brazis 2011, p. 89).

Why one root may numb so little

Neighboring dorsal roots overlap so much that cutting one of them causes little or no sensory loss. A normal sensory exam therefore does not exclude a root lesion (Brazis 2011, p. 89), and the motor and reflex findings deserve more trust than a numb patch. Root lesions more often show themselves through pain. The pain is sharp and electric, runs along the root's territory, and gets worse with coughing, straining or sneezing (Brazis 2011, p. 89).

Two rules for the limbs

The first rule is that if a nerve lesion weakens a muscle, every muscle that nerve supplies below the lesion must also be weak. Suppose brachioradialis is weak from a radial nerve lesion at the spiral groove. Wrist and finger extension must then be weak too. If they are not, the problem lies somewhere else (Morris 2012, p. xxix).

The second rule separates a root from a nerve. It tells you to test a muscle with the same root but a different nerve. Weak quadriceps could come from the femoral nerve or from the L2 to L4 roots. The hip adductors share those roots but travel in the obturator nerve, so their strength settles the question (Morris 2012, p. 23).

In a child

Root and nerve lesions in children are more often traumatic, compressive or present from birth than degenerative. Young children are also poor reporters of numbness. Both facts put more weight on strength and reflexes, which don't depend on the child's description. The rest of this module applies the two rules to the arm, the leg and the newborn plexus.

The dermatome map above marks the single point the international spinal cord injury standard uses to test each root, because whole dermatomes overlap too much to test reliably (Kirshblum 2011).

The figure above applies the two rules for you. Mark each muscle or reflex you tested as weak or normal. The figure then keeps only the roots, plexus trunks and nerves that explain everything you found, and notes why each rejected candidate dropped out. Behind it is a table that gives each muscle its predominant root. Sources differ by a segment here and there, so read the table as a teaching map whose boundaries may shift by a level.

Wrist drop

Start with the wrist-drop preset. The wrist and fingers drop and brachioradialis is weak, but triceps and its reflex are normal. The branches to triceps leave the radial nerve high in the arm, so a normal triceps puts the lesion at or below the spiral groove of the humerus (Morris 2012, p. 11). The radial muscles below the groove are weak and those above it are spared, which is the first rule at work (Morris 2012, p. xxix). A C7 root lesion could look similar, but triceps depends mainly on C7, so a normal triceps rules the root out.

Ulnar nerve or lower trunk

When the first dorsal interosseous is weak and wasted, check the thumb's short abductor next. Both depend on the C8 and T1 roots, but the interosseous travels in the ulnar nerve and the thumb abductor in the median nerve. A weak interosseous beside a strong thumb abductor points to the ulnar nerve. Weakness in both points higher, to the lower roots or the lower trunk of the plexus. This is the second rule again, and the figure works it out for you.

A screening sequence for the arm

A practical sweep of the arm needs only a few muscles, each chosen because it separates the candidates.

Then add the biceps, brachioradialis and triceps reflexes.

In a child

Strength testing goes better as a contest, done one side at a time so that the two sides can be compared. A child who won't cooperate with formal testing will often show you shoulder abduction, elbow extension and grip anyway while reaching for a toy held high, to the side, or just out of reach.

Foot drop is the commonest focal leg problem a neurologist is asked to localize, and two very different lesions produce it. The common peroneal nerve winds around the head of the fibula just under the skin, where it is easily compressed. The L5 root supplies the same ankle dorsiflexors. It also supplies muscles that the peroneal nerve never reaches, and those muscles tell the two lesions apart.

Inversion and hip abduction

When dorsiflexion, eversion and big-toe extension are weak and the reflexes are normal, the lesion is in the common peroneal nerve. Look for scars or bruising over the fibular head. When dorsiflexion and eversion are weak and so is inversion, sometimes with weak hip abduction, the lesion is at the L4/L5 root or plexus (Morris 2012, p. 26). Tibialis posterior, the main invertor, is an L5 muscle carried by the tibial nerve. Gluteus medius, the hip abductor, is an L5 muscle carried by the superior gluteal nerve. A peroneal lesion leaves both strong, because neither belongs to that nerve.

In a patient with sciatica, weak big-toe extension may be the only motor sign of an L5 root lesion (Morris 2012, p. 29). When everything at the ankle is weak and the ankle reflex is lost, widen the search to a neuropathy, a sciatic nerve lesion, or a root or plexus lesion affecting several segments (Morris 2012, p. 26).

