Fourth [trochlear] nerve palsy, left eye

This diagnosis code signifies a paralysis or weakness of the fourth cranial nerve (trochlear nerve) affecting the left eye. This nerve is responsible for innervating the superior oblique muscle, which controls downward and inward eye movement. Patients typically present with vertical diplopia (double vision) that worsens when looking down or to the right, and may exhibit a compensatory head tilt away from the affected eye.

Use this code when documentation clearly indicates a diagnosis of trochlear nerve palsy specifically affecting the left eye. This often follows trauma, vascular events, or congenital conditions. Supporting documentation should include findings from an ophthalmological or neurological examination confirming left superior oblique muscle weakness or paralysis.

• Ensure laterality is explicitly documented as "left eye" to support this specific code.
• Differentiate from other cranial nerve palsies, particularly oculomotor (CN III) or abducens (CN VI), which present with different patterns of eye movement deficits.
• Code any underlying cause, such as trauma or diabetes, as an additional diagnosis.