HS is a rare condition characterized by the classic symptoms of ptosis, miosis, and ipsilateral anhidrosis. The term “Horner’s syndrome” was coined by Swiss ophthalmologist Johann Friedrich Horner in 1869. The prevalence of HS is estimated to be approximately one in 6,000 individuals. However, postoperative HS following thyroidectomy is even more uncommon, with an incidence rate of approximately 0.2% among relevant postoperative complications, as reported in existing research (6).

Endoscopic thyroidectomy approaches include the gasless transaxillary endoscopic thyroidectomy (GTET), bilateral axillo-breast approach (BABA), retro-auricular (RA) facelift approach, and transoral endoscopic thyroidectomy via vestibular approach (TOETVA) (7). Laparoscopic surgery has been proven to be comparable to open surgery in terms of therapeutic effectiveness and safety. According to the latest meta-analysis, with the exception of GTET, the operation time for laparoscopic surgery is longer than that for open surgery. However, the two approaches have no significant difference in the overall length of hospital stay and the incidence of postoperative laryngeal nerve damage or hypocalcemia (7, 8). As a minimally invasive procedure, laparoscopic surgery can minimize postoperative neck scars and achieve cosmetic results (9), making it a preferred option for many patients. Surgical energy equipment plays a crucial role in laparoscopic thyroidectomy. This procedure involves operating within a confined surgical field through a small incision. The high-energy thermodynamic surgical machinery generates sufficient energy to quickly cut and separate blood vessels or tissues approximately 7 mm in diameter. In addition, it denatures collagen and elastin while promoting coagulation of blood vessels to effectively control bleeding.

HS is known to result from an impairment of the oculosympathetic nerve pathway. Understanding the anatomy of this pathway is crucial for comprehending the development of postoperative HS. The oculosympathetic nerve pathway consists of three-order neurons originating from the hypothalamus. The first-order neuron descends along the axon to the lower cervical or upper thoracic spinal cord at approximately the C8, T1, and T2 levels (10). The second-order neuron arises from the spinal cord nucleus and its axon ascends to the superior cervical ganglion near the bifurcation of the common carotid artery. The inferior thyroid artery is in close proximity to the intermediate ganglion (11). Subsequently, ascending fibers, vasomotor fibers, and sweat gland–distributed nerves form the oculosympathetic fiber, enabling them to independently carry out their specific functions (12). Various factors can lead to different levels of Horner’s syndrome based on the involvement of the first-, second-, or third-order neurons. Lesions within the intracranial or skull base region, such as brain tumors, cerebral ischemia, or cerebral hemorrhage, can potentially cause first-order neuron HS. Surgical procedures involving the cervical or thoracic regions can result in HS by affecting the second-order neuron of the cervical sympathetic nerve (13). Orbit diseases that affect the oculosympathetic fiber are associated with third-order neuron HS.

Although the risk factors can be conceptualized based on the anatomical structures involved, precise consequences can only be explained in a fraction of clinical cases. Therefore, further research is needed to fully understand the etiology and intricacies of post-thyroidectomy Horner’s syndrome (14).

With the increasing popularity of ETS, there is a growing demand for procedures that combine esthetic outcomes with radical cure. However, it is important to note that the incidence of postoperative HS following ETS is higher compared to open thyroid surgery. The association between HS and ETS primarily lies in iatrogenic injury during the thyroidectomy procedure. The consequences of such injuries must be considered based on the location and characteristics of the nerves involved in ETS (13). First, in endoscopic surgery, the surgical field is limited, making obtaining a comprehensive view of the complete branches of vessels and nerves within a narrow visual field challenging. Furthermore, retractors used to expose the thyroid gland can inadvertently cause direct, blunt damage to adjacent tissues or organs, including the carotid artery and sympathetic nerves surrounding the thyroid gland. In certain individuals, oculosympathetic branches may adhere to the thyroid gland, and there may also be communication branches between cervical nerves and recurrent laryngeal nerves, increasing the likelihood of unintentional injury to the oculosympathetic fiber. Ligating the inferior thyroid artery also poses a risk of damaging adjacent sympathetic nerves (4). Another contributing factor is the potential thermal injury caused by thermodynamic devices. Advanced surgical instruments such as ultrasonic knives and bipolar electrocautery provide convenient and precise operations. However, endoscopy relies more on heat conduction compared to conventional surgery. The cervical sympathetic nerve is particularly vulnerable to heat-related damage.

To minimize the risk of postoperative HS in patients with ETS, surgical techniques should be employed with meticulous precision and care to protect the delicate nerves and blood vessels in the surgical field. Further research and improvements in surgical approaches are necessary to reduce the occurrence of HS and other complications associated with ETS.

