S04.21 Injury of trochlear nerve, right side

ICD-10-CM Code S04.21: Injury of trochlear nerve, right side

ICD-10-CM code S04.21 designates an injury of the trochlear nerve on the right side. The trochlear nerve originates from the dorsal midbrain and terminates on the superior oblique muscle. Injury to this nerve can result in diplopia. Official coding references list specific encounter extensions for this diagnosis, such as initial encounters. 2 3 4

Plain-language overview

The trochlear nerve has the longest intracranial course of all cranial nerves and is the only one to originate from the dorsal midbrain. It decussates in the superior medullary velum and enters the cavernous sinus to terminate on the superior oblique muscle. 2

What this code represents

ICD-10-CM code S04.21 represents an injury of the trochlear nerve on the right side. Official diagnosis code listings include S04.21XA to specify an initial encounter for this right-sided trochlear nerve injury. 3 4

Clinical context

Trochlear nerve palsy resulting in diplopia is documented as a complication of percutaneous radiofrequency treatment of the gasserian ganglion. In one reported case, trochlear nerve function spontaneously recovered to normal five months after the procedure. 2

Coding details

A Centers for Medicare and Medicaid Services billing and coding article lists S04.21XD as a code for injury of the trochlear nerve, right side. An Extracorporeal Life Support Organization registry list of ICD-10 diagnosis codes includes S04.21XA for the initial encounter. 3 4

Documentation considerations

The CMS billing and coding article provides guidance that includes the diagnosis code S04.21XD for an injury of the trochlear nerve, right side. The ELSO registry support document lists S04.21 and S04.21XA within its ICD-10 diagnosis codes. 3 4

What the sources add

A clinical case report details the anatomical path of the trochlear nerve and notes that a decussating injury can occur separately from the trochlear nucleus. A CMS article and an ELSO registry list provide specific alphanumeric ICD-10 codes for right-sided trochlear nerve injuries. 2 3 4

Coverage and utilization context

The Centers for Medicare and Medicaid Services article provides billing and coding guidance that includes the code S04.21XD. The ELSO document provides a registry list of ICD-10 diagnosis codes that includes S04.21 and S04.21XA. 3 4

Questions to verify

Does the patient's clinical documentation specify whether the current encounter for the right-sided trochlear nerve injury is an initial encounter or a subsequent encounter? The sources list initial encounter codes but do not detail subsequent encounter codes. 3 4

Sources

1. Clinical outcomes of corneal neurotization using sural nerve graft in neurotrophic keratopathy — pmc.ncbi.nlm.nih.gov; accessed 2026-07-26.
2. Percutaneous radiofrequency treatment of the gasserian ganglion for trigeminal neuralgia complicated by trochlear nerve palsy: a case report — pmc.ncbi.nlm.nih.gov; accessed 2026-07-26.
3. Article - Billing and Coding: Nerve Conduction Studies and Electromyography (A57123) — cms.gov; accessed 2026-07-26.
4. ICD-10 Diagnosis Codes | Extracorporeal Life Support | ECMO — elso.org; accessed 2026-07-26.