Approach to differentiating lesions (nerve root, plexus, and peripheral nerve): Clinical sciences

Injuries involving different components of the peripheral nervous system, such as nerve roots, plexuses, and peripheral nerves, can result in symptoms such as muscle weakness and sensory deficits. Mechanisms of injury vary depending on the affected site and include trauma, compression, malignancy, and inflammatory disorders. Now, based on history and physical exam findings, you can differentiate between the most common causes of wrist drop, foot drop, and combined upper and lower extremity sensorimotor deficits.

Now, if your patient presents with chief concerns suggesting a nerve root, plexus, or peripheral nerve lesion, first obtain a focused history and physical examination starting with wrist drop.

Let’s start by discussing patients who present with wrist drop. History will reveal hand weakness and numbness; and on exam, you’ll notice weakness of wrist extension and loss of hand sensation. These findings are often referred to as wrist drop, which could result from radial neuropathy, C7 radiculopathy, or brachial plexopathy; so your next step is to assess the underlying cause.

Patients with radial neuropathy report numbness, tingling, and pain over the top of the hand, and sometimes, the back of the arm. In addition to wrist weakness, they may report weakness of the elbow. History might reveal prolonged positioning associated with radial nerve compression, or recent trauma to the humerus.

As for the exam, you'll typically notice weakness in wrist and finger extension, sensory loss in the dorsum of the hand, and decreased or absent brachioradialis reflexes.

These findings are consistent with a radial nerve injury at the spiral groove of the humerus. However, if the radial nerve lesion is located higher in the axilla, you'll also note weakness of elbow extension due to triceps weakness, sensory loss on the dorsal surface of the arm, and a decreased triceps reflex. With these findings, diagnose radial neuropathy.

Alright, let’s now discuss C7 radiculopathy. Affected patients report numbness and tingling of the middle finger, in addition to neck pain that may shoot down the arm. Past medical history may reveal a herniated cervical disc or spinal stenosis.

On exam, you’ll find weakness in wrist and finger extension, and in wrist flexion. You’ll also see weakness in elbow extension due to triceps involvement, with sensory loss along the third finger, and a decreased triceps reflex.

These findings are consistent with injury to the C7 spinal nerve, also known as C7 radiculopathy.

Here's a high-yield fact! Remember that each cervical spine nerve root exits above its correspondingly numbered vertebra, so the C7 spinal nerve exits between the C6 and C7 vertebrae. However, the C8 spinal nerve exits between C7 and T1 vertebrae; and the thoracic and lumbar spinal nerves exit below their correspondingly numbered vertebrae.

Next, let’s look at brachial plexopathy, which is associated with numbness, tingling, and weakness throughout the arm. Affected patients might also report shoulder and upper arm pain. History may reveal recent trauma in which the shoulder and head were pulled in separate directions; or it may be significant for recent sternotomy, apical lung tumor, also called a Pancoast tumor, or radiation therapy.

On exam, you'll find patchy weakness and sensory loss throughout the affected arm. Additionally, some or all reflexes in the affected arm will be decreased or absent; specifically, the triceps, biceps, and brachioradialis reflexes. With these findings, diagnose brachial plexopathy.

Now, here are a few clinical pearls! The brachial plexus is divided into upper, middle, and lower trunks; and depending on the injury’s location, the patient will present with specific clinical manifestations.

For example, an injury to the upper trunk involving C5 and C6 causes the affected arm to hang at the patient’s side, internally rotated, with wrist flexion, known as the “waiter’s tip” sign. This injury is called Erb-Duchenne palsy and is commonly seen in neonates after a delivery complicated by shoulder dystocia.

On the other hand, an injury to the lower trunk involving C8 and T1 results in Klumpke palsy, which is characterized by hand and finger weakness. Klumpke palsy can also be seen in neonates after difficult delivery involving traction to an abducted arm, as well as other conditions, like Pancoast tumor and thoracic outlet syndrome.

In thoracic outlet syndrome, the lower trunk of the brachial plexus is compressed between the first rib and clavicle. Sometimes the subclavian artery is also compressed, causing diminished arm pulses. Okay, let’s move on to lower extremity lesions that result in foot drop.

In this case, the patient reports foot weakness and numbness, while the physical exam reveals weakness of foot dorsiflexion and loss of sensation in the foot. These findings are known as foot drop, so your next step is to assess the underlying cause.

First, let’s focus on peroneal neuropathy, which is associated with numbness, tingling, and pain of the lateral shin and top of the foot. The patient might report risk factors like repeated or sustained leg crossing or squatting, prolonged immobility, a tight cast or clothing compressing the knee, or recent knee trauma.

Physical exam findings include weakness of ankle dorsiflexion and eversion, and weakness of toe extension. Your patient might demonstrate a steppage gait or a high-stepping walk on the affected side, to avoid dragging the toes. Finally, exam typically reveals sensory loss in the lateral shin and the dorsum of the foot. These findings suggest peroneal neuropathy.

In this case, the common peroneal nerve, or the common fibular nerve, is compressed as it courses laterally around the head of the proximal fibula.

Next up is L5 radiculopathy. In this condition, patients also describe numbness and tingling of the lateral shin and top of the foot, as well as low back pain that might shoot down the leg. History may reveal a herniated disc or spinal stenosis.

Just as in peroneal neuropathy, the exam will reveal weakness of ankle eversion and dorsiflexion, but you’ll also notice weakness of ankle inversion, as well as weakness in toe extension, hip abduction, and hip extension. Again, the patient will demonstrate steppage gait and sensory loss involving the lateral shin and dorsum of the foot. These findings are consistent with an L5 radiculopathy.