Testing inversion

Hip rotation can fake inversion. Hold the shin still with one hand, move the foot into inversion with the other, and then ask the child to hold it there (Morris 2012, p. 29).

Using the figure

The figure in the previous section has two foot-drop presets. Load them one after the other. In the first, inversion and hip abduction are normal, and the common peroneal nerve is the only candidate left. In the second, both are weak, and only the L5 root survives, because three different nerves are involved and the root is the one thing they share.

In a child

In a child, peroneal neuropathy tends to follow weight loss, a tight cast, prolonged squatting or kneeling, crossing the legs, or a knee injury. An L5 radiculopathy is unusual in a young child, so a convincing one deserves imaging to look for the cause.

A newborn whose arm lies still after a difficult delivery is a pure test of plexus localization. The baby cannot follow instructions, so everything is read from posture, spontaneous movement and reflexes.

Erb palsy and Klumpke palsy

In the plexus, C5 and C6 join at Erb's point to form the upper trunk, C7 forms the middle trunk, and C8 and T1 form the lower trunk (Fisch 2012, p. 30). An upper trunk (C5, C6) lesion, Erb palsy, usually follows shoulder dystocia. It weakens supraspinatus, infraspinatus, deltoid, biceps and supinator, so the arm hangs limply at the side, turned in and pronated (Pearl 2014, p. 80). A lower plexus injury, Klumpke palsy, weakens the intrinsic hand muscles and finger flexors (Pearl 2014, p. 80). Some define it as C7 to T1 (Pearl 2014, p. 80), but the lower trunk comes from C8 and T1 alone (Fisch 2012, p. 30).

Root or trunk

Two nerves leave the roots before the plexus forms. The dorsal scapular nerve runs from C5 to the rhomboids, and the long thoracic nerve runs from the upper roots to serratus anterior (Fisch 2012, p. 31). The suprascapular nerve leaves later, from the upper trunk (Fisch 2012, p. 35). These branches work like a ruler laid along the plexus. If the rhomboids and serratus work but the deltoid, biceps and infraspinatus do not, the injury lies beyond the roots, in the upper trunk. If they are weak too, the damage is at the roots. A Horner syndrome on the same side points to involvement of T1 near the spine (Pearl 2014, p. 80).

Examining the newborn arm

Watch spontaneous movement and the Moro response, which shows whether each shoulder abducts and elbow flexes (Module 2). Check the biceps reflex, which is absent in an upper trunk injury. A working grasp in an arm that otherwise hangs still shows that the lower trunk is intact. Winging of the scapula when the infant pushes against your hand means a weak serratus.

Not every still arm is a plexus

A clavicle or humerus fracture, or an infected shoulder joint or bone, can make a newborn refuse to move a painful arm. A perinatal stroke is unlikely in the first days, because it usually announces itself with seizures, and the arm weakness becomes obvious only months later. A plexus injury gives a lower motor neuron pattern in a root distribution, with the reflex lost.

A tendon reflex is one of the most objective tests in the limb exam, because it doesn't depend on the child's effort, cooperation or description. It tests one short loop made of a stretch receptor, a sensory fiber, one or two cord segments, a motor neuron and a muscle. A lost reflex therefore localizes better than almost any sensory complaint.

The segments

The table gives the values in common use.

Sources differ by a segment. The knee reflex is sometimes given as L2 to L4 and the ankle reflex as S1 to S2 (Brazis 2011, p. 117), and the figure in this module uses those wider ranges. For localization, the order of the reflexes from top to bottom matters more than exactly where each boundary falls.

Reflexes and the level of a cord lesion

In a cord lesion, reflexes are lost at the level of the lesion and increased below it (Brazis 2011, p. 12; Morris 2012, p. xxix). An absent biceps reflex with a brisk triceps and brisk knee and ankle jerks therefore puts the lesion in the cord at the C5 and C6 segments, a pattern that Module 6 develops.

Getting a reflex in a child

To a toddler a reflex hammer looks like a weapon, so bring it out late in the exam, on the parent's lap, and turn it into a toy (Module 2). Reinforcement often brings out a reflex that seems absent. Ask the child to clench their teeth or pull their hands apart while you tap. Compare the two sides every time, because asymmetry is often more informative than the absolute grade.

Reading a lost reflex

An absent ankle reflex on one side, with weak plantar flexion and numbness on the outer foot, points to S1. Absent ankle reflexes on both sides with stocking sensory loss point instead to a length-dependent neuropathy (Module 4). The reflex tells you which loop is involved, and the other findings tell you where in the loop the break lies.