acute foot drop

Introduction

acute foot drop is an uncommon presentation to the ED but often represents a genuine time critical emergency
foot drop is due to acute weakness of tibialis anterior (TA)
foot dorsiflexors are typically innervated by the L5 nerve root, but there may be considerable segmental overlap and anatomical variations, adding to the complexity of innervation
Most leg muscles have a dual root innervation, and usually TA is innervated primarily by L4 and secondarily by L5. EHL and EDL are primarily innervated by L5 and secondarily by L4 and S1.
TA and EHL are innervated by the L5 root in 90% of people
up to 10% of the population have an anatomical variant in which the S1 root innervates these muscles

it is important to differentiate from a flail foot in which NO movements at the ankle joint are possible and this represents a more extensive problem such as stroke (CVA) or cauda equina syndrome (CES)

there may be only a small window of opportunity to have a successful outcome
compartment syndrome must be recognized and treated WITHIN HOURS at most to minimise permanent deficits
cauda equina syndrome (CES) must be managed neurosurgically within 24-48hrs to minimise permanent deficits

ischaemia of tibialis anterior with local pain
- compartment syndrome
peroneal nerve mononeuropathy (the most common cause, and is usually painless)
- pressure on the nerve around the head of the fibula
- trauma
- nerve entrapment
- infection (eg. syphilis)
- space occupying lesions
- diabetes
- vasculitis
- iatrogenic injury
L4 or L5 nerve root radiculopathy with decreased reflexes and usually with back pain and rarely, may cause acute bilateral foot drop
- sciatica
  - lumbar disc disease
  - fracture L5 vertebra eg. Paget's disease
- cauda equina syndrome (CES)
UMN spinal cord pathology with brisk reflexes
- acute spinal cord compression
intracranial causes - usually with other neurology of the leg and possibly ipsilateral arm
- a parasagittal brain lesion may affect the leg area of the motor cortex and result in an isolated paraparesis
  - anterior cerebral artery stroke
  - parasagittal meningioma or other tumour
  - falling bullet hitting head in parasagittal region ¹⁾
- lesions in other areas where nerve fibres are highly condensed:
  - corona radiata
  - internal capsule (lacune)
  - cerebral peduncle (lacune)
  - medulla
  - spinal cord pyramidal tract (myelopathy)

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