Mononeuropathies Flashcards
A mononeuropathy refers to damage or dysfunction of a …
A mononeuropathy refers to damage or dysfunction of a single peripheral nerve.
Mononeuropathies are commonly seen in the … and ….
Mononeuropathies are commonly seen in the upper and lower limbs.
The peripheral nervous system sends and receives information from the brain and spinal cord that make up the central nervous system. The peripheral nervous system is divided into visceral and somatic fibres:
Visceral fibres: divided into visceral sensory fibres that predominantly carry information from thoracic and abdominal compartments and visceral motor fibres that form the autonomic nervous system (ANS). The ANS is further divided into the sympathetic and parasympathetic systems
Somatic fibres: these carry important sensory information from skin, muscles, bone and joints and motor information to glands and muscles
Mononeuropathies commonly affect cranial nerves and individual peripheral nerves of the upper and lower limbs. Therefore, we can broadly think of mononeuropathies in three groups:
Cranial mononeuropathies: relates to the 12 paired nerves arising from the brain and brainstem
Upper limb mononeuropathies: relates to the peripheral nerves involved in upper limb function
Lower limb mononeuropathies: relates to the peripheral nerves involved in lower limb function
The major peripheral nerves of the upper limb arise from the brachial plexus. They provide both sensory and motor function. There are many upper limb nerves including:
Median Ulnar Radial Axillary Other (e.g. musculocutaneous, long thoracic, suprascapular, spinal accessory)
The major peripheral nerves of the lower limb arise from the lumbosacral plexus. They provide both sensory and motor function. There are many lower limbs nerves including:
Common peroneal Tibial Femoral Sciatic Other (e.g. sural, obturator, lateral cutaneous nerve of the thigh)
Cranial nerve neuropathies may occur in isolation or in combination depending on the underlying aetiology and which part of the nerve is affected (e.g. cell body or axon). For example, diabetes mellitus may cause an isolated sixth cranial nerve neuropathy whereas a cavernous sinus thrombosis may affect the third, fourth, five and sixth nerves in combination.
In this note, we focus on mononeuropathies affecting the upper and lower limbs. For information on mononeuropathies affecting the cranial nerves check out the following notes:
Bell’s palsy
Sixth nerve palsy
Third nerve palsy
A single peripheral nerve injury is commonly due to …
A single peripheral nerve injury is commonly due to compression.
Peripheral nerves may be damaged from a variety of insults. However, damage or dysfunction of a single peripheral nerve commonly occurs secondary to a compressive aetiology. Other factors include transection, inflammation, ischaemia, or even radiation.
Compression of peripheral nerves is common as they travel along their course. Examples include:
Compression of peripheral nerves is common as they travel along their course. Examples include:
Entrapped in a ligamentous canal
Entrapped within a facial compartment
Local compression by a tumour
It is a combination of nerve compression and ischaemia that leads to damage and neurological symptoms. Compression may occur … (e.g. compartment syndrome) or occur … (e.g. carpal tunnel syndrome).
It is a combination of nerve compression and ischaemia that leads to damage and neurological symptoms. Compression may occur acutely (e.g. compartment syndrome) or occur chronically (e.g. carpal tunnel syndrome).
In mild cases, compression may occur when the limb is held in a certain position leading to temporary paraesthesia from ischaemia of the nerve. For example, leaning on the elbow leads to compression of the ulnar nerve. If compression becomes more consistent or chronic there is demyelination with additional pain and weakness. Over time, the distal segment of the nerve stops functioning and there is Wallerian degeneration that essentially refers to axonal damage distal to the problem. Functional loss of sensation and strength is seen when there is a complete conduction block in the nerve.
Transection - mononeuropathy
This refers to the complete separation of a nerve. It is commonly the result of trauma (e.g. knife injury). Without urgent reattachment, regrowth of the nerve is impossible.
Ischaemia - mononeuropathy
Damage to the blood vessels that supply peripheral nerves known as the vasa nervorum can lead to infarction and neuropathy, usually from axonal injury. This can be due to vasculitis, atherosclerotic disease, or other conditions such as diabetes mellitus.
A variety of other aetiologies can lead to single nerve damage including infections (e.g. … zoster virus), inflammation, radiation, or metabolic processes (e.g. hypothyroidism).
A variety of other aetiologies can lead to single nerve damage including infections (e.g. varicella zoster virus), inflammation, radiation, or metabolic processes (e.g. hypothyroidism).
