From the Guidelines
Increased tibial motor latency and decreased amplitude on EMG are typically caused by tibial nerve damage or dysfunction, with common causes including peripheral neuropathies, nerve compression, trauma, and systemic conditions, as well as metabolic disorders, which can be diagnosed through a thorough neurological examination, blood tests, and imaging, and treated based on the underlying cause. The causes of these EMG findings can be broadly categorized into:
- Peripheral neuropathies, such as diabetic neuropathy, which can cause demyelination and axonal loss 1
- Nerve compression, including tarsal tunnel syndrome, which can lead to slowed nerve conduction and muscle fiber reduction
- Trauma to the nerve, which can result in axonal loss and demyelination
- Systemic conditions, such as amyotrophic lateral sclerosis or Guillain-Barré syndrome, which can affect the tibial nerve and cause impaired electrical signal transmission
- Metabolic disorders, including vitamin B12 deficiency, hypothyroidism, and uremia, which can produce similar EMG findings 1 The evaluation of patients with these EMG findings should include a thorough neurological examination, blood tests for metabolic causes, and possibly additional imaging like MRI to identify compression, as recommended by the American College of Radiology 1. Treatment depends on the underlying cause, and may involve addressing diabetes control, surgical decompression for entrapment, vitamin supplementation for deficiencies, or immunotherapy for inflammatory conditions, with the goal of preventing permanent nerve damage and disability, as emphasized by the American College of Cardiology 1.
From the Research
Causes of Increased Tibial Motor Latency and Decreased Amplitude on EMG
- Increased tibial motor latency and decreased amplitude on EMG can be caused by various factors, including:
- Distal symmetric diabetic polyneuropathy, which can lead to slowed motor conduction velocity of the tibial nerve across the tarsal tunnel 2
- Tarsal tunnel syndrome, a neuropathy due to compression of the posterior tibial nerve and its branches, which can cause abnormally slow nerve conduction through the posterior tibial nerve 3
- Focal tibial nerve entrapment at the medial ankle, as occurs in posterior tarsal tunnel syndrome, which can be assessed using tibial motor and sensory nerve conduction studies with motor recording from the flexor hallucis brevis 4
- Normal values for tibial motor conduction to the abductor hallucis have been established, with mean onset latency of 4.5 ms and mean amplitude of 15.3 mV, and nerve conduction velocity ranging from 44 to 51 m/s depending on demographic groups of various ages and heights 5
- Reference values for peroneal nerve motor conduction to the tibialis anterior and for peroneal vs. tibial latencies have also been generated, with upper limits of normal latency increase of peroneal over tibial being 1.8 msec and tibial over peroneal being 1.5 msec 6
Electrophysiological Findings
- Electrophysiological studies, including nerve conduction studies and electromyography, can be used to assess tibial nerve function and diagnose conditions such as distal symmetric diabetic polyneuropathy and tarsal tunnel syndrome 2, 3
- Abnormal tibial motor conduction velocity across the tarsal tunnel can be a sensitive parameter in diagnosing distal symmetric diabetic polyneuropathy 2
- Motor recording from the flexor hallucis brevis can provide technically clear waveforms and improve the ability to assess tibial nerve function in the short segments used to assess tarsal tunnel syndrome 4