Focal Neurological Deficit: Definition
A focal neurological deficit is a clinical expression of structural or functional abnormality—whether transient or permanent—that can be attributed to a specific anatomical region or pathway of the brain, manifesting as motor weakness, sensory disturbances, visual field defects, language impairment, cranial nerve palsies, or other localized neurological dysfunction. 1, 2, 3
Core Defining Characteristics
The deficit must be lateralizing or localizable to a specific neuroanatomical region. 2 This distinguishes true focal deficits from generalized symptoms:
- Focal deficits include: unilateral motor weakness (hemiparesis/hemiplegia), hemisensory loss, visual field defects (hemianopia), aphasia, isolated cranial nerve palsies, or focal ataxia 1, 2
- Not focal deficits: diffuse confusion, global weakness, bilateral symptoms without clear localization, generalized lethargy, or isolated headache 2
Clinical Classification Systems
Focal deficits can be categorized by relationship to hemorrhage:
- Hemorrhagic FND (with radiological evidence of recent bleeding) 1
- Non-hemorrhagic FND (structural lesion without acute hemorrhage) 1
- FND not otherwise specified (NOS-FND), such as deficits from migraine or epileptic seizure with acute hemorrhage ruled out 4, 1
Focal deficits can also be classified by duration:
- Transient FND (resolves, but requires urgent evaluation as may represent warning hemorrhages or TIA) 1, 2
- Persistent FND (ongoing deficit) 1
- Progressive FND (worsening over time) 4, 1
Common Etiologies
The most frequent causes include:
- Stroke (ischemic or hemorrhagic): sudden onset due to interrupted blood supply or bleeding 4, 1
- Cavernous malformations: causing deficits through hemorrhage, mass effect, or edema 4, 1
- Subarachnoid hemorrhage: 10.3% of aneurysmal SAH patients present with focal deficits from intraparenchymal hematoma (45.5%), early cerebral infarction (22.7%), parenchymal compression by subarachnoid thrombus (18.2%), or Todd's paralysis (13.6%) 5
- Seizures: post-ictal Todd's paralysis causing temporary focal weakness 1, 2
- Traumatic brain injury: focal damage to specific brain regions 1
Standardized Assessment
Use the NIH Stroke Scale (NIHSS) to quantify deficits rapidly and uniformly. 4, 1 The NIHSS assesses:
- Level of consciousness and orientation 4
- Gaze, visual fields, and facial movement 4
- Motor function in arms and legs (scored 0-4 for each limb) 4
- Limb ataxia, sensory loss, language, and articulation 4
- Extinction or inattention 4
This standardized approach helps identify vessel occlusion location, provides early prognosis, facilitates communication between providers, and guides treatment selection. 4
Critical Diagnostic Pitfall
Do not dismiss transient focal deficits as benign—they may represent warning hemorrhages, TIA, or seizure activity requiring urgent evaluation. 2 Brain imaging (CT or MRI) should be performed as soon as possible after symptom onset to detect acute hemorrhage or non-hemorrhagic lesions. 1, 2