It is important to note that the closure of the carotid artery sheath with an ultrasonic knife for separating the thyroid gland from the surrounding lymphoid and adipose tissue may result in temporary or permanent nerve damage due to heat conduction (15). In addition, there are other factors that can potentially cause Horner’s syndrome, including but not limited to thyroid neoplasms or huge goiter compression (3), arteritis (16), central nervous system infection (3), peridural obstetric anesthesia (17, 18), and occasionally lung transplantation.

In conclusion, it is crucial to be aware of the potential risks of Horner’s syndrome during endoscopic thyroid surgery.

As mentioned earlier, HS is closely related to dysfunction of the sympathetic nervous system. It primarily manifests externally through oculosympathetic lesions at specific anatomical sites. The classical symptoms of HS include upper eyelid ptosis, miosis, and ipsilateral facial or chest wall anhidrosis, which can occur together or separately and serve as important indicators for clinical diagnosis. Organs located near the damaged nerves’ distribution areas are susceptible to impairment, and the severity of the impairment depends on the extent of damage to the sympathetic nerves. For example, the motion of pupil constriction is mainly regulated by parasympathetic nerves, while the activation of the sympathetic nerves controls dilation. Consequently, miosis, which results from parasympathetic function, prevails in 3-neuron oculosympathetic lesions. In addition to the example mentioned above, the activity of periocular muscles corresponds to the neural network distributed in the orbit. Activating the levator palpebrae superior muscle allows the upper eyelid to maintain a raised position. Therefore, if there is damage to the sympathetic nerves, the eyelid may droop, resulting in noticeably unequal palpebral fissure size. Another symptom of HS is anhidrosis; although uncommon, it is considered a classical clinical manifestation. The sympathetic system controls facial sweating, so perspiration dysfunction is a facial sign that aids diagnosis.

It is important to note that postoperative HS is usually reversible and not a significant obstacle. However, the long-term prognosis remains uncertain and requires careful monitoring during follow-up (19).

First and foremost, the patient's complaint raises an undeniable alarm, highlighting the necessity for a thorough diagnosis. It is crucial to consider both the present and past medical history to distinguish whether the condition is primary or secondary. The etiology of the condition requires careful investigation and collaboration with prominent neurologists. Potential underlying factors include infection, postoperative complications, anesthesia methods, and other intriguing factors. As an auxiliary examination, the implementation of apraclonidine is suggested. It is important to note that the downstream effects of sympathetic nerve lesions are not as sensitive as the upper ones. However, activation of the sympathetic nerves through apraclonidine (an alpha-adrenergic agonist) can alleviate ptosis and miosis by sensitizing these nerves (20). The development of artificial technology, particularly facial recognition, has shown limitless potential in assisting with the diagnosis of Horner’s syndrome. This technology automatically detects HS from facial images and has demonstrated promising results (21).

In cases of post-thyroidectomy HS, a conservative treatment approach is often preferred, involving neurotrophic drugs (22). In this particular case, it is recommended to prescribe Mecobalamin, along with vitamin B1, under the guidance of an experienced neurologist for neural repair. Remarkably, the effectiveness of this treatment can be validated based on the alleviation of symptoms, providing evidence for the availability of conservative treatment. However, it is important to acknowledge that long-term follow-up may show no improvement in some cases, leading to cosmetic disfigurement and potential mental disorders, which present significant challenges (23).

As a sporadic complication following endoscopic thyroid surgery, HS deserves significant attention. The surgical procedure holds a critical position and must be executed with utmost care, particularly regarding the preservation of anatomical structures, critical vessels, and nerves. A key factor in reducing the incidence of postoperative HS is the intact and precise separation of the prevertebral fascia and carotid sheath (24). As mentioned earlier, thermal damage caused by surgical instruments plays an essential role in the development of HS. Intermittent-intraoperative neural monitoring (I-IONM) effectively detects the distribution of important nerves around the thyroid gland, particularly facilitating the monitoring and protection of the recurrent laryngeal nerve from thermal damage (25, 26). Surgeons face the challenge of adapting to the limited vision of endoscopy, which lacks tactile feedback compared to conventional surgery. This poses a greater difficulty in preserving nerves during the procedure. It is recommended that surgeons carefully consider the merits and drawbacks of conventional and endoscopic surgery approaches to determine the optimal operative approach.

In summary, although the morbidity rate of postoperative HS is low, prompt diagnosis and therapy are crucial for improving therapeutic efficacy. Surgeons should be vigilant in their diagnosis and treatment to achieve the best possible outcomes.