1 APRIL 2022 ausdoc . com . au encountered metabolic / toxic neuropathies .

COMMON ENTRAPMENT NEUROPATHIES

THE MEDIAN , ulnar , radial and fibular nerves are most commonly affected by entrapment neuropathies . Early diagnosis and effective management are important for improving quality of life and preserving limb function . Neurophysiological studies form the cornerstone of diagnosis and enable stratification of nerve dysfunction with treatment directed to the individual level of impairment . Ancillary tests , including ultrasound of the nerve , aid confirmation and localisation . ⁵⁰

Figure 3 . Left foot of a patient with autonomic neuropathy and a plantar diabetic foot ulcer . The dry skin is prone to cracking , fissuring , and ulceration as illustrated beneath the first metatarsal head .

Del Core MA et al . Foot & Ankle Orthopaedics . July 2018 ; Reprinted by Permission of SAGE Publication

Carpal tunnel syndrome

Median neuropathy ( or carpal tunnel syndrome ) resulting from median nerve compression at the wrist is the most common mononeuropathy in adults . ⁵⁰

The prevalence of carpal tunnel syndrome may be significantly higher in certain occupations and among women . ⁵¹

Paraesthesia and pain affecting the thumb , index and middle fingers and the lateral aspect of the fourth finger is the classical presentation ( see figure 6 ). Patients may report sensory symptoms involving the entire hand and proximal radiation to the upper limb . Sensory symptoms are worse at night , often waking the patient , and are exacerbated by repetitive activities requiring wrist flexion .

Examination reveals sensory loss on the palmar aspect of the thumb , index and middle fingers and lateral fourth digit . ⁵⁰

Motor symptoms indicate an advanced stage and include weak grip and difficulty performing fine motor tasks . The specific finding of selective weakness of abductor pollicis brevis and opponens pollicis with preserved strength in the flexor pollicis longus helps localise the pathology to the carpal tunnel . Tinel sign ( pain / paraesthesia on nerve percussion ) has a low sensitivity ( 30-43 %) and a specificity up to 65 %. ⁵² The Phalen test , performed by asking the patient to flex the wrists and press the dorsum of both hands together for 30-60 seconds , has a sensitivity of 50-67 %, and lower specificity ( 15-17 %) because of false positives from musculoskeletal pain hindering the interpretation . ^{53 , 54} A negative Phalen test ( no reproduction of symptoms ) is highly predictive of normal nerve conduction studies . ⁵⁵

An increased median nerve cross-sectional area of more than 10mm ² on ultrasound has a high sensitivity and specificity for carpal tunnel syndrome . ⁵⁶

Treatment is stratified based on severity ( see table 5 ).

ACQUIRED IMMUNE DEMYELINATING NEUROPATHIES

IT IS important to recognise demyelinating polyneuropathies because , if detected in time , their immune-mediated aetiopathogenesis is often amenable to treatment . There is a specific set of clinical features that enables rapid clinical diagnosis ; further confirmation is with neurophysiological assessment and ancillary testing , including CSF examination . These conditions include subacute inflammatory demyelinating polyradiculoneuropathy ( SIDP , which progresses over 4-8 weeks ), chronic

Figure 2 . Distal symmetric polyneuropathy .

Table 4 . Metabolic and toxic axonal neuropathies

Aetiology Predominant pattern Affected fibres Evolution Comment

Amyloid neuropathy Sensory , autonomic Small fibres Carpal tunnel syndrome is common

inflammatory demyelinating polyradiculoneuropathy ( CIDP , which progresses beyond eight weeks and has several clinical types ) and Guillain-Barré syndrome where progression occurs over up to four weeks . The distinction is important as the type of immunomodulation and response to immunomodulation varies . ⁵⁷

Guillain-Barré syndrome

Guillain-Barré syndrome ( GBS ) is an acute polyneuropathy . It can occur at any age , from neonates to the elderly , with mean age of onset around 40 and a slight male preponderance . ⁵⁷

Common presentations include initial sensory symptoms ( acral paraesthesia ) followed shortly by ascending motor symptoms . The latter are often symmetrical and progressive , peaking by 3-4 weeks after onset . ⁵⁸ They may range from mild weakness to flaccid quadriplegia , and , in about a third of patients , respiratory failure . ⁵⁸ Bifacial lower motor neuron weakness , present in almost half of patients , often helps clinch the diagnosis . A significant

GBS mimic

Vitamin B12 deficiency ( see figure 5 )

Folate deficiency Sensory Large fibres Subacute-chronic Rarely occurs in isolation

Vinca alkaloids ( vinblastine , vincristine )

Figure 4 . Diabetic amyotrophy .

Isoniazid Sensorimotor Large fibres Acute-subacute Pyridoxine administration can be preventive

Heavy metals ( arsenic , thallium ) Sensorimotor Large fibre Acute-subacute GBS / porphyria mimic

number of patients have radicular pain , although the sensory loss is usually mild . ⁵⁸ There is a wide range of GBS variants with differing presentations .

Nerve conduction studies have varying sensitivity depending on their timing , ranging from 20 % at 1-2 weeks to 87 % at 4-5 weeks . ⁵⁹

The neurophysiological features evolve from early prolongation of F-wave latencies , followed by prolongation of distal motor latencies and subsequently temporal dispersion with or without conduction blocks , and

Reproduced from BMJ Case Reports , Khan ZU , et al ; 14 : e239869 ,

2021 with permission from BMJ Publishing Group Ltd .

then a drop in velocities of nerve conduction . ⁵⁹ It is important to be aware of both the sensitivity and evolution of neurophysiological findings to enable diagnosis based on clinical suspicion ; this enables early treatment despite neurophysiological findings not being supportive in the early stages of disease . Note that neurophysiological abnormalities can persist well beyond clinical recovery and do not indicate a diagnosis of CIDP .

Plasma exchange and IV immunoglobulin ( IVIg ) are the PAGE 48