Median neuropathy
The median nerve is derived from the lateral and medial cords of the brachial plexus (C5-T1). It enters the upper arm at the axilla and travels alongside the brachial artery into the cubital fossa. It then travels in the anterior forearm before passing through the carpal tunnel to supply some intrinsic muscles of the hand. Carpal tunnel syndrome is the most common cause of a median nerve neuropathy.
In the forearm, the median nerve provides innervation for:
Pronator teres Flexor carpi radialis Palmaris longus Flexor digitorum superficialis A branch of the median nerve known as the anterior interosseous arises within the forearm and provides innervation for:
Pronator quadratus
Flexor pollicis longus
Part of the flexor digitorum profundus
The … nerve enters the wrist through the carpal tunnel with nine flexor tendons. It goes on to provide motor innervation to the thenar eminence and two lateral lumbricals that can be remembered as ‘2LOAF’.
The median nerve enters the wrist through the carpal tunnel with nine flexor tendons. It goes on to provide motor innervation to the thenar eminence and two lateral lumbricals that can be remembered as ‘2LOAF’.
Two characteristic syndromes related to compression of the median nerve and/or its branches are pronator teres syndrome and anterior interosseus nerve syndrome.
Pronator teres syndrome: compression of the median nerve in the forearm as it passes through the pronator teres. A rare syndrome typically seen in physically active people (e.g. professional cyclists).
Anterior interosseous nerve syndrome: compression of the isolated motor branch of the median nerve known as the anterior interossoeus. Although this syndrome is rare, it has a very classic clinical examination finding of being unable to make the ‘okay’ sign.
The clinical features of median nerve neuropathy can vary depending on the underlying cause or level of compression/entrapment. Characteristic features include:
Sensory loss and/or paresthesia: typically seen over the palmar and distal dorsal aspects of the thumb, index, middle and half of the ring finger.
Weakness or clumsiness using the hand
Weak thumb abduction
Thenar eminence wasting
Hand pain: typically worse at night (carpal tunnel syndrome)
In the setting of carpal tunnel syndrome a few provocative manoeuvres can be performed, which are tests designed to elicit symptoms on examination. These include Phalen and Tinel.
Phalen manoeuvre: ask the patient to hyperflex both hands and hold dorsal surfaces together. This should be held for 1 minute. A positive test leads to pain and/or paraesthesia in the distribution of the median nerve
Tinel test: percuss over the median nerve just proximal to the carpal tunnel. A positive test leads to pain and/or paraesthesia in the distribution of the median nerve
Tinel’s test is a non-specific test to assess for peripheral nerve injury by tapping proximal to the damaged nerve. Commonly used in … tunnel syndrome, but may be used in other peripheral nerve injuries (e.g. tarsal tunnel syndrome)
Tinel’s test is a non-specific test to assess for peripheral nerve injury by tapping proximal to the damaged nerve. Commonly used in Carpal tunnel syndrome, but may be used in other peripheral nerve injuries (e.g. tarsal tunnel syndrome)
Ulnar neuropathy
The ulnar nerve may become trapped at the elbow or the wrist.
The ulnar nerve is a continuation of the medial cord of the brachial plexus (C8-T1). The ulnar nerve lies medial to the brachial artery in the upper arm. At the elbow, the ulnar nerve passes between the medial epicondyle of the humerus and olecranon of the ulna. Posterior to the medial epicondyle the ulnar nerve is easily palpable.
The ulnar nerve subsequently runs through two main tunnels that can lead to compression:
Cubital tunnel at the elbow: bordered by the medial epicondyle, olecranon and arcuate ligament (connects two heads of flexor carpi ulnaris)
Guyon’s (ulna) canal at the wrist: a groove between the pisiform carpal bone and the hook of hamate that are joined by the palmar carpal ligament
The axillary nerve - sensory
The axillary nerve supplies a small oval-shaped patch of cutaneous skin on the lateral aspect of the shoulder. This is often known as the ‘regimental badge area’. It also carries sensory information from the glenohumeral joint (i.e. the shoulder joint).
Meralgia paraesthetica
Meralgia paraesthetica refers to pain and/or sensory loss over the anterolateral thigh.
Meralgia paraesthetica is due to neuropathy of the lateral femoral cutaneous nerve which is a small sensory nerve supplying cutaneous tissue over the anterolateral portion of the thigh. The nerve branches from the lumbar plexus roots L2/L3 and runs lateral to the psoas muscle and then passes underneath the inguinal ligament.
